None A. Barth
Internet-Draft I. Hickson
Expires: July 28, 2011 Google, Inc.
January 24, 2011
Media Type Sniffing
draft-ietf-websec-mime-sniff-01
Abstract
Many web servers supply incorrect Content-Type header fields with
their HTTP responses. In order to be compatible with these servers,
user agents consider the content of HTTP responses as well as the
Content-Type header fields when determining the effective media type
of the response. This document describes an algorithm for
determining the effective media type of HTTP responses that balances
security and compatibility considerations.
Please send feedback on this draft to apps-discuss@ietf.org.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on July 28, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Metadata . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Web Pages . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Text or Binary . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Unknown Type . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Image . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. Feed or HTML . . . . . . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
The HTTP Content-Type header field indicates the media type of an
HTTP response. However, many HTTP servers supply a Content-Type that
does not match the actual contents of the response. Historically,
web browsers have tolerated these servers by examining the content of
HTTP responses in addition to the Content-Type header field to
determine the effective media type of the response.
Without a clear specification of how to "sniff" the media type, each
user agent implementor was forced to reverse engineer the behavior of
the other user agents and to develop their own algorithm. These
divergent algorithms have lead to a lack of interoperability between
user agents and to security issues when the server intends an HTTP
response to be interpreted as one media type but some user agents
interpret the responses as another media type.
These security issues are most severe when an "honest" server lets
potentially malicious users upload files and then serves the contents
of those files with a low-privilege media type (such as text/plain or
image/jpeg). (Malicious servers, of course, can specify an arbitrary
media type in the Content-Type header field.) In the absence of
media type sniffing, this user-generated content would not be
interpreted as a high-privilege media type, such as text/html.
However, if a user agent does interpret a low-privilege media type,
such as image/gif, as a high-privilege media type, such as text/html,
the user agent has created a privilege escalation vulnerability in
the server. For example, a malicious user might be able to leverage
content sniffing to mount a cross-site script attack by including
JavaScript code in the uploaded file that a user agent treats as
text/html.
This document describes a content sniffing algorithm that carefully
balances the compatibility needs of user agent implementors with the
security constraints. The algorithm has been constructed with
reference to content sniffing algorithms present in popular user
agents, an extensive database of existing web content, and metrics
collected from implementations deployed to a sizable number of users
[BarthCaballeroSong2009].
WARNING! Whenever possible, user agents SHOULD NOT employ a content
sniffing algorithm. However, if a user agent does employ a content
sniffing algorithm, the user agent SHOULD use the algorithm in this
document because using a different content sniffing algorithm than
servers expect causes security problems. 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 user agent believes the content
to be HTML (and thus privileged to execute any scripts contained
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therein), an attacker might be able to steal the user's
authentication credentials and mount other cross-site scripting
attacks.
Conformance requirements phrased as algorithms or specific steps MAY
be implemented in any manner, so long as the end result is
equivalent. (In particular, the algorithms defined in this
specification are intended to be easy to follow, and not intended to
be performant.)
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2. Metadata
The explicit media type metadata information associated with sequence
of octets depends on the protocol that was used to fetch the octets.
For octets received via HTTP, the Content-Type HTTP header field, if
present, indicates the media type. Let the official-type be the
media type indicted by the HTTP Content-Type header field, if
present. If the Content-Type header field is absent or if its value
cannot be interpreted as a media type (e.g. because its value doesn't
contain a U+002F SOLIDUS ('/') character), then there is no official-
type.
Note: If an HTTP response contains multiple Content-Type header
fields, the user agent MUST use the textually last Content-Type
header field to the official-type. For example, if the last
Content-Type header field contains the value "foo", then there is
no official media type because "foo" cannot be interpreted as a
media type (even if the HTTP response contains another Content-
Type header field that could be interpreted as a media type).
For octets fetched from the file system, user agents should use
platform-specific conventions (e.g., operating system file extension/
type mappings) to determine the official-type.
