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Re: ADsafe, Take 6

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  • Douglas Crockford
    ... The set of HTML confusions is vast, but not infinite. An advantage here is that JSLint/ADsafe does not have to pass all valid HTML. I can be semidraconian
    Message 1 of 30 , Oct 19, 2007
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      --- In caplet@yahoogroups.com, "Mike Samuel" <mikesamuel@...> wrote:

      > There are two problems here:
      > (1) Identifying a safe subset of HTML/CSS and Javascript -- without
      > obscure extensions like expression()
      > (2) The other is making sure the browser interprets it the same way
      > you do -- rejecting malformed markup, conditional compilation
      > comments, etc.
      >
      > The first is not solved by translating. You can filter to a safe
      > subset just as easily as translating to a safe subset.
      >
      > The second is an easier problem. If you start with a validating XHTML
      > parser, and relax constraints as you convince yourself it's safe, then
      > you can be confident that the browser will produce the same parse
      > tree.

      The set of HTML confusions is vast, but not infinite. An advantage
      here is that JSLint/ADsafe does not have to pass all valid HTML. I can
      be semidraconian in rejecting forms on the grounds that they look
      suspicious or faulty. That permits some alexandrian solutions. For
      example, I don't have to understand the amazing complexity of
      entities. I can simply forbid the use of ampersand in some contexts.
      Ampersands are sometimes useful and sometimes valid, but I can still
      exclude them from my subset simply because I don't want to deal with
      them. Having done that, I feel more confident that I won't be surprised.
    • collin_jackson
      Here s another one:
      Message 2 of 30 , Oct 19, 2007
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        Here's another one:

        <iframe/src="javascript:alert(42)"></iframe>

        --- In caplet@yahoogroups.com, "Douglas Crockford" <douglas@...> wrote:
        >
        > --- In caplet@yahoogroups.com, "collin_jackson" <collinj@> wrote:
        > >
        > > <div x="\"><img onload=alert(42)
        > > src=http://json.org/img/json160.gif>"></div>
        >
        > Excellent. Keep them coming.
        >
      • Adam Barth
        Why is ADsafe allowing invalid HTML at all? It seems like requiring the HTML to be well-formed is a good first step in trying to understand how it will be
        Message 3 of 30 , Oct 19, 2007
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          Why is ADsafe allowing invalid HTML at all? It seems like requiring
          the HTML to be well-formed is a good first step in trying to
          understand how it will be executed in different browsers.

          On 10/19/07, collin_jackson <collinj@...> wrote:
          > Here's another one:
          >
          > <iframe/src="javascript:alert(42)"></iframe>
          >
          > --- In caplet@yahoogroups.com, "Douglas Crockford" <douglas@...> wrote:
          > >
          > > --- In caplet@yahoogroups.com, "collin_jackson" <collinj@> wrote:
          > > >
          > > > <div x="\"><img onload=alert(42)
          > > > src=http://json.org/img/json160.gif>"></div>
          > >
          > > Excellent. Keep them coming.
          > >
          >
        • Adam Barth
          One simple way to approximate this (if you didn t want to reuse someone else s code for validating HTML) would be to serialize your parsed HTML back to an
          Message 4 of 30 , Oct 19, 2007
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            One simple way to approximate this (if you didn't want to reuse
            someone else's code for validating HTML) would be to serialize your
            parsed HTML back to an octet-stream and compare it with the input
            (probably being tolerate of whitespace and capitalization in the
            appropriate places).

            On 10/19/07, Adam Barth <hk9565@...> wrote:
            > Why is ADsafe allowing invalid HTML at all? It seems like requiring
            > the HTML to be well-formed is a good first step in trying to
            > understand how it will be executed in different browsers.
            >
            > On 10/19/07, collin_jackson <collinj@...> wrote:
            > > Here's another one:
            > >
            > > <iframe/src="javascript:alert(42)"></iframe>
            > >
            > > --- In caplet@yahoogroups.com, "Douglas Crockford" <douglas@...> wrote:
            > > >
            > > > --- In caplet@yahoogroups.com, "collin_jackson" <collinj@> wrote:
            > > > >
            > > > > <div x="\"><img onload=alert(42)
            > > > > src=http://json.org/img/json160.gif>"></div>
            > > >
            > > > Excellent. Keep them coming.
            > > >
            > >
            >
          • Larry Masinter
            I think you got it backward: URIs are sequences of characters, not bytes. and in (X)HTML, URI is really IRI – the XHTML spec allows full Unicode (10646)
            Message 5 of 30 , Oct 19, 2007
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              I think you got it backward: URIs are sequences of characters, not bytes.  and in (X)HTML, "URI" is really "IRI" – the XHTML spec allows full Unicode (10646) characters which are UTF8 and then hex-encoded if you need an (old-fashioned) URI.

               

               

               

              From: caplet@yahoogroups.com [mailto:caplet@yahoogroups.com] On Behalf Of Mike Samuel
              Sent: Wednesday, October 17, 2007 4:16 PM
              To: caplet@yahoogroups.com
              Subject: Re: [caplet] Re: ADsafe, Take 6

               

              RFC 3986 disallows the null byte in URIs, and says URIs are sequences of bytes, not characters, so 65533 is out of range.

              In your attribute whitelist, can't you identify all whose value is a URI or URI Reference, and restrict the unescaped value to the union of the reserved and unreserved characters and '%'.

                    unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"

                    reserved    = gen-delims / sub-delims

                    gen-delims  = ":" / "/" / "?" / "#" / "[" / "]" / "@"


                    sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
                                / "*" / "+" / "," / ";" / "="
                


              cheers,
              mike

              On 17/10/2007, collin_jackson <collinj@...> wrote:

              I'm not pasting. I'm reading the value of a textarea into JSLint
              directly using JavaScript.

              See http://crypto.stanford.edu/jsonrequest/nullbyte2.html

              It looks like Firefox is converting null bytes to Unicode character
              65533, which isn't rejected by JSLint. So all you need to do is reject
              Unicode character 65533 to defeat this attack.

              (Note that null bytes vanish in IE, which is fine as long as Firefox
              rejects them.)

              --- In caplet@yahoogroups.com, "Douglas Crockford" <douglas@...> wrote:

              >
              > --- In
              target="_blank">caplet@yahoogroups.com, "collin_jackson" <collinj@> wrote:
              > >
              > > Null byte between "java" and
              "script" passes JSLint on Firefox
              despite
              > > being an attack on IE
              >
              > I scan every line for null and other characters. I am guessing
              that
              > the null is lost in the browser's paste process. In
              production,
              > inspection will be done on files, so I don't think that will
              be a
              problem.
              >

               

            • Douglas Crockford
              ... It shouldn t. So I am grateful to Collin for reporting a case where it did.
              Message 6 of 30 , Oct 19, 2007
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                --- In caplet@yahoogroups.com, "Adam Barth" <hk9565@...> wrote:

                > Why is ADsafe allowing invalid HTML at all?

