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Versions: 00 01 02 03 04 05 06 draft-ietf-websec-mime-sniff

Working Group                                                   A. Barth
Internet-Draft                                             U.C. Berkeley
Expires: July 13, 2009                                        I. Hickson
                                                            Google, Inc.
                                                         January 9, 2009


                     Content-Type Processing Model
                       draft-abarth-mime-sniff-00

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 13, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.







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Abstract

   Many Web servers supply incorrect Content-Type headers with their
   HTTP responses.  In order to be compatible with these Web servers,
   Web browsers must consider the content of HTTP responses as well as
   the Content-Type header when determining the effective mime type of
   the response.  This document describes an algorithm for determining
   the effective mime type of HTTP responses that balances security and
   compatibility considerations.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Metadata . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Web Pages  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Text or Binary . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Unknown Type . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Image  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Feed or HTML . . . . . . . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18






























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1.  Introduction

   The HTTP Content-Type header indicates the mime type of an HTTP
   responses.  However, many HTTP servers supply a Content-Type that
   does not match the actual contents of the response.  Historically,
   Web browsers have been tolerated these servers by examining the
   content of HTTP responses in addition to the Content-Type header to
   determine the effective mime type of the response.

   Without a clear specification of how to "sniff" the mime type, each
   browser vendor was forced to reverse engineer the behavior of the
   other borwsers and to developed their own algorithm.  These divergent
   algorithms have lead to a lack of interoperability between browsers
   and to security issues when the site intends an HTTP response to be
   interpreted as one mime type but the browser interpretes the
   responses as another mime type.

   These security issues are must severe when a Web site lets users
   upload files and then serves the contents of those files with a low-
   privilege mime type (such as text/plain or image/jpeg).  In the
   absense of mime sniffing, this user-generated content will not be
   able to run JavaScript, but if the browser treats the response as
   text/html, then the user can mount a cross-site scripting attack by
   including JavaScript code in the uploaded file.

   This document describes a mime sniffing algorithm that carefully
   balances the compatibility needs of browser vendors with the security
   constraints.  The algorithm has been constructed with reference to
   mime sniffing algorithms present in popular Web browsers, an
   extensive database of Web content, and metrics collected from
   implementations deployed to a sizable number of Web users.

   Warning!  It is imperative that the algorithm in this document be
   followed exactly.  When a user agent uses different heuristics for
   content type detection than the server expects, security problems can
   occur.  For example, if a server believes that the client will treat
   a contributed file as an image (and thus treat it as benign), but a
   Web browser believes the content to be HTML (and thus execute any
   scripts contained therein), the end user can be exposed to malicious
   content, making the user vulnerable to cookie theft attacks and other
   cross-site scripting attacks.










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2.  Metadata

   What explicit Content-Type metadata is associated with the resource
   (the resource's type information) depends on the protocol that was
   used to fetch the resource.

   For HTTP resources, only the first Content-Type HTTP header, if any,
   contributes any type information; the explicit type of the resource
   is then the value of that header, interpreted as described by the
   HTTP specifications.  If the Content-Type HTTP header is present but
   the value of the first such header cannot be interpreted as described
   by the HTTP specifications (e.g. because its value doesn't contain a
   U+002F SOLIDUS ('/') character), then the resource has no type
   information (even if there are multiple Content-Type HTTP headers and
   one of the other ones is syntactically correct).  [HTTP]

   For resources fetched from the file system, user agents should use
   platform-specific conventions, e.g. operating system extension/type
   mappings.

   Extensions must not be used for determining resource types for
   resources fetched over HTTP.

   For resources fetched over most other protocols, e.g.  FTP, there is
   no type information.

   The algorithm for extracting an encoding from a Content-Type, given a
   string s, is as follows.  It either returns an encoding or nothing.

   1.  Find the first seven characters in s that are an ASCII case-
       insensitive match for the word "charset".  If no such match is
       found, return nothing.

   2.  Skip any U+0009, U+000A, U+000C, U+000D, or U+0020 characters
       that immediately follow the word 'charset' (there might not be
       any).

   3.  If the next character is not a U+003D EQUALS SIGN ('='), return
       nothing.

   4.  Skip any U+0009, U+000A, U+000C, U+000D, or U+0020 characters
       that immediately follow the equals sign (there might not be any).