Note: It is essential that file extensions are not used for
determining the media type for octets fetched over HTTP because,
in some cases, file extensions can be supplied by malicious
parties. For example, most PHP installations let the attacker
append arbitrary path information to URLs (e.g.,
http://example.com/foo.php/bar.html) and thereby determine the
file extension.
For octets fetched over some other protocols, e.g. FTP, there is no
type information.
Note: Comparisons between media types, as defined by MIME
specifications, are done in an ASCII case-insensitive manner.
[RFC2046]
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3. Web Pages
The user agent MUST use the following algorithm to determine the
sniffed-type of a sequence of octets:
1. If the user agent is configured to strictly obey the official-
type, then let the sniffed-type be the official-type and abort
these steps.
2. If the octets were fetched via HTTP and there is an HTTP Content-
Type header field and the value of the last such header field has
octets that *exactly* match the octets contained in 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. If there is no official-type, jump to the "unknown type" section
below.
4. If the official-type is "unknown/unknown", "application/unknown",
or "*/*", jump to the "unknown" type section below.
5. If the official-type ends in "+xml", or if it is either "text/
xml" or "application/xml", then let the sniffed-type be the
official-type and abort these steps.
6. If the 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.
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7. If the official-type is "text/html", then jump to the "feed or
HTML" section below.
8. Let the sniffed-type be the official type.
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4. Text or Binary
This section defines the *rules for distingushing if a resource is
text or binary*.
1. The user agent MAY wait for 512 or more octets be to arrive.
Note: Waiting for 512 octets octets to arrive causes the text-
or-binary algorithm to be deterministic for a given sequence
of octets. However, in some cases, the user agent might need
to wait an arbitrary length of time for these octets to
arrive. User agents SHOULD wait for 512 octets to arrive,
when feasible.
2. Let n be the smaller of either 512 or the number of octets that
have already arrived.
3. If n is greater than or equal to 3, and the first 2 or 3 octets
match one of the following octet sequences:
+----------------------+--------------+
| Bytes in Hexadecimal | Description |
+----------------------+--------------+
| FE FF | UTF-16BE BOM |
| FF FE | UTF-16LE BOM |
| EF BB BF | UTF-8 BOM |
+----------------------+--------------+
...then let the sniffed-type be "text/plain" and abort these
steps.
4. If none of the first n octets are binary data octets then let the
sniffed-type be "text/plain" and abort these steps.
+-------------------------+
| Binary Data Byte Ranges |
+-------------------------+
| 0x00 -- 0x08 |
| 0x0B |
| 0x0E -- 0x1A |
| 0x1C -- 0x1F |
+-------------------------+
5. If the first octets match one of the octet 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 let the sniffed-type be the type
given in the corresponding cell in the "sniffed type" column on
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that row and abort these steps.
WARNING! It is critical that this step not ever return a
scriptable type (e.g., text/html), because otherwise that
would allow a privilege escalation attack.
6. Otherwise, let the sniffed-type be "application/octet-stream" and
abort these steps.
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5. Unknown Type
1. The user agent MAY wait for 512 or more octets to arrive for the
same reason as in the "text or binary" section above.
2. Let n be the smaller of either 512 or the number of octets that
have already arrived.
3. For each row in the table below:
* If the row has no "WS" octets:
1. Let pattern-length be the length of the pattern.
2. If n is smaller than pattern-length then skip this row.
3. Apply the bit-wise "and" operator to the first pattern-
length octets and the given mask, and let the result be
the masked-data.
4. If the octets of the masked-data matches the given pattern
octets exactly, then let the sniffed-type be the type
given in the cell of the third column in that row and
abort these steps.
* If the row has a "WS" octet or a "_>" octet:
1. Let index-pattern be an index into the mask and pattern
octet strings of the row.
2. Let index-stream be an index into the octet stream being
examined.
3. LOOP: If index-stream points beyond the end of the octet
stream, then this row doesn't match and skip this row.