                It shouldn't. So I am grateful to Collin for reporting a case where it
                did.
              • Mike Samuel
                Sorry. I was reading 2396 (not 3986) which says An escaped octet is encoded as a character triplet, consisting of the percent character % followed by the
                Message 7 of 30 , Oct 19, 2007
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                  Sorry.  I was reading 2396 (not 3986) which says
                     An escaped octet is encoded as a character triplet, consisting of the
                  percent character "%" followed by the two hexadecimal digits
                  representing the octet code. For example, "%20" is the escaped
                  encoding for the US-ASCII space character.

                  I think that says that each part of the URI is a sequence of bytes (since only octets can be encoded as hex pairs) which is converted to a sequence of ASCII characters, so you're right in that those characters then need to be encoded for transport.

                  But I don't understand where full unicode characters are allowed.  Ignoring non-standard extensions like %uxxxx, where are you seeing those?




                  On 19/10/2007, Larry Masinter <lmm@...> wrote:

                  I think you got it backward: URIs are sequences of characters, not bytes.  and in (X)HTML, "URI" is really "IRI" – the XHTML spec allows full Unicode (10646) characters which are UTF8 and then hex-encoded if you need an (old-fashioned) URI.

                   

                   

                   

                  From: caplet@yahoogroups.com [mailto:caplet@yahoogroups.com ] On Behalf Of Mike Samuel
                  Sent: Wednesday, October 17, 2007 4:16 PM
                  To: caplet@yahoogroups.com
                  Subject: Re: [caplet] Re: ADsafe, Take 6

                   

                  RFC 3986 disallows the null byte in URIs, and says URIs are sequences of bytes, not characters, so 65533 is out of range.

                  In your attribute whitelist, can't you identify all whose value is a URI or URI Reference, and restrict the unescaped value to the union of the reserved and unreserved characters and '%'.

                        unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"

                        reserved    = gen-delims / sub-delims

                        gen-delims  = ":" / "/" / "?" / "#" / "[" / "]" / "@"


                        sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
                                    / "*" / "+" / "," / ";" / "="
                   


                  cheers,
                  mike

                  On 17/10/2007, collin_jackson <collinj@...> wrote:

                  I'm not pasting. I'm reading the value of a textarea into JSLint
                  directly using JavaScript.

                  See http://crypto.stanford.edu/jsonrequest/nullbyte2.html

                  It looks like Firefox is converting null bytes to Unicode character
                  65533, which isn't rejected by JSLint. So all you need to do is reject
                  Unicode character 65533 to defeat this attack.

                  (Note that null bytes vanish in IE, which is fine as long as Firefox
                  rejects them.)

                  --- In caplet@yahoogroups.com, "Douglas Crockford" <douglas@...> wrote:
                  >
                  > --- In caplet@yahoogroups.com, "collin_jackson" <collinj@> wrote:
                  > >
                  > > Null byte between "java" and "script" passes JSLint on Firefox
                  despite
                  > > being an attack on IE
                  >
                  > I scan every line for null and other characters. I am guessing that
                  > the null is lost in the browser's paste process. In production,
                  > inspection will be done on files, so I don't think that will be a
                  problem.
                  >

                   


                • Adam Barth
                  ... It seems to be accepting lots of invalid HTML. For example, the simple seems to pass, whereas http://validator.w3.org/check
                  Message 8 of 30 , Oct 19, 2007
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                    On 10/19/07, Douglas Crockford <douglas@...> wrote:
                    > --- In caplet@yahoogroups.com, "Adam Barth" <hk9565@...> wrote:
                    > > Why is ADsafe allowing invalid HTML at all?
                    >
                    > It shouldn't. So I am grateful to Collin for reporting a case where it
                    > did.

                    It seems to be accepting lots of invalid HTML. For example, the simple

                    <iframe xx="yy"></iframe>

                    seems to pass, whereas http://validator.w3.org/check rejects

                    <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
                    "http://www.w3.org/TR/html4/loose.dtd">
                    <HTML>
                    <HEAD>
                    <TITLE>title</TITLE>
                    </HEAD>
                    <BODY>
                    <iframe xx="yy"></iframe>
                    </BODY>
                    </HTML>

                    You might believe the xx attribute isn't dangerous, but the HTML spec
                    doesn't give it semantics, so we don't really know what it does. It
                    seems much safer to restrict fragments to the HTML DTD.
                  • David Hopwood
                    ... URIs are sequences of characters that encode a sequence of bytes, which *may* in turn encode a sequence of Unicode characters. For URIs that have some
                    Message 9 of 30 , Oct 19, 2007
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                      Larry Masinter wrote:
                      > I think you got it backward: URIs are sequences of characters, not bytes.

                      URIs are sequences of characters that encode a sequence of bytes, which
                      *may* in turn encode a sequence of Unicode characters.

                      For URIs that have some server-specific part, the interpretation of the byte
                      sequence is up to that server. RFC 3986 *recommends* that, where they encode
                      a string, the encoding used should be UTF-8. However, there's no way to
                      enforce this (and no particular reason to enforce it). So it is valid, for
                      example, to have "%FF" in an URL, even though that is always an invalid byte
                      in UTF-8.

                      > and in (X)HTML, "URI" is really "IRI" – the XHTML spec allows full
                      > Unicode (10646) characters which are UTF8 and then hex-encoded if you need
                      > an (old-fashioned) URI.

                      XHTML still doesn't require that the sequence of bytes is valid UTF-8.

                      In any case, the immediate question was whether it is reasonable to reject
                      any input that contains 65533 (U+FFFD REPLACEMENT CHARACTER). IMHO it is:
                      this isn't a useful character in its own right; it indicates that an
                      encoding error occurred in producing the input.

                      --
                      David Hopwood <david.hopwood@...>
                    • Mike Samuel
                      ... I still don t understand. My reading of the spec says that the first sequence of characters is in ASCII. If that s the case, then an HTML validator should
                      Message 10 of 30 , Oct 19, 2007
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                        On 19/10/2007, David Hopwood <david.hopwood@...> wrote:
                        >
                        >
                        >
                        >
                        >
                        >
                        > Larry Masinter wrote:
                        > > I think you got it backward: URIs are sequences of characters, not bytes.
                        >
                        > URIs are sequences of characters that encode a sequence of bytes, which
                        > *may* in turn encode a sequence of Unicode characters.

                        I still don't understand.

                        My reading of the spec says that the first sequence of characters is in ASCII.

                        If that's the case, then an HTML validator should be able to reject
                        any HTML attribute of type URI whose value contains a codepoint
                        outside [0, 255] without making it possibly to express any valid URI.
                        Does that sound right?

                        If that's right, would it be appropriate for the error message to
                        recommend re-encoding the out of range characters using a %-encoding
                        of UTF-8? So "�" -> "%EF%BF%BD".



                        Also, on terminology, is the below right?
                        * An escaping is an n:1 mapping from strings in an alphabet A to
                        strings in an alphabet which is a subset of A.
                        * An encoding is a 1:1 mapping of strings over one alphabet to strings
                        over another alphabet.



