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Posted Feb 25, 2006
Authored by Amit Klein

Whitepaper entitled "HTTP Response Smuggling". It discusses evasion techniques to bypass anti-HTTP response splitting strategies.

tags | paper, web
SHA-256 | ee3a42dce4b4f8bc8c2ae652525c238be609475a31e10db164e4648e1e6a3f2f


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                     HTTP Response Smuggling

Or "HTTP Response Splitting is [still] Mostly Harmful" ;-)

Amit Klein, February 2006


Recently, several anti- HTTP Response Splitting strategies has
been suggested and/or put to use by various individuals and
vendors. Apparently, those individuals and vendors did not
subscribe to the somewhat strict approach recommended in [1],
which is, to simply disallow CR and LF in data embedded in HTTP
response headers. Rather, the recent anti-HTTP Response Splitting
suggestions attempt to take a more granular approach. However, it
seems that unfortunately, this approach is basically flawed,
because it does not take into account variations and tolerance in
the parsing of HTTP responses among proxy servers and clients.
This paper presents HTTP Response Smuggling - a way to evade
those anti- HTTP response splitting strategies. HTTP Response
Smuggling makes use of HTTP Request Smuggling -like techniques
([2]) to exploit the discrepancies between what an anti- HTTP
Response Splitting mechanism would consider to be the HTTP
response stream, and the response stream as parsed by a proxy
server (or a browser).

Technique #1 - Who needs a CRLF anyway?

In [3] and [4], it seems that the major defense line against HTTP
Response Splitting is disallowing the CRLF sequence ([4]
recommends also disallowing the string "HTTP/1.", as well as
other strings - this will be covered below). Apart from the
serious false positive problem this inflicts (forms with TEXTAREA
fields expect multi-line submission, which has CRLF in it), it is
also quite ineffective against HTTP Response Splitting.

Many proxy servers (e.g. Apache 2.0.55 mod_proxy and Sun Java
System Web Proxy Server 4.0, DeleGate 8.11.5) simply allow LF
where CRLF is expected. This is also true for Microsoft IE 6.0
SP2 and Mozilla Firefox 1.5. As such, an HTTP Response Splitting
attack can be devised containing LFs only (and was indeed
demonstrated on Apache 2.0.55 mod_cache+mod_proxy). Note that
treating LF as an end of line marker is in violation of the
"strict" RFC 2616 [5] section 2.2, which defines the CRLF
sequence as the end of line marker, yet at the same time, the RFC
(in section 19.3) recommends parsing LF as CRLF.

Poisoning the cache of Apache 2.0.55 and Sun Java System Web
Proxy Server 4.0 (see appendix) succeeded when only LFs were

Technique #2 - The oldest trick in the Smuggling book

In [6], the author suggest anti- HTTP Response Splitting
technique based on the server marking where it considers the
start of headers and end of headers are (using a marker such as a
random string which is unknown to the attacker at the injection
time). The HTTP client (proxy or browser) then has to verify that
the start of headers and end of headers markers match. Putting
aside usability issues such as header reordering (note that the
RFC [5] section 4.2 states that "The order in which header fields
with differing field names are received is not significant.",
meaning that RFC compliant implementations are not required to
maintain order among different headers, and indeed some are known
to reorder headers), the fact of the matter is that still, some
HTTP Response Splitting attacks are possible. In this case, the
double Content-Length technique (a classic smuggling trick) comes
in handy. Let us assume that the injection point occurs before
the original Content-Length in the headers section. In such case,
the attacker injects a Content-Length header of his/her own. As
it happens, Microsoft IE 6.0 SP2 and Apache 2.0.55 mod_proxy will
use the first Content-Length header, and ignore any additional
Content-Length headers (while Mozilla Firefox 1.5, Sun Java
System Web Proxy Server 4.0 and Delegate 8.11.5 will use the last
Content-Length header, and ignore any preceding headers - so if
the injection point occurs after the original Content-Length
header, they can be exploited).
The injected Content-Length header terminates the first request
at a location of the attacker's choice. The attacker needs to
carefully choose this location to point at another injection
point (this time in the response body) in which he/she can embed
a complete HTTP response, including a spoofed start of headers
marker and end of headers marker. This second injection is an
additional requirement, and as such, arguably limits the attack,
however - there are cases wherein a second injection is native to
the situation (see below). Anyway, the importance here is to show
that the anti-HTTP Response Splitting can be bypassed under some

Note that an HTTP (response) message containing multiple Content-
Length headers is in violation of the HTTP/1.1 RFC [5].

Poisoning the cache of Apache 2.0.55 succeeded with multiple
Content-Length headers were provided in the first HTTP response
message (the injected header was the first one, of course).