4. Examine the index-stream-th octet of the octet stream as
follows:
- If the index-pattern-th octet of the pattern is a
normal hexadecimal octet and not a "WS" octet or a "SB"
octet:
If the bit-wise "and" operator, applied to the
index-stream-th octet of the stream and the index-
pattern-th octet of the mask, yield a value
different than the index-pattern-th octet of the
pattern, then skip this row.
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Otherwise, increment index-pattern to the next octet
in the mask and pattern and index-stream to the next
octet in the octet stream.
- Otherwise, if the index-pattern-th octet of the pattern
is a "WS" octet:
"WS" means "whitespace", and allows insignificant
whitespace to be skipped when sniffing for a type
signature.
If the index-stream-th octet 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 octet in
the octet stream.
Otherwise, increment only the index-pattern to the
next octet in the mask and pattern.
- Otherwise, if the index-pattern-th octet of the pattern
is a "_>" octet:
"_>" means "space-or-bracket", and allows HTML tag
names to terminate with either a space or a greater
than sign.
If index-stream-th octet of the stream different
than 0x20 (ASCII space) or 0x3E (ASCII ">"), then
skip this row.
Otherwise, increment index-pattern to the next octet
in the mask and pattern and index-stream to the next
octet in the octet stream.
5. If index-pattern does not point beyond the end of the mask
and pattern octet strings, then jump back to the LOOP step
in this algorithm.
6. Otherwise, let the sniffed-type be the type given in the
cell of the third column in that row and abort these
steps.
4. If none of the first n octets are binary data (as defined in the
"text or binary" section), then let the sniffed-type be "text/
plain" and abort these steps.
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5. Otherwise, let the sniffed-type be "application/octet-stream" and
abort these steps.
The table used by the above algorithm is:
+-------------------+-------------------+-----------------+------------+
| Mask in Hex | Pattern in Hex | Sniffed Type | Security |
+-------------------+-------------------+-----------------+------------+
| FF FF FF DF DF DF | WS 3C 21 44 4F 43 | text/html | Scriptable |
| DF DF DF DF FF DF | 54 59 50 45 20 48 | | |
| DF DF DF FF | 54 4D 4C _> | | |
| Comment: <!DOCTYPE HTML |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 48 54 4D 4C | text/html | Scriptable |
| FF | _> | | |
| Comment: <HTML |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 48 45 41 44 | text/html | Scriptable |
| FF | _> | | |
| Comment: <HEAD |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 53 43 52 49 | text/html | Scriptable |
| DF DF FF | 50 54 _> | | |
| Comment: <SCRIPT |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 49 46 52 41 | text/html | Scriptable |
| DF DF FF | 4d 45 _> | | |
| Comment: <IFRAME |
+-------------------+-------------------+-----------------+------------+
| FF FF DF FF FF | WS 3C 48 31 _> | text/html | Scriptable |
| Comment: <H1 |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF FF | WS 3C 44 49 56 _> | text/html | Scriptable |
| Comment: <DIV |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 46 4f 4e 54 | text/html | Scriptable |
| FF | _> | | |
| Comment: <FONT |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 54 41 42 4c | text/html | Scriptable |
| DF FF | 45 _> | | |
| Comment: <TABLE |
+-------------------+-------------------+-----------------+------------+
| FF FF DF FF | WS 3C 41 _> | text/html | Scriptable |
| Comment: <A |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 53 54 59 4c | text/html | Scriptable |
| DF FF | 45 _> | | |
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| Comment: <STYLE |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 54 49 54 4c | text/html | Scriptable |
| DF FF | 45 _> | | |
| Comment: <TITLE |
+-------------------+-------------------+-----------------+------------+
| FF FF DF FF | WS 3C 42 _> | text/html | Scriptable |
| Comment: <B |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF DF DF | WS 3C 42 4f 44 59 | text/html | Scriptable |
| FF | _> | | |
| Comment: <BODY |
+-------------------+-------------------+-----------------+------------+
| FF FF DF DF FF | WS 3C 42 52 _> | text/html | Scriptable |
| Comment: <BR |
+-------------------+-------------------+-----------------+------------+
| FF FF DF FF | WS 3C 50 _> | text/html | Scriptable |
| Comment: <P |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | WS 3C 21 2d 2d _> | text/html | Scriptable |