                        >
                        >
                        >
                        >
                        >
                        >
                        > For URIs that have some server-specific part, the interpretation of the byte
                        > sequence is up to that server. RFC 3986 *recommends* that, where they encode
                        > a string, the encoding used should be UTF-8. However, there's no way to
                        > enforce this (and no particular reason to enforce it). So it is valid, for
                        > example, to have "%FF" in an URL, even though that is always an invalid byte
                        > in UTF-8.
                        >
                        > > and in (X)HTML, "URI" is really "IRI" – the XHTML spec allows full
                        > > Unicode (10646) characters which are UTF8 and then hex-encoded if you need
                        > > an (old-fashioned) URI.
                        >
                        > XHTML still doesn't require that the sequence of bytes is valid UTF-8.
                        >
                        > In any case, the immediate question was whether it is reasonable to reject
                        > any input that contains 65533 (U+FFFD REPLACEMENT CHARACTER). IMHO it is:
                        > this isn't a useful character in its own right; it indicates that an
                        > encoding error occurred in producing the input.
                        >
                        > --
                        > David Hopwood <david.hopwood@...>
                      • Larry Masinter
                        To answer your direct questions: I don t know any formal definition for escaping except as a part of encoding -- you encode a sequence of bytes into (a
                        Message 11 of 30 , Oct 21, 2007
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                          To answer your direct questions:

                          I don't know any formal definition for "escaping" except as a part of "encoding" -- you encode a sequence of bytes into (a subset of) US-ASCII by translating each (allowed) byte into its corresponding ASCII character, but translating some (disallowed) bytes into a different sequence.

                          I think it's reasonable to (a) reject Javascript that contains 65533 (U+FFFD REPLACEMENT CHARACTER) and (b) to not look for, parse, or handle in any special way URI references within (X)HTML attributes or content.

                          -----------------------------------------

                          Talking about this is complicated because the same concept appears with different encodings:

                          * The HTTP protocol does its work in sequences of bytes, but the spec is written in terms of "characters".


                          * The XML specification defines XML as a sequence of (Unicode) characters, encoded in UTF8 or UTF16 (or some other encoding) but also with &char; character entities and ￝ numeric character references. XHTML as an XML language follows this; whether HTML follows this depends on the HTML version.

                          * the Javascript specification (at least ECMA-262) defines Javascript as a sequence of (Unicode) characters encoded in UTF8 or UTF16. E4X (ECMA 357) seems to follow XML in using character entity references and numeric character references to encode characters that would otherwise be disallowed.

                          * The URI specification defines a URI as a sequence of characters, taken from the repertoire of US-ASCII characters, with the encoding chosen by the protocol/format that embeds it; it also defines an encoding (%xx) for bytes that would otherwise correspond to disallowed or reserved characters.

                          * The IRI specification defines an IRI similarly, but allows a larger repertoire of characters.

                          Parsing an XML stream into an XML DOM (including HTML) will translate the UTF8, UTF16 (or other encoding) as well as character and numeric character entity references, into a sequence of characters. Parsing a Javascript string (using E4X) will apparently do the same, even though Javascript-per-se parts and XHTML-constant parts use different escaping mechanisms.

                          A (X)HTML validator for content embedded within Javascript should likely perform the same entity and numeric character reference decoding logic as would apply when the Javascript was read and interpreted -- resolve character entity references and numeric character references -- and then validate the results.

                          There are many troublesome syntactically valid URIs (and IRIs) that could appear within a URI reference in (X)HTML and (X)HTML embedded within Javascript, but I think it is part of the security requirements of the (X)HTML interpreter runtime to manage and prevent those references. Because URIs (and IRIs) can and sometimes do encode non-character byte streams, looking for or managing the URI-encoding level would be inappropriate.

                          Larry








                          -----Original Message-----
                          From: caplet@yahoogroups.com [mailto:caplet@yahoogroups.com] On Behalf Of Mike Samuel
                          Sent: Friday, October 19, 2007 10:30 PM
                          To: caplet@yahoogroups.com
                          Subject: Re: [caplet] Re: ADsafe, Take 6

                          On 19/10/2007, David Hopwood <david.hopwood@...> wrote:
                          >
                          >
                          >
                          >
                          >
                          >
                          > Larry Masinter wrote:
                          > > I think you got it backward: URIs are sequences of characters, not bytes.
                          >
                          > URIs are sequences of characters that encode a sequence of bytes, which
                          > *may* in turn encode a sequence of Unicode characters.

                          I still don't understand.

                          My reading of the spec says that the first sequence of characters is in ASCII.

                          If that's the case, then an HTML validator should be able to reject
                          any HTML attribute of type URI whose value contains a codepoint
                          outside [0, 255] without making it possibly to express any valid URI.
                          Does that sound right?

                          If that's right, would it be appropriate for the error message to
                          recommend re-encoding the out of range characters using a %-encoding
                          of UTF-8? So "�" -> "%EF%BF%BD".



                          Also, on terminology, is the below right?
                          * An escaping is an n:1 mapping from strings in an alphabet A to
                          strings in an alphabet which is a subset of A.
                          * An encoding is a 1:1 mapping of strings over one alphabet to strings
                          over another alphabet.



                          >
                          >
                          >
                          >
                          >
                          >
                          > For URIs that have some server-specific part, the interpretation of the byte
                          > sequence is up to that server. RFC 3986 *recommends* that, where they encode
                          > a string, the encoding used should be UTF-8. However, there's no way to
                          > enforce this (and no particular reason to enforce it). So it is valid, for
                          > example, to have "%FF" in an URL, even though that is always an invalid byte
                          > in UTF-8.
                          >
                          > > and in (X)HTML, "URI" is really "IRI" – the XHTML spec allows full
                          > > Unicode (10646) characters which are UTF8 and then hex-encoded if you need
                          > > an (old-fashioned) URI.
                          >
                          > XHTML still doesn't require that the sequence of bytes is valid UTF-8.
                          >
                          > In any case, the immediate question was whether it is reasonable to reject
                          > any input that contains 65533 (U+FFFD REPLACEMENT CHARACTER). IMHO it is:
                          > this isn't a useful character in its own right; it indicates that an
                          > encoding error occurred in producing the input.
                          >
                          > --
                          > David Hopwood <david.hopwood@...>
                        • Mike Samuel
                          ... Ok. I think it s useful to make a distinction between the n:1 mappings and the 1:1 mappings. If you re escaping (which I defined as n:1), you have to
                          Message 12 of 30 , Oct 21, 2007
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                            On 21/10/2007, Larry Masinter <lmm@...> wrote:
                            >
                            >
                            >
                            >
                            >
                            >
                            > To answer your direct questions:
                            >
                            > I don't know any formal definition for "escaping" except as a part of "encoding" -- you encode a sequence of bytes into (a subset of) US-ASCII by translating each (allowed) byte into its corresponding ASCII character, but translating some (disallowed) bytes into a different sequence.

                            Ok. I think it's useful to make a distinction between the n:1
                            mappings and the 1:1 mappings.
                            If you're escaping (which I defined as n:1), you have to unescape
                            before comparing strings, while you can check against an encoded
                            string by either decoding the one or encoding the other.

                            One way to check attribute content in a markup language is to keep a
                            stack of escaping and encoding conventions as you examine the document
                            in the left to right pass.
                            To check whether an iframe's src's protocol is javascript: you deal
                            with the following stack
                            protocol <(%-escaped)> uri <(html-entity-escaped)> html-attribute
                            <(UTF-8 encoding)> bytes

                            If you're sending a message and you want it to be interpreted as you
                            intend, then you have to make sure that the recipient of the message
                            will use the same escaping/encodings, and if you want to verify
                            properties of the message, then you have to consider every escaping,
                            but not necessarily every encoding.

