One document matched: draft-ietf-websec-mime-sniff-02.xml
<?xml version="1.0"?>
<?rfc toc="yes"?>
<?xml-stylesheet type="text/xml" href="rfc2629.xslt"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<rfc ipr="trust200902" docName="draft-ietf-websec-mime-sniff-02">
<front>
<title abbrev="Media Type Sniffing">
Media Type Sniffing
</title>
<author initials="A." surname="Barth" fullname="Adam Barth">
<organization>
Google, Inc.
</organization>
<address>
<email>ietf@adambarth.com</email>
<uri>http://www.adambarth.com/</uri>
</address>
</author>
<author initials="I." surname="Hickson" fullname="Ian Hickson">
<organization>
Google, Inc.
</organization>
<address>
<email>ian@hixie.ch</email>
<uri>http://ln.hixie.ch/</uri>
</address>
</author>
<date month="February" year="2011" />
<workgroup>None</workgroup>
<keyword>Internet-Draft</keyword>
<abstract>
<t>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.</t>
<t>Please send feedback on this draft to websec@ietf.org.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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 <xref target="BarthCaballeroSong2009" />.</t>
<t>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 therein), an attacker might be able to steal the user's
authentication credentials and mount other cross-site scripting
attacks.</t>
<t>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.)</t>
</section>
<section anchor="metadata" title="Metadata">
<t>The explicit media type metadata information associated with sequence
of octets depends on the protocol that was used to fetch the octets.</t>
<t>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.
<list style="empty">
<t>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).</t>
</list>
</t>
<t>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.
<list style="empty">
<t>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.</t>
</list>
</t>
<t>For octets fetched over some other protocols, e.g. FTP, there is no
type information.</t>
<t>Note: Comparisons between media types, as defined by MIME
specifications, are done in an ASCII case-insensitive manner.
[RFC2046]</t>
</section>
<section anchor="web-pages" title="Web Pages">
<t>The user agent MUST use the following algorithm to determine the
sniffed-type of a sequence of octets:
<list style="numbers">
<t>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.</t>
<t>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:
<figure>
<artwork>
+-------------------------------+--------------------------------+
| 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 | |
+-------------------------------+--------------------------------+
</artwork>
<postamble>
...then jump to the "text or binary" section below.
</postamble>
</figure>
</t>
<t>If there is no official-type, jump to the "unknown type" section
below.</t>
<t>If the official-type is "unknown/unknown", "application/unknown", or
"*/*", jump to the "unknown type" section below.</t>
<t>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.</t>
<t>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.</t>
<t>If the official-type is "text/html", then jump to the "feed or HTML"
section below.</t>
<t>Let the sniffed-type be the official type.</t>
</list>
</t>
</section>
<section anchor="text-or-binary" title="Text or Binary">
<t>This section defines the *rules for distinguishing if a resource is
text or binary*.</t>
<t>
<list style="numbers">
<t>The user agent MAY wait for 512 or more octets be to arrive.
<list style="empty">
<t>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.</t>
</list>
</t>
<t>Let n be the smaller of either 512 or the number of octets that
have already arrived.</t>
<t>If n is greater than or equal to 3, and the first 2 or 3 octets
match one of the following octet sequences:
<figure>
<artwork>
+----------------------+--------------+
| Bytes in Hexadecimal | Description |
+----------------------+--------------+
| FE FF | UTF-16BE BOM |
| FF FE | UTF-16LE BOM |
| EF BB BF | UTF-8 BOM |
+----------------------+--------------+
</artwork>
<postamble>
...then let the sniffed-type be "text/plain" and abort these
steps.
</postamble>
</figure>
</t>
<t>If none of the first n octets are binary data octets then let the
sniffed-type be "text/plain" and abort these steps.
<figure>
<artwork>
+-------------------------+
| Binary Data Byte Ranges |
+-------------------------+
| 0x00 -- 0x08 |
| 0x0B |
| 0x0E -- 0x1A |
| 0x1C -- 0x1F |
+-------------------------+
</artwork>
</figure>
</t>
<t>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 that row and
abort these steps.