| Comment: <!-- |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | WS 3C 3f 78 6d 6c | text/xml | Scriptable |
| Comment: <?xml (Note the case sensitivity and lack of trailing _>) |
+-------------------+-------------------+-----------------+------------+
| 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 FE 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 |
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| 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 octet of another marker. |
+-------------------+-------------------+-----------------+------------+
| FF FF | 42 4D | image/bmp | Safe |
| Comment: The string "BM", a BMP signature. |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF 00 00 | 52 49 46 46 00 00 | image/webp | Safe |
| 00 00 FF FF FF FF | 00 00 57 45 42 50 | | |
| FF FF | 56 50 | | |
| Comment: "RIFF" followed by four bytes, followed by "WEBPVP". |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF | 00 00 01 00 | image/vnd. | Safe |
| | | microsoft.icon | |
| Comment: A Windows Icon signature. |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF | 4F 67 67 53 00 | application/ogg | Safe |
| Comment: An Ogg Vorbis audio or video signature. |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF 00 00 | 52 49 46 46 00 00 | audio/x-wave | Safe |
| 00 00 FF FF FF FF | 00 00 57 41 56 45 | | |
| Comment: "RIFF" followed by four bytes, followed by "WAVE". |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF | 1A 45 DF A3 | vidow/webm | Safe |
| Comment: The WebM signature [TODO: Use more octets?] |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF FF FF | 52 61 72 20 1A 07 | application/ | Safe |
| FF | 00 | x-rar-compressed| |
| Comment: A RAR archive. |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF | 50 4B 03 04 | application/zip | Safe |
| Comment: A ZIP archive. |
+-------------------+-------------------+-----------------+------------+
| FF FF FF | 1F 8B 08 | application/ | Safe |
| | | x-gzip | |
| Comment: A GZIP archive. |
+-------------------+-------------------+-----------------+------------+
[TODO: MP3 audio and H.264 video.]
User agents MAY support additional types if necessary, by implicitly
adding to the above table. However, user agents SHOULD NOT not use
any other patterns for types already mentioned in the table above
because 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 cross-site scripting attacks, but then a user
agent detects it as HTML anyway and allows script to execute). In
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extending this table, user agents SHOULD NOT introduce any privilege
escalation vulnerabilities.
Note: 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
This section defines the *rules for sniffing images specifically*.
If the official-type is "image/svg+xml", then let the sniffed-type be
the official-type (an XML type) and abort these steps.
If the first octets match one of the octet sequences in the first
column of the following table, then let the sniffed-type be the type
given in the corresponding cell in the second column on the same row
and abort these steps:
+-------------------------+--------------------------+-----------------+
| 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 | |
| (see Section ??) | image/webp | "RIFF????WEBPVP |
+-------------------------+--------------------------+-----------------+
Otherwise, let the sniffed-type be the official-type and abort these
steps.
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7. Feed or HTML
1. The user agent MAY wait for 512 or more octets to arrive for the
same reason as in the "text or binary" section above.
2. Let s be the stream of octets, and let s[i] represent the octet
in s with position i, treating s as zero-indexed (so the first
octet is at i=0).
3. If at any point this algorithm requires the user agent to
determine the value of a octet in s which has not yet arrived,
or which is past the first 512 octets, or which is beyond the
end of the octet stream, the algorithm stops and the sniffed-
type is "text/html".
Note: User agents are allowed, by the first step of this
algorithm, to wait until the first 512 octets have arrived.
4. Initialize pos to 0.
5. If s[0] equals 0xEF, s[1] equals 0xBB, and s[2] equals 0xBF,
then set pos to 3. (This skips over a leading UTF-8 BOM, if
any.)
6. LOOP: Examine s[pos].
* If it equals 0x09 (ASCII tab), 0x20 (ASCII space), 0x0A
(ASCII LF), or 0x0D (ASCII CR)
Increase pos by 1 and repeat this step.