   5.  Process the next character as follows:

       *  If it is a U+0022 QUOTATION MARK ('"') and there is a later
          U+0022 QUOTATION MARK ('"') in s, or




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       *  If it is a U+0027 APOSTROPHE ("'") and there is a later U+0027
          APOSTROPHE ("'") in s

             Return the string between this character and the next
             earliest occurrence of this character.

       *  If it is an unmatched U+0022 QUOTATION MARK ('"'),

       *  If it is an unmatched U+0027 APOSTROPHE ("'"), or

       *  If there is no next character

             Return nothing.

       *  Otherwise

             Return the string from this character to the first U+0009,
             U+000A, U+000C, U+000D, U+0020, or U+003B character or the
             end of s, whichever comes first.

   Note: The above algorithm is a willful violation of the HTTP
   specification.  [RFC2616]





























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3.  Web Pages

   The sniffed type of a resource must be found as follows:

   1.  If the user agent is configured to strictly obey Content-Type
       headers for this resource, then jump to the last step in this set
       of steps.

   2.  If the resource was fetched over an HTTP protocol and there is an
       HTTP Content-Type header and the value of the first such header
       has bytes that exactly match one of the following lines:

      +-------------------------------+--------------------------------+
      | Bytes in Hexadecimal          | Textual representation         |
      +-------------------------------+--------------------------------+
      | 74 65 78 74 2f 70 6c 61 69 6e | text/plain                     |
      +-------------------------------+--------------------------------+
      | 74 65 78 74 2f 70 6c 61 69 6e | text/plain; charset=ISO-8859-1 |
      | 3b 20 63 68 61 72 73 65 74 3d |                                |
      | 49 53 4f 2d 38 38 35 39 2d 31 |                                |
      +-------------------------------+--------------------------------+
      | 74 65 78 74 2f 70 6c 61 69 6e | text/plain; charset=iso-8859-1 |
      | 3b 20 63 68 61 72 73 65 74 3d |                                |
      | 69 73 6f 2d 38 38 35 39 2d 31 |                                |
      +-------------------------------+--------------------------------+
      | 74 65 78 74 2f 70 6c 61 69 6e | text/plain; charset=UTF-8      |
      | 3b 20 63 68 61 72 73 65 74 3d |                                |
      | 55 54 46 2d 38                |                                |
      +-------------------------------+--------------------------------+

       ...then jump to the "text or binary" section below.

   3.  Let official type be the type given by the Content-Type metadata
       for the resource, ignoring parameters.  If there is no such type,
       jump to the unknown type step below.  Comparisons with this type,
       as defined by MIME specifications, are done in an ASCII case-
       insensitive manner.  [RFC2046]

   4.  If official type is "unknown/unknown" or "application/unknown",
       jump to the unknown type step below.

   5.  If official type ends in "+xml", or if it is either "text/xml" or
       "application/xml", then the sniffed type of the resource is
       official type; return that and abort these steps.

   6.  If official type is an image type supported by the user agent
       (e.g. "image/png", "image/gif", "image/jpeg", etc), then jump to
       the "images" section below, passing it the official type.



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   7.  If official type is "text/html", then jump to the feed or HTML
       section below.

   8.  The sniffed type of the resource is official type.















































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4.  Text or Binary

   1.  The user agent may wait for 512 or more bytes of the resource to
       be available.

   2.  Let n be the smaller of either 512 or the number of bytes already
       available.

   3.  If n is 4 or more, and the first bytes of the resource match one
       of the following byte sets:

                   +----------------------+--------------+
                   | Bytes in Hexadecimal | Description  |
                   +----------------------+--------------+
                   | FE FF                | UTF-16BE BOM |
                   | FF FE                | UTF-16LE BOM |
                   | EF BB BF             | UTF-8 BOM    |
                   +----------------------+--------------+

       ...then the sniffed type of the resource is "text/plain".  Abort
       these steps.

   4.  If none of the first n bytes of the resource are binary data
       bytes then the sniffed type of the resource is "text/plain".
       Abort these steps.