                            So for the javascript: check, there are 3 points of attack, but the
                            encoding can be considered entirely separately leaving you two.




                            >
                            > I think it's reasonable to (a) reject Javascript that contains 65533 (U+FFFD REPLACEMENT CHARACTER) and (b) to not look for, parse, or handle in any special way URI references within (X)HTML attributes or content.

                            How would you detect urls that can execute or import scripts without
                            distinguishing attributes that contain URIs or URI references, given
                            that it is a goal of ADSafe to allow iframes to external srcs?



                            >
                            > Talking about this is complicated because the same concept appears with different encodings:
                            >
                            > * The HTTP protocol does its work in sequences of bytes, but the spec is written in terms of "characters".
                            >
                            > * The XML specification defines XML as a sequence of (Unicode) characters, encoded in UTF8 or UTF16 (or some other encoding) but also with &char; character entities and ￝ numeric character references. XHTML as an XML language follows this; whether HTML follows this depends on the HTML version.
                            >
                            > * the Javascript specification (at least ECMA-262) defines Javascript as a sequence of (Unicode) characters encoded in UTF8 or UTF16. E4X (ECMA 357) seems to follow XML in using character entity references and numeric character references to encode characters that would otherwise be disallowed.
                            >
                            > * The URI specification defines a URI as a sequence of characters, taken from the repertoire of US-ASCII characters, with the encoding chosen by the protocol/format that embeds it; it also defines an encoding (%xx) for bytes that would otherwise correspond to disallowed or reserved characters.
                            >
                            > * The IRI specification defines an IRI similarly, but allows a larger repertoire of characters.
                            >
                            > Parsing an XML stream into an XML DOM (including HTML) will translate the UTF8, UTF16 (or other encoding) as well as character and numeric character entity references, into a sequence of characters. Parsing a Javascript string (using E4X) will apparently do the same, even though Javascript-per-se parts and XHTML-constant parts use different escaping mechanisms.
                            >
                            > A (X)HTML validator for content embedded within Javascript should likely perform the same entity and numeric character reference decoding logic as would apply when the Javascript was read and interpreted -- resolve character entity references and numeric character references -- and then validate the results.
                            >
                            > There are many troublesome syntactically valid URIs (and IRIs) that could appear within a URI reference in (X)HTML and (X)HTML embedded within Javascript, but I think it is part of the security requirements of the (X)HTML interpreter runtime to manage and prevent those references. Because URIs (and IRIs) can and sometimes do encode non-character byte streams, looking for or managing the URI-encoding level would be inappropriate.
                            >
                            > Larry
                            >
                            > -----Original Message-----
                            > From: caplet@yahoogroups.com [mailto:caplet@yahoogroups.com] On Behalf Of Mike Samuel
                            > Sent: Friday, October 19, 2007 10:30 PM
                            > To: caplet@yahoogroups.com
                            > Subject: Re: [caplet] Re: ADsafe, Take 6
                            >
                            >
                            > On 19/10/2007, David Hopwood <david.hopwood@...> wrote:
                            > >
                            > >
                            > >
                            > >
                            > >
                            > >
                            > > Larry Masinter wrote:
                            > > > I think you got it backward: URIs are sequences of characters, not bytes.
                            > >
                            > > URIs are sequences of characters that encode a sequence of bytes, which
                            > > *may* in turn encode a sequence of Unicode characters.
                            >
                            > I still don't understand.
                            >
                            > My reading of the spec says that the first sequence of characters is in ASCII.
                            >
                            > If that's the case, then an HTML validator should be able to reject
                            > any HTML attribute of type URI whose value contains a codepoint
                            > outside [0, 255] without making it possibly to express any valid URI.
                            > Does that sound right?
                            >
                            > If that's right, would it be appropriate for the error message to
                            > recommend re-encoding the out of range characters using a %-encoding
                            > of UTF-8? So "�" -> "%EF%BF%BD".
                            >
                            > Also, on terminology, is the below right?
                            > * An escaping is an n:1 mapping from strings in an alphabet A to
                            > strings in an alphabet which is a subset of A.
                            > * An encoding is a 1:1 mapping of strings over one alphabet to strings
                            > over another alphabet.
                            >
                            > >
                            > >
                            > >
                            > >
                            > >
                            > >
                            > > For URIs that have some server-specific part, the interpretation of the byte
                            > > sequence is up to that server. RFC 3986 *recommends* that, where they encode
                            > > a string, the encoding used should be UTF-8. However, there's no way to
                            > > enforce this (and no particular reason to enforce it). So it is valid, for
                            > > example, to have "%FF" in an URL, even though that is always an invalid byte
                            > > in UTF-8.
                            > >
                            > > > and in (X)HTML, "URI" is really "IRI" – the XHTML spec allows full
                            > > > Unicode (10646) characters which are UTF8 and then hex-encoded if you need
                            > > > an (old-fashioned) URI.
                            > >
                            > > XHTML still doesn't require that the sequence of bytes is valid UTF-8.
                            > >
                            > > In any case, the immediate question was whether it is reasonable to reject
                            > > any input that contains 65533 (U+FFFD REPLACEMENT CHARACTER). IMHO it is:
                            > > this isn't a useful character in its own right; it indicates that an
                            > > encoding error occurred in producing the input.
                            > >
                            > > --
                            > > David Hopwood <david.hopwood@...>
                          • Freeman, Tim
                            ... Okay, I ll try to say the obvious here -- although no one individual is responsible, we find ourselves in the middle of a big hacked-up pile of conventions
                            Message 13 of 30 , Oct 22, 2007
                            • 0 Attachment
                              Quoting Mike Samuel:
                              > To check whether an iframe's src's protocol is javascript: you deal
                              > with the following stack
                              > protocol <(%-escaped)> uri <(html-entity-escaped)> html-attribute
                              > <(UTF-8 encoding)> bytes

                              Okay, I'll try to say the obvious here -- although no one individual is
                              responsible, we find ourselves in the middle of a big hacked-up pile of
                              conventions that were put together with insufficient forethought. It
                              shouldn't
                              be this complicated to fetch information from a few computers and
                              display
                              it on another.