Example response stream:

HTTP/1.1 200 OK
Termination-Token: cvb098srnwe23
Content-Length: 1234 <-- Injected header (first injection
Content-Length: 5678
Termination-Header: cvb098srnwe23

[... HTML data from the original response, 1234 bytes ...]
HTTP/1.1 200 OK <-- Injected complete HTTP response (second
injection point)
Termination-Token: gotcha
Content-Length: 46
Termination-Header: gotcha

<html>I can still do response splitting</html> <-- End of
[... more HTML data from the original response ...]

Technique #3 - The PHP way - close, but no cigar

An impressive fine grained mechanism that attempts to prevent
HTTP header injection, with HTTP Response Splitting as a special
case ([7], [8], [9]), is implemented in the latest versions of
PHP (5.1.2 and 4.4.2). The code in /main/SAPI.c (function
sapi_header_op) performs the following:

1. Removal of the trailing sequence of CRs, LFs, SPs, and HTs,
if such sequence exists.

2. Aborting if any LF found is not followed by SP or HT.

This really looks fine, except that Sun Java System Web Proxy
Server 4.0 happily accepts CR as an end of line marker. This
means that this proxy server can be exploited using CR only (no
LF whatsoever), so this anti- HTTP Response Splitting is not full
proof. Quite likely several other proxy servers are that liberal,
although strictly speaking, an HTTP message that has CR as an end
of line marker instead of CRLF is in violation of the RFC.
Using CR-only response, and a successful cache poisoning with CR-
only was demonstrated.

Handling additional patterns

[4] suggests the following additional patterns for detecting HTTP
Response Splitting (on top of CRLF):




Now, "<html" and "<meta" are located in the body of the injected
2nd response. Therefore, they can be easily hidden using UTF-7
encoding tricks [1] or UTF-16 encoding tricks, compression and
chunked-encoding [11]. Moreover, a malicious payload doesn't have
to use any of these. It suffices for most purposes to have a
payload such as:



<script src=...></script>

Both IE 6.0 SP2 and Firefox 1.5 parse the <script> tag and
execute its code even if it is not nested inside an <html> tag.

As for the "http/1." pattern - some proxy servers are willing to
accept slight deviations from this pattern. For example, for Sun
Java System Web Proxy Server 4.0 and DeleGate 8.11.5, "HTTP/" is
enough for the response to be served nicely (and cached). So in
their case, "HTTP/0.9", "HTTP/2.0" and "HTTP/01.0" can all be
used successfully. In DeleGate's case, it's even possible to use
"HTTP/ 1.0" (the Sun proxy server will not cache it - it probably
needs an alphanumeric character after the forward slash).

Thus, Sun Java System Web Proxy Server 4.0 can be poisoned
without using CRLF (i.e. using LF only) and without using the
string "HTTP/1." (and instead, using "HTTP/2.1") and "<html" and
"<meta". This was indeed demonstrated. In fact, it's even better
- Sun Java System Web Proxy Server 4.0 will convert the response
into a valid HTTP/1.1 response (i.e. convert the first line into
"HTTP/1.1 ...").

Even if a proxy server won't cache the response if it is doesn't
begin with HTTP/1.0 or HTTP/1.1, it may still treat the response
as an HTTP/0.9 response [10] and send it back to the client (e.g.
Apache 2.0.55, Sun Java System Web Proxy Server 4.0). Of course,
it would have to wait until the connection is closed though, as
there's no other way for the web server to inform the client of
the end of the response message. In such case, the content is
unlikely to be cached, but still, other tricks from [1] (such as
cross site scripting, sending malicious data to an arbitrary
client or receiving pages destined for another client) may be

As for the major browsers - IE 6.0 SP2 will parse and cache
responses starting with "HTTP/2.0", "HTTP/0.9" and "HTTP/01.1",
and Mozilla Firefox 1.5 will parse and cache these, as well as
"HTTP/foobar" and "HTTP /1.0".

Native double injection

On some application servers (e.g. IIS ASP), a redirection results
in a 3xx response with a Location header containing the URL to
redirect to, and an HTML body containing a reference to this same
URL. In this case, a double injection is trivial. Yet one should
keep in mind that the data injected is identical. An HTTP/0.9
response may be the only way to get around this, i.e. the
injection may be:

Content-Length: N
Foo: <script>...</script>

Where N should be calculated so that it terminates the first
response's body just after the "Foo:" injected in the body (the
second injection).

Therefore, on such servers, double injection (needed for
technique #2) is natively available.

