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

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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
2.  Metadata
3.  Web Pages
4.  Text or Binary
5.  Unknown Type
6.  Image
7.  Feed or HTML
§  Authors' Addresses

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.

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
   • 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]

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.
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.

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.

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

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 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             |                 |            |
|                                                                      |
| 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.                       |
+-------------------+-------------------+-----------------+------------+
| 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.

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.

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 "-->"), 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.

    +----------------------+-----------------------------------+-----------+
    | 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.

Authors' Addresses