                         +-------------------------+
                         | Binary data byte ranges |
                         +-------------------------+
                         | 0x00 -- 0x08            |
                         | 0x0B                    |
                         | 0x0E -- 0x1A            |
                         | 0x1C -- 0x1F            |
                         +-------------------------+

   5.  If the first bytes of the resource match one of the byte
       sequences in the "pattern" column of the table in the unknown
       type section below, ignoring any rows whose cell in the
       "security" column says "scriptable" (or "n/a"), then the sniffed
       type of the resource is the type given in the corresponding cell
       in the "sniffed type" column on that row; abort these steps.

          Warning!  It is critical that this step not ever return a
          scriptable type (e.g. text/html), as otherwise that would
          allow a privilege escalation attack.






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   6.  Otherwise, the sniffed type of the resource is "application/
       octet-stream".

















































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5.  Unknown Type

   1.  The user agent may wait for 512 or more bytes of the resource to
       be available.

   2.  Let stream length be the smaller of either 512 or the number of
       bytes already available.

   3.  For each row in the table below:

       *  If the row has no "WS" bytes:

          1.  Let pattern length be the length of the pattern (number of
              bytes described by the cell in the second column of the
              row).

          2.  If stream length is smaller than pattern length then skip
              this row.

          3.  Apply the "and" operator to the first pattern length bytes
              of the resource and the given mask (the bytes in the cell
              of first column of that row), and let the result be the
              data.

          4.  If the bytes of the data matches the given pattern bytes
              exactly, then the sniffed type of the resource is the type
              given in the cell of the third column in that row; abort
              these steps.

       *  If the row has a "WS" byte:

          1.  Let index_pattern be an index into the mask and pattern
              byte strings of the row.

          2.  Let index_stream be an index into the byte stream being
              examined.

          3.  Loop: If indexstream points beyond the end of the byte
              stream, then this row doesn't match, skip this row.

          4.  Examine the indexstreamth byte of the byte stream as
              follows:

              -  If the index_patternth byte of the pattern is a normal
                 hexadecimal byte and not a "WS" byte:

                    If the "and" operator, applied to the index_streamth
                    byte of the stream and the index_patternth byte of



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                    the mask, yield a value different that the
                    index_patternth byte of the pattern, then skip this
                    row.

                    Otherwise, increment index_pattern to the next byte
                    in the mask and pattern and index_stream to the next
                    byte in the byte stream.

              -  Otherwise, if the indexpatternth byte of the pattern is
                 a "WS" byte:

                    "WS" means "whitespace", and allows insignificant
                    whitespace to be skipped when sniffing for a type
                    signature.

                    If the index_streamth byte of the stream is one of
                    0x09 (ASCII TAB), 0x0A (ASCII LF), 0x0C (ASCII FF),
                    0x0D (ASCII CR), or 0x20 (ASCII space), then
                    increment only the index_stream to the next byte in
                    the byte stream.

                    Otherwise, increment only the index_pattern to the
                    next byte in the mask and pattern.

          5.  If index_pattern does not point beyond the end of the mask
              and pattern byte strings, then jump back to the loop step
              in this algorithm.

          6.  Otherwise, the sniffed type of the resource is the type
              given in the cell of the third column in that row; abort
              these steps.

   4.  If none of the first n bytes of the resource are binary data
       bytes then the sniffed type of the resource is "text/plain".
       Abort these steps.

   5.  Otherwise, the sniffed type of the resource is "application/
       octet-stream".

   The table used by the above algorithm is:

+-------------------+-------------------+-----------------+------------+
| Mask in Hex       | Pattern in Hex    | Sniffed type    | Security   |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | 3C 21 44 4F 43 54 | text/html       | Scriptable |
| DF DF DF FF DF DF | 59 50 45 20 48 54 |                 |            |
| DF DF             | 4D 4C             |                 |            |
|                                                                      |