<list style="empty">
<t>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.</t>
</list>
</t>
<t>Otherwise, let the sniffed-type be "application/octet-stream" and
abort these steps.</t>
</list>
</t>
</section>
<section anchor="unknown-type" title="Unknown Type">
<t>
<list style="numbers">
<t>The user agent MAY wait for 512 or more octets to arrive for the
same reason as in the "text or binary" section above.</t>
<t>Let n be the smaller of either 512 or the number of octets that
have already arrived.</t>
<t>For each row in the table below:
<list style="symbols">
<t>If the row has no "WS" octets:
<list style="numbers">
<t>Let pattern-length be the length of the pattern.</t>
<t>If n is smaller than pattern-length then skip this row.</t>
<t>Apply the bit-wise "and" operator to the first pattern-length
octets and the given mask, and let the result be the
masked-data.</t>
<t>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.</t>
</list>
</t>
<t>If the row has a "WS" octet or a "_>" octet:
<list style="numbers">
<t>Let index-pattern be an index into the mask and pattern octet
strings of the row.</t>
<t>Let index-stream be an index into the octet stream being
examined.</t>
<t>LOOP: If index-stream points beyond the end of the octet
stream, then this row doesn't match and skip this row.</t>
<t>Examine the index-stream-th octet of the octet stream as
follows:
<list style="symbols">
<t>If the index-pattern-th octet of the pattern is a normal
hexadecimal octet and not a "WS" octet or a "_>" octet:
<list style="empty">
<t>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.</t>
<t>Otherwise, increment index-pattern to the next octet in
the mask and pattern and index-stream to the next octet in
the octet stream.</t>
</list>
</t>
<t>Otherwise, if the index-pattern-th octet of the pattern is a
"WS" octet:
<list style="empty">
<t>"WS" means "whitespace", and allows insignificant
whitespace to be skipped when sniffing for a type
signature.</t>
<t>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.</t>
<t>Otherwise, increment only the index-pattern to the next
octet in the mask and pattern.</t>
</list>
</t>
<t>Otherwise, if the index-pattern-th octet of the pattern is a
"_>" octet:
<list style="empty">
<t>"_>" means "space-or-bracket", and allows HTML tag names to
terminate with either a space or a greater than sign.</t>
<t>If index-stream-th octet of the stream different than 0x20
(ASCII space) or 0x3E (ASCII ">"), then skip this row.</t>
<t>Otherwise, increment index-pattern to the next octet in
the mask and pattern and index-stream to the next octet in
the octet stream.</t>
</list>
</t>
</list>
</t>
<t>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.</t>
<t>Otherwise, let the sniffed-type be the type given in the cell
of the third column in that row and abort these steps.</t>
</list>
</t>
</list>
</t>
<t>If the first n octets match the signature for H264 (as define in
<xref target="h264-signature" />), then let the sniffed-type be
video/H264 and abort these steps.</t>
<t>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.</t>
<t>Otherwise, let the sniffed-type be "application/octet-stream" and
abort these steps.</t>
</list>
</t>
<t>The table used by the above algorithm is:
<figure>
<artwork>
+-------------------+-------------------+-----------------+------------+
| 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 _> | | |
| 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 |
| 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 audio or video signature. |
+-------------------+-------------------+-----------------+------------+
| FF FF FF FF 00 00 | 52 49 46 46 00 00 | audio/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 | video/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.]
</artwork>
</figure>
</t>
<t>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 extending this table, user
agents SHOULD NOT introduce any privilege escalation
vulnerabilities.</t>
<t>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.</t>
<section anchor="h264-signature" title="Signature for H.264">
<t>This section defines whether a sequence of n octets *matches the
signature for H.264*.</t>
<t>If n is less than 4, then the sequence does not match the signature
for H264 and abort these steps.</t>
<t>Let box-size be the value of the first four octets, interpreted
as a 32 bit unsigned, little-endian integer.</t>
<t>If n is less than box-size or if box-size is not evenly divisible
by 4, then the sequence does not match the signature for H264 and
abort these steps.</t>
<t>If octets 5 through 8 (inclusive) of the sequence are not 0x66 0x74
0x79 0x70 (the ASCII string "ftyp"), then the sequence does not match
the signature for H264 and abort these steps.</t>
<t>For each i from 2 to box-size/4 - 1 (inclusive):
<list style="numbers">
<t>If i is equal to 3, continue to the next i, if any. (These octets
correspond to the minor version number.)</t>
<t>If octets 4*i through 4*i + 3 (inclusive) of the sequence are
0x6D 0x70 0x34 (the ASCII string "mp4"), then the sequence *does*
match the signature for H264 and abort these steps.</t>
</list>
</t>
<t>The sequence does not match the signature for H264.</t>
</section>
</section>
<section anchor="image" title="Image">
<t>This section defines the *rules for sniffing images
specifically*.</t>
<t>If the official-type is "image/svg+xml", then let the sniffed-type be
the official-type (an XML type) and abort these steps.</t>
<t>If the first octets match one of the signatures in <xref
target="unknown-type" /> for one of the following media types, then let
the sniffed-type be the corresponding media type and abort these steps:
<list style="symbols">
<t>image/gif</t>
<t>image/png</t>
<t>image/jpeg</t>
<t>image/bmp</t>
<t>image/vnd.microsoft.icon</t>
<t>image/webp</t>
</list>
</t>
<t>Otherwise, let the sniffed-type be the official-type
and abort these steps.</t>
</section>
<section anchor="video" title="Video">
<t>This section defines the *rules for sniffing videos
specifically*.</t>
<t>If the first octets match one of the signatures in <xref
target="unknown-type" /> for one of the following media types, then let
the sniffed-type be the corresponding media type and abort these steps:
<list style="symbols">
<t>video/H264</t>
<t>video/webm</t>
</list>
</t>
<t>Otherwise, let the sniffed-type be the official-type
and abort these steps.</t>
</section>
<section anchor="fonts" title="Fonts">
<t>This section defines the *rules for sniffing fonts
specifically*.</t>
<t>TODO</t>
<t>Otherwise, let the sniffed-type be the official-type
and abort these steps.</t>
</section>
<section anchor="feed-or-html" title="Feed or HTML">
<t>
<list style="numbers">
<t>The user agent MAY wait for 512 or more octets to arrive for the
same reason as in the "text or binary" section above.</t>
<t>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).</t>
<t>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".