                              Has anyone put some thought into figuring out how the web should have
                              been and
                              writing it down? I feel in need of a comforting fantasy to read just
                              before
                              going to bed at night. Such a document would also help to visualize
                              some desired
                              destination so we can move in that direction when we make new stuff.
                              -----
                              Tim Freeman
                              Email: tim.freeman@...
                              Desk in Palo Alto: (650) 857-2581
                              Home: (408) 774-1298
                              Cell: (408) 348-7536


                              > -----Original Message-----
                              > From: caplet@yahoogroups.com [mailto:caplet@yahoogroups.com]
                              > On Behalf Of Mike Samuel
                              > Sent: Sunday, October 21, 2007 21:11
                              > To: caplet@yahoogroups.com
                              > Subject: Re: [caplet] Re: ADsafe, Take 6
                              >
                              > On 21/10/2007, Larry Masinter <lmm@...> wrote:
                              > >
                              > >
                              > >
                              > >
                              > >
                              > >
                              > > To answer your direct questions:
                              > >
                              > > I don't know any formal definition for "escaping" except
                              > as a part of "encoding" -- you encode a sequence of bytes
                              > into (a subset of) US-ASCII by translating each (allowed)
                              > byte into its corresponding ASCII character, but translating
                              > some (disallowed) bytes into a different sequence.
                              >
                              > Ok. I think it's useful to make a distinction between the n:1
                              > mappings and the 1:1 mappings.
                              > If you're escaping (which I defined as n:1), you have to unescape
                              > before comparing strings, while you can check against an encoded
                              > string by either decoding the one or encoding the other.
                              >
                              > One way to check attribute content in a markup language is to keep a
                              > stack of escaping and encoding conventions as you examine the document
                              > in the left to right pass.
                              > To check whether an iframe's src's protocol is javascript: you deal
                              > with the following stack
                              > protocol <(%-escaped)> uri <(html-entity-escaped)> html-attribute
                              > <(UTF-8 encoding)> bytes
                              >
                              > If you're sending a message and you want it to be interpreted as you
                              > intend, then you have to make sure that the recipient of the message
                              > will use the same escaping/encodings, and if you want to verify
                              > properties of the message, then you have to consider every escaping,
                              > but not necessarily every encoding.
                              >
                              > So for the javascript: check, there are 3 points of attack, but the
                              > encoding can be considered entirely separately leaving you two.
                              >
                              >
                              >
                              >
                              > >
                              > > I think it's reasonable to (a) reject Javascript that
                              > contains 65533 (U+FFFD REPLACEMENT CHARACTER) and (b) to not
                              > look for, parse, or handle in any special way URI references
                              > within (X)HTML attributes or content.
                              >
                              > How would you detect urls that can execute or import scripts without
                              > distinguishing attributes that contain URIs or URI references, given
                              > that it is a goal of ADSafe to allow iframes to external srcs?
                              >
                              >
                              >
                              > >
                              > > Talking about this is complicated because the same concept
                              > appears with different encodings:
                              > >
                              > > * The HTTP protocol does its work in sequences of bytes,
                              > but the spec is written in terms of "characters".
                              > >
                              > > * The XML specification defines XML as a sequence of
                              > (Unicode) characters, encoded in UTF8 or UTF16 (or some other
                              > encoding) but also with &char; character entities and
                              > ￝ numeric character references. XHTML as an XML
                              > language follows this; whether HTML follows this depends on
                              > the HTML version.
                              > >
                              > > * the Javascript specification (at least ECMA-262) defines
                              > Javascript as a sequence of (Unicode) characters encoded in
                              > UTF8 or UTF16. E4X (ECMA 357) seems to follow XML in using
                              > character entity references and numeric character references
                              > to encode characters that would otherwise be disallowed.
                              > >
                              > > * The URI specification defines a URI as a sequence of
                              > characters, taken from the repertoire of US-ASCII characters,
                              > with the encoding chosen by the protocol/format that embeds
                              > it; it also defines an encoding (%xx) for bytes that would
                              > otherwise correspond to disallowed or reserved characters.
                              > >
                              > > * The IRI specification defines an IRI similarly, but
                              > allows a larger repertoire of characters.
                              > >
                              > > Parsing an XML stream into an XML DOM (including HTML)
                              > will translate the UTF8, UTF16 (or other encoding) as well as
                              > character and numeric character entity references, into a
                              > sequence of characters. Parsing a Javascript string (using
                              > E4X) will apparently do the same, even though
                              > Javascript-per-se parts and XHTML-constant parts use
                              > different escaping mechanisms.
                              > >
                              > > A (X)HTML validator for content embedded within Javascript
                              > should likely perform the same entity and numeric character
                              > reference decoding logic as would apply when the Javascript
                              > was read and interpreted -- resolve character entity
                              > references and numeric character references -- and then
                              > validate the results.
                              > >
                              > > There are many troublesome syntactically valid URIs (and
                              > IRIs) that could appear within a URI reference in (X)HTML and
                              > (X)HTML embedded within Javascript, but I think it is part of
                              > the security requirements of the (X)HTML interpreter runtime
                              > to manage and prevent those references. Because URIs (and
                              > IRIs) can and sometimes do encode non-character byte streams,
                              > looking for or managing the URI-encoding level would be inappropriate.
                              > >
                              > > Larry
                              > >
                              > > -----Original Message-----
                              > > From: caplet@yahoogroups.com
                              > [mailto:caplet@yahoogroups.com] On Behalf Of Mike Samuel
                              > > Sent: Friday, October 19, 2007 10:30 PM
                              > > To: caplet@yahoogroups.com
                              > > Subject: Re: [caplet] Re: ADsafe, Take 6
                              > >
                              > >
                              > > On 19/10/2007, David Hopwood
                              > <david.hopwood@...> wrote:
                              > > >
                              > > >
                              > > >
                              > > >
                              > > >
                              > > >
                              > > > Larry Masinter wrote:
                              > > > > I think you got it backward: URIs are sequences of
                              > characters, not bytes.
                              > > >
                              > > > URIs are sequences of characters that encode a sequence
                              > of bytes, which
                              > > > *may* in turn encode a sequence of Unicode characters.
                              > >
                              > > I still don't understand.
                              > >
                              > > My reading of the spec says that the first sequence of
                              > characters is in ASCII.
                              > >
                              > > If that's the case, then an HTML validator should be able to reject
                              > > any HTML attribute of type URI whose value contains a codepoint
                              > > outside [0, 255] without making it possibly to express any
                              > valid URI.
                              > > Does that sound right?
                              > >
                              > > If that's right, would it be appropriate for the error message to
                              > > recommend re-encoding the out of range characters using a
                              > %-encoding
                              > > of UTF-8? So "�" -> "%EF%BF%BD".
                              > >
                              > > Also, on terminology, is the below right?
                              > > * An escaping is an n:1 mapping from strings in an alphabet A to
                              > > strings in an alphabet which is a subset of A.
                              > > * An encoding is a 1:1 mapping of strings over one
                              > alphabet to strings
                              > > over another alphabet.
                              > >
                              > > >
                              > > >
                              > > >
                              > > >
                              > > >
                              > > >
                              > > > For URIs that have some server-specific part, the
                              > interpretation of the byte
                              > > > sequence is up to that server. RFC 3986 *recommends*
                              > that, where they encode
                              > > > a string, the encoding used should be UTF-8. However,
                              > there's no way to
                              > > > enforce this (and no particular reason to enforce it).
                              > So it is valid, for
                              > > > example, to have "%FF" in an URL, even though that is
                              > always an invalid byte
                              > > > in UTF-8.
                              > > >
                              > > > > and in (X)HTML, "URI" is really "IRI" - the XHTML
                              > spec allows full
                              > > > > Unicode (10646) characters which are UTF8 and then
                              > hex-encoded if you need
                              > > > > an (old-fashioned) URI.
                              > > >
                              > > > XHTML still doesn't require that the sequence of bytes
                              > is valid UTF-8.
                              > > >
                              > > > In any case, the immediate question was whether it is
                              > reasonable to reject
                              > > > any input that contains 65533 (U+FFFD REPLACEMENT
                              > CHARACTER). IMHO it is:
                              > > > this isn't a useful character in its own right; it
                              > indicates that an
                              > > > encoding error occurred in producing the input.
                              > > >
                              > > > --
                              > > > David Hopwood <david.hopwood@...>
                              >
                              >
                              >
                              > Yahoo! Groups Links
                              >
                              >
                              >
                              >
                            • Mike Samuel
                              Ok. I think the time for debate has passed, but it s a slow Monday so I ll bite :) There s a few problems: (1) Documents embed other documents using a melange
                              Message 14 of 30 , Oct 22, 2007
                              • 0 Attachment
                                Ok. I think the time for debate has passed, but it's a slow Monday so
                                I'll bite :)