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| Comment: The string "<!DOCTYPE HTML" in US-ASCII or compatible       |
|          encodings, case-insensitively.                              |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 48 54 4D 4C | text/html       | Scriptable |
|                                                                      |
| Comment: The string "<HTML" in US-ASCII or compatible encodings,     |
|          case-insensitively, possibly with leading spaces.           |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 48 45 41 44 | text/html       | Scriptable |
|                                                                      |
| Comment: The string "<HEAD" in US-ASCII or compatible encodings,     |
| case-insensitively, possibly with leading spaces.                    |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 53 43 52 49 | text/html       | Scriptable |
| DF DF             | 50 54             |                 |            |
|                                                                      |
| Comment: The string "<SCRIPT" in US-ASCII or compatible              |
|          encodings, case-insensitively, possibly with leading        |
|          spaces.                                                     |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF    | 25 50 44 46 2D    | application/pdf | Scriptable |
|                                                                      |
| Comment: The string "%PDF-", the PDF signature.                      |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | 25 21 50 53 2D 41 | application/    | Safe       |
| FF FF FF FF FF    | 64 6F 62 65 2D    |      postscript |            |
|                                                                      |
| Comment: The string "%!PS-Adobe-", the PostScript signature.         |
+-------------------+-------------------+-----------------+------------+
| FF FF 00 00       | FE FF 00 00       | text/plain      | n/a        |
|                                                                      |
| Comment: UTF-16BE BOM                                                |
+-------------------+-------------------+-----------------+------------+
| FF FF 00 00       | FF FF 00 00       | text/plain      | n/a        |
|                                                                      |
| Comment: UTF-16LE BOM                                                |
+-------------------+-------------------+-----------------+------------+
| FF FF FF 00       | EF BB BF 00       | text/plain      | n/a        |
|                                                                      |
| Comment: UTF-8 BOM                                                   |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | 47 49 46 38 37 61 | image/gif       | Safe       |
|                                                                      |
| Comment: The string "GIF87a", a GIF signature.                       |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | 47 49 46 38 39 61 | image/gif       | Safe       |
|                                                                      |
| Comment: The string "GIF89a", a GIF signature.                       |



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+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | 89 50 4E 47 0D 0A | image/png       | Safe       |
| FF FF             | 1A 0A             |                 |            |
|                                                                      |
| Comment: The PNG signature.                                          |
+-------------------+-------------------+-----------------+------------+
| FF FF FF          | FF D8 FF          | image/jpeg      | Safe       |
|                                                                      |
| Comment: A JPEG SOI marker followed by a byte of another marker.     |
+-------------------+-------------------+-----------------+------------+
| FF FF             | 42 4D             | image/bmp       | Safe       |
|                                                                      |
| Comment: The string "BM", a BMP signature.                           |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF       | 00 00 01 00       | image/vnd.      | Safe       |
|                   |                   |  microsoft.icon |            |
|                                                                      |
| Comment: A 0 word following by a 1 word, a Windows Icon signature.   |
+-------------------+-------------------+-----------------+------------+

   Note: I'd like to add types like MPEG, AVI, Flash, Java, etc, to the
   above table.

   User agents may support further types if desired, by implicitly
   adding to the above table.  However, user agents should not use any
   other patterns for types already mentioned in the table above, as
   this could then be used for privilege escalation (where, e.g., a
   server uses the above table to determine that content is not HTML and
   thus safe from XSS attacks, but then a user agent detects it as HTML
   anyway and allows script to execute).

   The column marked "security" is used by the algorithm in the "text or
   binary" section, to avoid sniffing text/plain content as a type that
   can be used for a privilege escalation attack.

















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6.  Image

   If the resource's official type is "image/svg+xml", then the sniffed
   type of the resource is its official type (an XML type).

   Otherwise, if the first bytes of the resource match one of the byte
   sequences in the first column of the following table, then the
   sniffed type of the resource is the type given in the corresponding
   cell in the second column on the same row:

     +-------------------------+--------------------------+----------+
     | Bytes in Hexadecimal    | Sniffed type             | Comment  |
     +-------------------------+--------------------------+----------+
     | 47 49 46 38 37 61       | image/gif                | "GIF87a" |
     | 47 49 46 38 39 61       | image/gif                | "GIF89a" |
     | 89 50 4E 47 0D 0A 1A 0A | image/png                |          |
     | FF D8 FF                | image/jpeg               |          |
     | 42 4D                   | image/bmp                | "BM"     |
     | 00 00 01 00             | image/vnd.microsoft.icon |          |
     +-------------------------+--------------------------+----------+

   Otherwise, the sniffed type of the resource is the same as its
   official type.




























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7.  Feed or HTML

   1.   The user agent may wait for 512 or more bytes of the resource to
        be available.

   2.   Let s be the stream of bytes, and let s[i] represent the byte in
        s with position i, treating s as zero-indexed (so the first byte
        is at i=0).