<list style="empty">
<t>Note: User agents are allowed, by the first step of this
algorithm, to wait until the first 512 octets have arrived.</t>
</list>
</t>
<t>Initialize pos to 0.</t>
<t>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.)</t>
<t>LOOP: Examine s[pos].
<list style="symbols">
<t>If it equals 0x09 (ASCII tab), 0x20 (ASCII space), 0x0A (ASCII
LF), or 0x0D (ASCII CR)
<list style="empty">
<t>Increase pos by 1 and repeat this step.</t>
</list>
</t>
<t>If it equals 0x3C (ASCII "<")
<list style="empty">
<t>Increase pos by 1 and go to the next step.</t>
</list>
</t>
<t>If it is anything else
<list style="empty">
<t>Let the sniffed-type be "text/html" and abort these steps.</t>
</list>
</t>
</list>
</t>
<t>If the octets with positions pos to pos+2 in s are exactly equal
to 0x21, 0x2D, 0x2D respectively (ASCII for "!--"), then:
<list style="numbers">
<t>Increase pos by 3.</t>
<t>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 this section.</t>
<t>Otherwise, increase pos by 1.</t>
<t>Return to step 2 in these substeps.</t>
</list>
</t>
<t>If s[pos] equals 0x21 (ASCII "!"):
<list style="numbers">
<t>Increase pos by 1.</t>
<t>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.</t>
<t>Otherwise, return to step 1 in these substeps.</t>
</list>
</t>
<t>If s[pos] equals 0x3F (ASCII "?"):
<list style="numbers">
<t>Increase pos by 1.</t>
<t>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.</t>
<t>Otherwise, return to step 1 in these substeps.</t>
</list>
</t>
<t>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.
<figure>
<artwork>
+----------------------+------------------------------------+---------+
| 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. | |
+----------------------+------------------------------------+---------+
</artwork>
<postamble>
If none of the octet sequences above match the octets in s
starting at pos, then let the sniffed-type be "text/html" and
abort these steps.
</postamble>
</figure>
</t>
<t>Initialize RDF-flag to 0.</t>
<t>Initialize RSS-flag to 0.</t>
<t>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:
<list style="numbers">
<t>Increase pos by 23.</t>
<t>Set RSS-flag to 1.</t>
</list>
</t>
<t>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:
<list style="numbers">
<t>Increase pos by 42.</t>
<t>Set RDF-flag to 1.</t>
</list>
</t>
<t>Increase pos by 1.</t>
<t>If RDF-flag is 1 and RSS-flag is 1, then let the sniffed-type be
"application/rss+xml" and abort these steps.</t>
<t>If pos points beyond the end of the octet stream s, then continue
to step 19 of this algorithm.</t>
<t>Jump back to step 13 of this algorithm.</t>
<t>Let the sniffed-type be "text/html" and abort these steps.</t>
</list>
</t>
<t>For efficiency reasons, implementations might wish to implement this
algorithm and the algorithm for detecting the character encoding of HTML
documents in parallel.</t>
</section>
</middle>
<back>
<references>
<reference anchor="BarthCaballeroSong2009" target="http://www.adambarth.com/papers/2009/barth-caballero-song.pdf">
<front>
<title>Secure Content Sniffing for Web Browsers, or How to Stop
Papers from Reviewing Themselves</title>
<author initials="A." surname="Barth" fullname="Adam Barth">
<organization>UC Berkeley</organization>
</author>
<author initials="J." surname="Caballero" fullname="Juan Caballero">
<organization>UC Berkeley and CMU</organization>
</author>
<author initials="D." surname="Song" fullname="Dawn Song">
<organization>UC Berkeley</organization>
</author>
<date year="2009"/>
</front>
</reference>
</references>
<!--
TODO:
* Transcribe the tables into C and auto generate the tables.
-->
<!-- Ack Alfred HÎnes, Mark Pilgrim -->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 10:56:38 |