Alternately, it is possible to inject data (without double CRLF,
in order not to interfere with technique #2) such that at the
second embedding point, it will be parsed as a partial HTTP
response (incomplete header section). In such case, a double CRLF
at the body of the web-server's first response, or the double
CRLF in the web-server's second response (which terminates the
second response's header section) would terminate this HTTP
response. Consider the following injection text:

Content-Length: N
Foo: HTTP/1.1 200 OK
Content-Length: 0
Content-Type: text/html
Last-Modified: ...
Refresh: 0; URL=http://www.evil.site/

At the first injection point, this will be interpreted (by a
proxy server that uses the first Content-Length header) as an
HTTP response whose size is N. N should be calculated such that
it will position the proxy server right the string "Foo: " of the
second injection. The proxy will therefore read the second
injection and wait for the terminating double CRLF, which will
complete the response and make it cacheable.

Of course, this method may fail if the HTML page contains a CRLF
followed by data which the proxy cannot accept as an HTTP
response header.

If the 2 latter alternatives (i.e. using HTTP/1.x response) are
used, a problem arises: the injection string cannot directly use
the two headers (Termination-Token and Termination-Header
response) because it will flag the first response as invalid, and
thus may alter the processing of the rest of the data. But if the
double CRLF is provided someplace in the body of the first server
response, then evading [6] can still succeed. It is assumed that
a response from a non-compliant web server (i.e. a response that
does not contain any one of the Termination-Token and
Termination-Header response headers) should be accepted as valid
by [6]. Otherwise, [6] would be impractical due to the majority
of web-servers being non-compliant. Therefore, simply not
providing those headers in the second (spoofed) response header
section should evade [6].

On the other hand, if the terminating double CRLF is the one
provided by the server as the termination sequence of the second
response's header section, evading [6] will fail. After all, we
assume that the web server complies with [6], and therefore each
response header section would contain the two server generated
headers (Termination-Token and Termination-Header). Hence, if the
second injection relies on the second response header section to
provide the double CRLF, it will fail to evade [6] due to the
existence of those 2 server generated headers later in the second
response headers. This method (of relying on the termination of
the second server response header section), while not being a
workaround for [6], may serve as a good counter-example to some
other anti- HTTP Response Splitting ideas.

Detecting HTTP Response Splitting/Smuggling using the browser

As suggested in [12], a browser can be used in many cases to
easily determine if a specific parameter in a specific script is
vulnerable to HTTP Response Splitting. With the introduction of
the above "anti HTTP Response Splitting" methods, the method
presented in [12] may not work. However, as will be shown below,
the same techniques employed above can also be used to verify the
existence of HTTP Response Splitting/Smuggling.

Header injection at large (needed for technique #2) can still be
verified the way [12] subscribes, i.e. injecting


Injecting LF only headers (technique #1) is a simple matter of
trying the following string, and would succeed in both Microsoft
IE 6.0 SP2 and Mozilla Firefox 1.5:


Injecting CR only headers (technique #3) is slightly more
problematic. Both IE and Firefox in general do not parse CR as an
end of header line. However, in some headers (probably those less
critical for understanding the HTTP stream), IE is willing to
accept CR as an end of headers. Fortunately, Location and Set-
Cookie are such headers. Therefore, using IE 6.0 SP2 (but not
Firefox, unfortunately), we can use the following string to



Providers of anti- HTTP response splitting solutions
Do not rely on double CRLF patterns, on the existence of even a
single CRLF, or on existence of the "HTTP/1." string or of HTML
tags. Do not assume that HTTP Response Splitting requires
"breaking out" of the header section (consult the above counter-
examples to make sure that the solution indeed covers at least
those scenarios). Instead, focus on what is by definition invalid
in HTTP responses, such as CRs and LFs in header names and values
(as recommended in [1]). When doing so, keep in mind that the
logical form of the data (the characters CR and LF) may be
represented in various ways as physical characters (e.g. raw
characters, URL-encoded, the IIS-specific %uHHHH encoding, UTF-8
overlong/invalid encoding, etc.), and may be delivered not only
in the URL, but also in the body parameters and possibly in other
locations as well (e.g. headers and path). Simply blocking
dangerous characters in the URL, in their raw form and their URL-
encoded form (as hinted in [3]) is insufficient.

HTTP client vendors (including browsers and proxy servers)
Disallow invalid or ambiguous responses such as discussed above
(but do not limit this treatment to those examples, it's very
likely that there are more such problems). Ideally, convert such
response to an error response (perhaps with 5xx status code) and
terminate the TCP connection.
Also, consider the detection method described in [13].

Security testers
Pay heed to the extension of the detection method in [12] to HTTP
Response Smuggling, and learn the patterns suggested above.


HTTP Response Smuggling is possible because some of the
protection mechanisms suggested address symptoms, not root cause,
of the injection into the HTTP response headers problem. It also
exploits the liberal and tolerant parsing exhibited by several
proxy servers and. The net result is that the classic HTTP
Response Splitting is still at large possible, requiring minimal
modifications to overcome the current protection mechanism.