   3.   If at any point this algorithm requires the user agent to
        determine the value of a byte in s which is not yet available,
        or which is past the first 512 bytes of the resource, or which
        is beyond the end of the resource, the user agent must stop this
        algorithm, and assume that the sniffed type of the resource is
        "text/html".

           Note: User agents are allowed, by the first step of this
           algorithm, to wait until the first 512 bytes of the resource
           are available.

   4.   Initialize pos to 0.

   5.   If s[0] is 0xEF, s[1] is 0xBB, and s[2] is 0xBF, then set pos to
        3.  (This skips over a leading UTF-8 BOM, if any.)

   6.   Loop start: Examine s[pos].

        *  If it is 0x09 (ASCII tab), 0x20 (ASCII space), 0x0A (ASCII
           LF), or 0x0D (ASCII CR)

              Increase pos by 1 and repeat this step.

        *  If it is 0x3C (ASCII "<")

              Increase pos by 1 and go to the next step.

        *  If it is anything else

              The sniffed type of the resource is "text/html".  Abort
              these steps.

   7.   If the bytes with positions pos to pos+2 in s are exactly equal
        to 0x21, 0x2D, 0x2D respectively (ASCII for "!--"), then:

        1.  Increase pos by 3.

        2.  If the bytes with positions pos to pos+2 in s are exactly
            equal to 0x2D, 0x2D, 0x3E respectively (ASCII for "-->"),



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            then increase pos by 3 and jump back to the previous step
            (the step labeled loop start) in the overall algorithm in
            this section.

        3.  Otherwise, increase pos by 1.

        4.  Return to step 2 in these substeps.

   8.   If s[pos] is 0x21 (ASCII "!"):

        1.  Increase pos by 1.

        2.  If s[pos] equal 0x3E, then increase pos by 1 and jump back
            to the step labeled loop start in the overall algorithm in
            this section.

        3.  Otherwise, return to step 1 in these substeps.

   9.   If s[pos] is 0x3F (ASCII "?"):

        1.  Increase pos by 1.

        2.  If s[pos] and s[pos+1] equal 0x3F and 0x3E respectively,
            then increase pos by 1 and jump back to the step labeled
            loop start in the overall algorithm in this section.

        3.  Otherwise, return to step 1 in these substeps.

   10.  Otherwise, if the bytes in s starting at pos match any of the
        sequences of bytes in the first column of the following table,
        then the user agent must follow the steps given in the
        corresponding cell in the second column of the same row.



















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+----------------------+-----------------------------------+-----------+
| Bytes in Hexadecimal | Requirement                       | Comment   |
+----------------------+-----------------------------------+-----------+
| 72 73 73             | The sniffed type of the resource  | "rss"     |
|                      | is "application/rss+xml"; abort   |           |
|                      | these steps.                      |           |
+----------------------+-----------------------------------+-----------+
| 66 65 65 64          | The sniffed type of the resource  | "feed"    |
|                      | si "application/atom+xml"; abort  |           |
|                      | these steps.                      |           |
+----------------------+-----------------------------------+-----------+
| 72 64 66 3A 52 44 46 | Continue to the next step in this | "rdf:RDF" |
|                      | algorithm.                        |           |
+----------------------+-----------------------------------+-----------+

        If none of the byte sequences above match the bytes in s
        starting at pos, then the sniffed type of the resource is "text/
        html".  Abort these steps.

   11.  ????  If, before the next ">", you find two xmlns* attributes
        with http://www.w3.org/1999/02/22-rdf-syntax-ns# and
        http://purl.org/rss/1.0/ as the namespaces, then the sniffed
        type of the resource is "application/rss+xml", abort these
        steps. (maybe we only need to check for http://purl.org/rss/1.0/
        actually) ????

   12.  Otherwise, the sniffed type of the resource is "text/html".

   For efficiency reasons, implementations may wish to implement this
   algorithm and the algorithm for detecting the character encoding of
   HTML documents in parallel.




















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Authors' Addresses

   Adam Barth
   Univeristy of California, Berkeley

   Email: abarth@eecs.berkeley.edu
   URI:   http://www.adambarth.com/


   Ian Hickson
   Google, Inc.

   Email: ian@hixie.ch
   URI:   http://ln.hixie.ch/





































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