* If it equals 0x3C (ASCII "<")
Increase pos by 1 and go to the next step.
* If it is anything else
Let the sniffed-type be "text/html" and abort these steps.
7. If the octets 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 octets 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
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this section.
3. Otherwise, increase pos by 1.
4. Return to step 2 in these substeps.
8. If s[pos] equals 0x21 (ASCII "!"):
1. Increase pos by 1.
2. If s[pos] equals 0x3E, then increase pos by 1 and jump back
to the step labeled LOOP in the overall algorithm in this
section.
3. Otherwise, return to step 1 in these substeps.
9. If s[pos] equals 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 in the overall algorithm in this section.
3. Otherwise, return to step 1 in these substeps.
10. Otherwise, if the octets in s starting at pos match any of the
sequences of octets 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 | Let the sniffed-type be | rss |
| | "application/rss+xml" and abort | |
| | these steps. | |
+----------------------+------------------------------------+---------+
| 66 65 65 64 | Let the sniffed-type be | feed |
| | "application/atom+xml" and abort | |
| | these steps. | |
+----------------------+------------------------------------+---------+
| 72 64 66 3A 52 44 46 | Continue to the next step in this | rdf:RDF |
| | algorithm. | |
+----------------------+------------------------------------+---------+
If none of the octet sequences above match the octets in s
starting at pos, then let the sniffed-type be "text/html" and
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abort these steps.
11. Initialize RDF-flag to 0.
12. Initialize RSS-flag to 0.
13. If the octets with positions pos to pos+23 in s are exactly
equal to 0x68, 0x74, 0x74, 0x70, 0x3A, 0x2F, 0x2F, 0x70, 0x75,
0x72, 0x6C, 0x2E, 0x6F, 0x72, 0x67, 0x2F, 0x72, 0x73, 0x73,
0x2F, 0x31, 0x2E, 0x30, 0x2F respectively (ASCII for
"http://purl.org/rss/1.0/"), then:
1. Increase pos by 23.
2. Set RSS-flag to 1.
14. If the octets with positions pos to pos+42 in s are exactly
equal to 0x68, 0x74, 0x74, 0x70, 0x3A, 0x2F, 0x2F, 0x77, 0x77,
0x77, 0x2E, 0x77, 0x33, 0x2E, 0x6F, 0x72, 0x67, 0x2F, 0x31,
0x39, 0x39, 0x39, 0x2F, 0x30, 0x32, 0x2F, 0x32, 0x32, 0x2D,
0x72, 0x64, 0x66, 0x2D, 0x73, 0x79, 0x6E, 0x74, 0x61, 0x78,
0x2D, 0x6E, 0x73, 0x23 respectively (ASCII for
"http://www.w3.org/1999/02/22-rdf-syntax-ns#"), then:
1. Increase pos by 42.
2. Set RDF-flag to 1.
15. Increase pos by 1.
16. If RDF-flag is 1 and RSS-flag is 1, then let the sniffed-type be
"application/rss+xml" and abort these steps.
17. If pos points beyond the end of the octet stream s, then
continue to step 19 of this algorithm.
18. Jump back to step 13 of this algorithm.
19. Let the sniffed-type be "text/html" and abort these steps.
For efficiency reasons, implementations might wish to implement this
algorithm and the algorithm for detecting the character encoding of
HTML documents in parallel.
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8. References
[BarthCaballeroSong2009]
Barth, A., Caballero, J., and D. Song, "Secure Content
Sniffing for Web Browsers, or How to Stop Papers from
Reviewing Themselves", 2009, <http://www.adambarth.com/
papers/2009/barth-caballero-song.pdf>.
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Authors' Addresses
Adam Barth
Google, Inc.
Email: ietf@adambarth.com
URI: http://www.adambarth.com/
Ian Hickson
Google, Inc.
Email: ian@hixie.ch
URI: http://ln.hixie.ch/
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