                                There's a few problems:
                                (1) Documents embed other documents using a melange of separators,
                                delimiters, and escaping conventions.
                                (2) Developers don't understand escaping and encoding issues, and so
                                apps are rife with injection vulnerabilities.
                                (3) Protocol designers have an incentive to gloss over such issues
                                because a protocol that attracts developers will survive longer than a
                                protocol that doesn't.

                                XML promised a uniform way of representing s-expressions, so any
                                document representable as an s-expression could embed another
                                document. XML runs into problems with embedding -- you have to do
                                tricks to use XSL to generate XSL; it's horribly verbose; and it makes
                                an arbitrary distinction between elements and attributes which kills
                                extensibility.

                                That aside, some terse form of consistent S-expression document
                                representation for all of (HTTP request/response, markup languages,
                                stylesheets, and ideally code) would be a good start.

                                Lisp style S-expressions avoid escaping conventions entirely, so
                                address 2 to some degree. Which is easier to suss out?
                                Content-type:text/html

                                <html><script>document.write('<head><title>foo<\/head><\/title>')</script></html>
                                where from the DOM you have to treat the content of script as an
                                opaque string, or
                                ('http-response,
                                ('headers, ('Content-type, 'text/html)),
                                ('body
                                ('html, ('script, ('operator-call, 'document, 'write, '('head,
                                ('title, 'foo)))))))

                                The problems with that single consistent representation is that you
                                need an envelope to specify charset and any other encoding steps.
                                Even if we reworked all clients to use UTF-8 of unicode, or had
                                gateways to handle legacy formats, the envelope still needs to specify
                                compression schemes, signatures, and the like.

                                But it should be possible to move all the information not needed to
                                decode the body from a byte string out of the envelope without
                                requiring a hard distinction between transport and persistence
                                formats. Lightweight envelopes are nice, because they let you
                                sidestep the debate between those who want a human-readable protocols
                                and those who care about size.

                                The other major problem is efficient representation of binary data.
                                One way is the content: url -- the reference to the binary can specify
                                the binary which is horrible and brings encoding and reintroduces
                                escaping into all layers. The other way is the email way -- the
                                envelope contains multiple parts which share a namespace, and that can
                                reference one another using relative URIs. Apple's data and resource
                                forks might also suggest ways to allow a uniform structured part to
                                reference blobs.

                                cheers,
                                mike



                                On 22/10/2007, Freeman, Tim <tim.freeman@...> wrote:
                                >
                                >
                                >
                                >
                                >
                                >
                                > Quoting Mike Samuel:
                                > > To check whether an iframe's src's protocol is javascript: you deal
                                > > with the following stack
                                > > protocol <(%-escaped)> uri <(html-entity-escaped)> html-attribute
                                > > <(UTF-8 encoding)> bytes
                                >
                                > Okay, I'll try to say the obvious here -- although no one individual is
                                > responsible, we find ourselves in the middle of a big hacked-up pile of
                                > conventions that were put together with insufficient forethought. It
                                > shouldn't
                                > be this complicated to fetch information from a few computers and
                                > display
                                > it on another.
                                >
                                > Has anyone put some thought into figuring out how the web should have
                                > been and
                                > writing it down? I feel in need of a comforting fantasy to read just
                                > before
                                > going to bed at night. Such a document would also help to visualize
                                > some desired
                                > destination so we can move in that direction when we make new stuff.