Additional research directions

While HTTP Response Smuggling was developed to bypass several
anti- HTTP Response Splitting mechanisms, no doubt it has many
other applications. One such direction is bypassing content
filtering. A malicious web server can send HTTP responses that
may be interpreted in one manner (as innocent responses) by a
content filtering gateway, and in another manner completely (as
malicious pages) by the end client (browser). Another direction
is spoofed indexing, wherein a search engine parses the data
stream one way, while the actual clients may parse it
differently. Likewise, it may be possible to generalize and serve
different content to different clients, based on difference in
the way clients parse the HTTP response stream. Finally, it
should be noted that there are many more response smuggling
techniques, of which only 3 were discussed in the paper due to
brevity and clarity considerations. Such techniques can be
explored to exploit other scenarios (combinations of servers,
proxy servers, browsers and protection techniques).

A note about terminology

This paper concludes the HTTP {Request,Response} x
{Splitting,Smuggling} quartet. Since the first work was
introduced ([1], almost 2 years ago), there was some
misunderstanding of the terms and concepts in the various works
(e.g. the difference between [1] and [2]). In order to clarify
the terminology, here are two definitions:

Splitting - the act of forcing a sender of (HTTP) messages to
emit data stream consisting of more messages than the sender's
intension. The messages sent are 100% valid and RFC compliant.

Smuggling - the act of forcing a sender of (HTTP) messages to
emit data stream which may be parsed as a different set of
messages (i.e. dislocated message boundaries) than the sender's
intention. This is done by virtue of forcing the sender to emit
non-standard messages which can be interpreted in more than one

Both terms (when applied to HTTP requests/responses) belong to
the peripheral web security world, as described in [14].


[1] "Divide and Conquer - HTTP Response Splitting, Web Cache
Poisoning Attacks, and Other Topics", Amit Klein, March 2004

[2] "HTTP Request Smuggling", Chaim Linhart, Amit Klein, Ronen
Heled, Steve Orrin, June 2005

[3] "Blocking HTTP Attacks Using CPL", BlueCoat Technical Brief

[4] "Learn How To Configure Your ISA 2004 Server To Block HTTP
Response Splitting Attacks", Microsoft document

[5] "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June

[6] "Effective Countermeasure to HTTP Response Splitting", Aaron
Emigh, anti-fraud@lists.cacert.org mailing list submission,
September 11th, 2005.

[7] "PHP 5.1.2. Release Announcement", PHP website

[8] "PHP 4.4.2. Release Announcement", PHP website

[9] "Goodbye HTTP Response Splitting, and thanks for all the
fish", Stefan Esser, "PHP Security Blog" blog post, January 12th,

[10] "The Original HTTP As Defined in 1991", Tim Berners-Lee,

[11] "Bypassing content filtering whitepaper", 3APA3A

[12] "Detecting and Testing HTTP Response Splitting Using a
Browser", Amit Klein, WebAppSem mailing list submission, October
14th, 2004

[13] "Detecting and Preventing HTTP Response Splitting and HTTP
Request Smuggling Attacks at the TCP Level", Amit Klein, BugTraq
mailing list submission, August 15th, 2005

[14] "Meanwhile, on the other side of the web server", Amit
Klein, June 10th, 2005


Web Cache Poisoning with Sun Java System Web Proxy Server 4.0

Here are some practical considerations to be taken into account
when poisoning the cache of Sun Java System Web Proxy Server 4.0
(B05/10/2005) via HTTP Response Splitting (or Smuggling).

1. The Sun proxy server has some kind of buffering or packet-
boundary parsing of the HTTP response stream. Therefore,
padding of few thousand bytes is required between the end
of the first response and the beginning of the second
response. In the author's experience, 3000-6000 bytes
usually suffice.

2. The Sun proxy server has a unique parsing mechanism wherein
it scans for the first response line ("HTTP/..."), so the
exact position of the response is less critical (compared
to the precision required to poison other cache servers).

3. Due to some timing issues, it's much easier to poison Sun
proxy server's cache with a (second) HTTP message whose
Content-Length is 0. This is still interesting because a
redirection can be forced (e.g. via a Refresh header). That
is, it's possible to poison the cache with a fake 0 length
homepage of the target website, refreshing itself
immediately to the attacker's website (classic defacement).
Poisoning the cache with 0-length header has high rate of
success (>50%), while for non-empty response, it's lower
(though was demonstrated several times). It seems that the
reason is that Sun proxy server terminates a 0-length
response right after the headers, regardless of what's
following. When facing a non-empty response, it will not
cache the response if superfluous data exists. This means
that in order to successfully poison the cache with non-
empty response, the real second response from the web
server should be taken into account, and even then, there
are some timing issues.

4. It seems that Sun proxy server will cache all URLs except
root resources (e.g. http://www.some.site/).

5. Forcing a cache revalidation is done using the "Pragma: no-
cache" HTTP request header. This header should therefore be
included with the second request (the one made for the
poisoned resource).

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