                                >
                                >
                                >
                                >
                                >
                                >
                                > -----
                                > Tim Freeman
                                > Email: tim.freeman@...
                                > Desk in Palo Alto: (650) 857-2581
                                > Home: (408) 774-1298
                                > Cell: (408) 348-7536
                                >
                                >
                                > > -----Original Message-----
                                > > From: caplet@yahoogroups.com [mailto:caplet@yahoogroups.com]
                                > > On Behalf Of Mike Samuel
                                > > Sent: Sunday, October 21, 2007 21:11
                                > > To: caplet@yahoogroups.com
                                > > Subject: Re: [caplet] Re: ADsafe, Take 6
                                > >
                                > > On 21/10/2007, Larry Masinter <lmm@...> wrote:
                                > > >
                                > > >
                                > > >
                                > > >
                                > > >
                                > > >
                                > > > To answer your direct questions:
                                > > >
                                > > > I don't know any formal definition for "escaping" except
                                > > as a part of "encoding" -- you encode a sequence of bytes
                                > > into (a subset of) US-ASCII by translating each (allowed)
                                > > byte into its corresponding ASCII character, but translating
                                > > some (disallowed) bytes into a different sequence.
                                > >
                                > > Ok. I think it's useful to make a distinction between the n:1
                                > > mappings and the 1:1 mappings.
                                > > If you're escaping (which I defined as n:1), you have to unescape
                                > > before comparing strings, while you can check against an encoded
                                > > string by either decoding the one or encoding the other.
                                > >
                                > > One way to check attribute content in a markup language is to keep a
                                > > stack of escaping and encoding conventions as you examine the document
                                > > in the left to right pass.
                                > > To check whether an iframe's src's protocol is javascript: you deal
                                > > with the following stack
                                > > protocol <(%-escaped)> uri <(html-entity-escaped)> html-attribute
                                > > <(UTF-8 encoding)> bytes
                                > >
                                > > If you're sending a message and you want it to be interpreted as you
                                > > intend, then you have to make sure that the recipient of the message
                                > > will use the same escaping/encodings, and if you want to verify
                                > > properties of the message, then you have to consider every escaping,
                                > > but not necessarily every encoding.
                                > >
                                > > So for the javascript: check, there are 3 points of attack, but the
                                > > encoding can be considered entirely separately leaving you two.
                                > >
                                > >
                                > >
                                > >
                                > > >
                                > > > I think it's reasonable to (a) reject Javascript that
                                > > contains 65533 (U+FFFD REPLACEMENT CHARACTER) and (b) to not
                                > > look for, parse, or handle in any special way URI references
                                > > within (X)HTML attributes or content.
                                > >
                                > > How would you detect urls that can execute or import scripts without
                                > > distinguishing attributes that contain URIs or URI references, given
                                > > that it is a goal of ADSafe to allow iframes to external srcs?
                                > >
                                > >
                                > >
                                > > >
                                > > > Talking about this is complicated because the same concept
                                > > appears with different encodings:
                                > > >
                                > > > * The HTTP protocol does its work in sequences of bytes,
                                > > but the spec is written in terms of "characters".
                                > > >
                                > > > * The XML specification defines XML as a sequence of
                                > > (Unicode) characters, encoded in UTF8 or UTF16 (or some other
                                > > encoding) but also with &char; character entities and
                                > > ￝ numeric character references. XHTML as an XML
                                > > language follows this; whether HTML follows this depends on
                                > > the HTML version.
                                > > >
                                > > > * the Javascript specification (at least ECMA-262) defines
                                > > Javascript as a sequence of (Unicode) characters encoded in
                                > > UTF8 or UTF16. E4X (ECMA 357) seems to follow XML in using
                                > > character entity references and numeric character references
                                > > to encode characters that would otherwise be disallowed.
                                > > >
                                > > > * The URI specification defines a URI as a sequence of
                                > > characters, taken from the repertoire of US-ASCII characters,
                                > > with the encoding chosen by the protocol/format that embeds
                                > > it; it also defines an encoding (%xx) for bytes that would
                                > > otherwise correspond to disallowed or reserved characters.
                                > > >
                                > > > * The IRI specification defines an IRI similarly, but
                                > > allows a larger repertoire of characters.
                                > > >
                                > > > Parsing an XML stream into an XML DOM (including HTML)
                                > > will translate the UTF8, UTF16 (or other encoding) as well as
                                > > character and numeric character entity references, into a
                                > > sequence of characters. Parsing a Javascript string (using
                                > > E4X) will apparently do the same, even though
                                > > Javascript-per-se parts and XHTML-constant parts use
                                > > different escaping mechanisms.
                                > > >
                                > > > A (X)HTML validator for content embedded within Javascript
                                > > should likely perform the same entity and numeric character
                                > > reference decoding logic as would apply when the Javascript
                                > > was read and interpreted -- resolve character entity
                                > > references and numeric character references -- and then
                                > > validate the results.
                                > > >
                                > > > There are many troublesome syntactically valid URIs (and
                                > > IRIs) that could appear within a URI reference in (X)HTML and
                                > > (X)HTML embedded within Javascript, but I think it is part of
                                > > the security requirements of the (X)HTML interpreter runtime
                                > > to manage and prevent those references. Because URIs (and
                                > > IRIs) can and sometimes do encode non-character byte streams,
                                > > looking for or managing the URI-encoding level would be inappropriate.
                                > > >
                                > > > Larry
                                > > >
                                > > > -----Original Message-----
                                > > > From: caplet@yahoogroups.com
                                > > [mailto:caplet@yahoogroups.com] On Behalf Of Mike Samuel
                                > > > Sent: Friday, October 19, 2007 10:30 PM
                                > > > To: caplet@yahoogroups.com
                                > > > Subject: Re: [caplet] Re: ADsafe, Take 6
                                > > >
                                > > >
                                > > > On 19/10/2007, David Hopwood
                                > > <david.hopwood@...> wrote:
                                > > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > > Larry Masinter wrote:
                                > > > > > I think you got it backward: URIs are sequences of
                                > > characters, not bytes.
                                > > > >
                                > > > > URIs are sequences of characters that encode a sequence
                                > > of bytes, which
                                > > > > *may* in turn encode a sequence of Unicode characters.
                                > > >
                                > > > I still don't understand.
                                > > >
                                > > > My reading of the spec says that the first sequence of
                                > > characters is in ASCII.
                                > > >
                                > > > If that's the case, then an HTML validator should be able to reject
                                > > > any HTML attribute of type URI whose value contains a codepoint
                                > > > outside [0, 255] without making it possibly to express any
                                > > valid URI.
                                > > > Does that sound right?
                                > > >
                                > > > If that's right, would it be appropriate for the error message to
                                > > > recommend re-encoding the out of range characters using a
                                > > %-encoding
                                > > > of UTF-8? So "�" -> "%EF%BF%BD".
                                > > >
                                > > > Also, on terminology, is the below right?
                                > > > * An escaping is an n:1 mapping from strings in an alphabet A to
                                > > > strings in an alphabet which is a subset of A.
                                > > > * An encoding is a 1:1 mapping of strings over one
                                > > alphabet to strings
                                > > > over another alphabet.
                                > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > >
                                > > > > For URIs that have some server-specific part, the
                                > > interpretation of the byte
                                > > > > sequence is up to that server. RFC 3986 *recommends*
                                > > that, where they encode
                                > > > > a string, the encoding used should be UTF-8. However,
                                > > there's no way to
                                > > > > enforce this (and no particular reason to enforce it).
                                > > So it is valid, for
                                > > > > example, to have "%FF" in an URL, even though that is
                                > > always an invalid byte
                                > > > > in UTF-8.
                                > > > >
                                > > > > > and in (X)HTML, "URI" is really "IRI" - the XHTML
                                > > spec allows full
                                > > > > > Unicode (10646) characters which are UTF8 and then
                                > > hex-encoded if you need
                                > > > > > an (old-fashioned) URI.
                                > > > >
                                > > > > XHTML still doesn't require that the sequence of bytes
                                > > is valid UTF-8.
                                > > > >
                                > > > > In any case, the immediate question was whether it is
                                > > reasonable to reject
                                > > > > any input that contains 65533 (U+FFFD REPLACEMENT
                                > > CHARACTER). IMHO it is:
                                > > > > this isn't a useful character in its own right; it
                                > > indicates that an
                                > > > > encoding error occurred in producing the input.
                                > > > >
                                > > > > --
                                > > > > David Hopwood <david.hopwood@...>
                                > >
                                > >
                              • Larry Masinter
                                On standards: The benefit of HTTP and XML and HTML is not that they are well-designed protocol and syntax and language, but that there are many different and
                                Message 15 of 30 , Oct 23, 2007
                                • 0 Attachment
                                  On standards:

                                  The benefit of HTTP and XML and HTML is not that they are well-designed protocol and syntax and language, but that there are many different and (more-or-less) interoperable implementations for many operating systems and languages and well-deployed support infrastructure; with enough general agreement about them at the lower layers that you can get on with it defining the next layer up. So a "let's redesign them to be cleaner" effort isn't helpful, really. You'd have to be 10 times or 100 times better before getting traction.

                                  Protocol designers don't "gloss over" escaping; protocol designers are software developers (or maybe developers-gone-bad) for whom escaping is generally an ugly after-the-fact design addition or compromise between allowing everything to be encoded but letting simple cases be encoded concisely. Think of it as Huffman coding at the design level.

                                  <p><html>... </html></p>

                                  is how you write html in html, not because &#xxx; and &symbolname; are wonderful quoting mechanisms, but because the &entity; syntax was already there, and inventing another one for what was seemed like an uncommon case appeared unnecessary.

                                  On quoting:

                                  No matter what your escaping and encoding system, developers will have problems with them: you either understand the general principle or you don't.

                                  I know (from ancient experience) that most programmers learning LISP had trouble with thinking about X, (QUOTE X) and (EVAL X) when learning Lisp. The problem is keeping track of the different layers of interpretation – it isn't the syntax.

                                  Common Lisp added many other escaping conventions: `(let ((,x "abc\"def") (y ',z)) ,w) so it's hard to claim that S-expressions have consistent delimiters.

                                  On layering of escaping/quoting:

                                  The multiple layered quoting systems work well enough, because each layer does its own escaping/encoding and unescaping/unencoding and tools either hide or assist with the process. It's only when you're writing a program trying to process multiple layers simultaneously that you have trouble.

                                  The reason that ADsafe is having trouble is that it is trying to do filtering without actually using the normal layer software for parsing and interpretation, and skip what turns out to be necessary complexity. Try to write a regular expression that will determines whether a Lisp program might divide by zero, and you'd have similar problems.


                                  On checking URLs:

                                  I think you can't check for invalid URLs by examining a program's syntax because (a) programs can construct URLs, and you can't check for invalid (vs. valid) URLs any more than you can do all array bounds checking at compile time and (b) the rules for what constitute a "safe" URL are complicated and evolving. After all, a URL is just a reference to a registry of protocols, which requires the registering body define some syntax for how the URL syntax might identify something or invoke some protocol or process. Each URL scheme has its own syntax and story for what might be "safe" to execute in different contexts, but that depends as much on the implementation of the URL-interpreter as anything else.

                                  If you're going to do dynamic URL safety checking, there's not much point in doing syntactic checking, because you'll get lots of false positives ("this is unsafe" when it isn't) and won't catch any more problems syntactically than would be caught by the run-time check.

                                  Larry
                                • Mike Samuel
                                  ... Maybe I m being horribly unfair to protocol designers, but implementors do. An example is entities in URIs embedded in HTML. is
                                  Message 16 of 30 , Oct 23, 2007
                                  • 0 Attachment
                                    On 23/10/2007, Larry Masinter <lmm@...> wrote:
                                    >
                                    >
                                    >
                                    >
                                    >
                                    >
                                    > On standards:
                                    >
                                    > The benefit of HTTP and XML and HTML is not that they are well-designed protocol and syntax and language, but that there are many different and (more-or-less) interoperable implementations for many operating systems and languages and well-deployed support infrastructure; with enough general agreement about them at the lower layers that you can get on with it defining the next layer up. So a "let's redesign them to be cleaner" effort isn't helpful, really. You'd have to be 10 times or 100 times better before getting traction.
                                    >
                                    > Protocol designers don't "gloss over" escaping; protocol designers are software developers (or maybe developers-gone-bad) for whom escaping is generally an ugly after-the-fact design addition or compromise between allowing everything to be encoded but letting simple cases be encoded concisely. Think of it as Huffman coding at the design level.

                                    Maybe I'm being horribly unfair to protocol designers, but implementors do.

                                    An example is entities in URIs embedded in HTML.
                                    <a href="foo?bar=a&baz=b">
                                    is invalid HTML, but browser implementors, faced with a bunch of
                                    markup writers who don't understand escaping decided that they should
                                    guess what it means, so it means something vastly different from
                                    <a href="foo?bar=a&=c">


                                    > <p><html>... </html></p>
                                    >
                                    > is how you write html in html, not because &#xxx; and &symbolname; are wonderful quoting mechanisms, but because the &entity; syntax was already there, and inventing another one for what was seemed like an uncommon case appeared unnecessary.

                                    That's how it's done today, and that's the source of a huge number of problems.

                                    And a point I was trying to make (don't remember if I made it) was
                                    that embedding one structured language in another shouldn't require
                                    escaping. You should be able to include the parse tree directly using
                                    some kind of consistent (quote ...) syntax.


                                    > On quoting:
                                    >
                                    > No matter what your escaping and encoding system, developers will have problems with them: you either understand the general principle or you don't.

                                    Agreed. But there's a third possibility:

                                    I understand them and I've got deadlines and it'll usually work unless
                                    some lunatic gives their kid a name with apostrophes, so I'll not
                                    think through it now, and then come back to it once I've gotten some
                                    sleep which'll happen never.


                                    > I know (from ancient experience) that most programmers learning LISP had trouble with thinking about X, (QUOTE X) and (EVAL X) when learning Lisp. The problem is keeping track of the different layers of interpretation – it isn't the syntax.

                                    I think there's something fundamentally different about embedding by
                                    escaping and embedding by injecting attaching another languages parse
                                    tree to your own. Escaping requires clients of your language to be
                                    able to also parse any embeddable languages, and agree on the parse
                                    tree that any given string should produce.


                                    > Common Lisp added many other escaping conventions: `(let ((,x "abc\"def") (y ',z)) ,w) so it's hard to claim that S-expressions have consistent delimiters.

                                    Fair enough, but that's only a concern if you are embedding using escaping.

                                    > On layering of escaping/quoting:
                                    >
                                    > The multiple layered quoting systems work well enough, because each layer does its own escaping/encoding and unescaping/unencoding and tools either hide or assist with the process. It's only when you're writing a program trying to process multiple layers simultaneously that you have trouble.

                                    Quite right. If you're trying to identify a "secure" subset of any
                                    system (like web applications) which use multiple languages then you
                                    have two choices:
                                    - identify a safe subset of each language individually
                                    - deal with all languages at once and try to identify a safe subset of the union

                                    The first approach is certainly easier but has
                                    least-common-denominator problems -- you have to exclude things that
                                    might be allowable under the second approach.

                                    An example
                                    <a href="javascript:foo()">clicky</a>

                                    There's three languages in play here, Javascript, URIs, and HTML.

                                    If you consider the three individually, you have to conclude that the
                                    HTML is safe, the URI is not, and you never consider the javascript
                                    itself.

                                    If you deal with all languages at once, then you can apply your
                                    javascript verification recursively to the URI.

                                    To do that you have to parse all three languages and then hope that
                                    your parse trees agree with browsers' interpretations. My point is
                                    that's easier if you have a unified parse tree representation for all
                                    languages that appear in a document.


                                    > The reason that ADsafe is having trouble is that it is trying to do filtering without actually using the normal layer software for parsing and interpretation, and skip what turns out to be necessary complexity. Try to write a regular expression that will determines whether a Lisp program might divide by zero, and you'd have similar problems.
                                    >
                                    >
                                    > On checking URLs:
                                    >
                                    > I think you can't check for invalid URLs by examining a program's syntax because (a) programs can construct URLs, and you can't check for invalid (vs. valid) URLs any more than you can do all array bounds checking at compile time and (b) the rules for what constitute a "safe" URL are complicated and evolving. After all, a URL is just a reference to a registry of protocols, which requires the registering body define some syntax for how the URL syntax might identify something or invoke some protocol or process. Each URL scheme has its own syntax and story for what might be "safe" to execute in different contexts, but that depends as much on the implementation of the URL-interpreter as anything else.

                                    I disagree. URLs are more than a pointer into a registry of handlers.
                                    Specifically, javascript: and data: URLs contain data that can be
                                    classified.

                                    Again, you'll get fewer false negatives if you do that classification
                                    in the context of the larger document.


                                    > If you're going to do dynamic URL safety checking, there's not much point in doing syntactic checking, because you'll get lots of false positives ("this is unsafe" when it isn't) and won't catch any more problems syntactically than would be caught by the run-time check.

                                    There's always reason to do static checking since it let's you skip
                                    runtime checks :) But runtime checks are out of scope for ADSafe.


                                    > Larry
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