Technical Note by Amit Klein: Detecting and Preventing HTTP Response Splitting and HTTP Request Smuggling Attacks at the TCP Le
From: Amit Klein (AKsecurity) (aksecurity_at_hotpop.com)
Date: Mon, 15 Aug 2005 20:31:00 +0200 To: bugTraq <email@example.com>
Technical Note: Detecting and Preventing HTTP Response Splitting
and HTTP Request Smuggling Attacks at the TCP Level
Amit Klein, August 2005
This technical note describes a detection/prevention technique that
works in many cases both with HTTP Response Splitting and with HTTP
Request Smuggling. This technique makes use of implicit information
found in the TCP stream, namely the segmentation into packets and
the TCP PSH bit. In HTTP Response Splitting, this technique needs
to be applied at the proxy server, the one closest to the web
server, and to the response stream. In HTTP Request Smuggling, this
technique needs to be applied at the entity closest to the attacked
proxy server/device (i.e. implemented in another proxy server, or
the web server itself), and to the request stream (note, however,
that this second server may be off the premises of the organization
wherein the web server is, see also "Can HTTP Request Smuggling be
blocked by Web Application Firewalls?",
TCP PSH bit and PUSH flag
Before describing the technique, the reader is reminded what are
the TCP PSH bit and PUSH flag. RFC 793 (Transmission Control
Protocol, http://www.ietf.org/rfc/rfc793.txt) defines the push
functionality as following (from section 1.5):
Sometimes users need to be sure that all the data they have
submitted to the TCP has been transmitted. For this purpose
a push function is defined. To assure that data submitted to
a TCP is actually transmitted the sending user indicates that
it should be pushed through to the receiving user. A push
causes the TCPs to promptly forward and deliver data up to
that point to the receiver.
And in section 2.8:
The sending user indicates in each SEND call whether the data
in that call (and any preceeding [sic] calls) should be
immediately pushed through to the receiving user by the
setting of the PUSH flag.
This is realized by a PSH bit in the TCP header.
RFC 793 also mandates that the socket API provides means for the
caller to set this bit via a PUSH flag in the SEND function, and to
likewise receive it in the RECV function.
However, this requirement was later waived in RFC 1122
(Requirements for Internet Hosts - Communication Layers,
http://www.ietf.org/rfc/rfc1122.txt), section 22.214.171.124:
A TCP MAY implement PUSH flags on SEND calls. If PUSH flags
are not implemented, then the sending TCP: (1) must not
buffer data indefinitely, and (2) MUST set the PSH bit in the
last buffered segment (i.e., when there is no more queued
data to be sent).
The discussion in RFC-793 on pages 48, 50, and 74 erroneously
implies that a received PSH flag must be passed to the
application layer. Passing a received PSH flag to the
application layer is now OPTIONAL.
And indeed, the two implementations of sockets, the UNIX BSD
sockets and WinSock, do not implement the PUSH flag neither in SEND
nor in RECEIVE:
*) FreeBSD (example of standard BSD sockets)
send(2) man page:
recv(2) man page:
*) Sun Solaris 8 (another example of standard BSD sockets)
send(3SOCKET) man page: http://docs.sun.com/app/docs/doc/806-0628/6j9vie803?a=view
recv(3SOCKET) man page: http://docs.sun.com/app/docs/doc/806-0628/6j9vie7u0?a=view
*) Microsoft Windows WinSock (2.0) sockets:
This means that according to RFC 1122, these implementations should
simply set the TCP PSH flag in the last TCP segment of the caller's
data buffer. Unfortunately, this also means that socket
applications do not have direct access to the PSH bit (in other
words, it would take some hacking around the standard sockets API
to get this information - thus making the whole idea presented
below much harder to implement in pure sockets applications).
To illustrate: a sender uses the sockets send() function to send a
payload of 2000 bytes over a TCP circuit on a LAN. The call is
(assuming C API):
send(socket, buffer, 2000, ...);
This results in 2 TCP segments (IP packets), the first carrying a
payload of 1448 bytes (maximal LAN packet, minus level 2, IP and
TCP headers), the second carrying a payload of the remaining 552
bytes, with a PSH bit set.
In contrast, having the caller invoke send twice to send the same
send(socket, buffer, 1000, ...);
send(socket,buffer+1000, 1000, ...);
Results in 2 TCP segments, both having 1000 bytes of payload, and
both having the PSH bit set.
Therefore, by inspecting the TCP header, one can figure out (in
this rather simplistic example) whether the data was sent via a
single call to send(), or via two calls.
This observation is the basis for the following technique described
Detecting HTTP Response Splitting and HTTP Request Smuggling
Detection of HTTP Response Splitting attack can be realized by
fulfilling the following requirements (the technique can be applied
to HTTP Request Smuggling with some obvious modifications):
Requirement #1: Deny "pipelined" traffic, that is, do not accept a
second response before a first response was fully served, and a
second request was fully received. Particularly, a packet
containing data for the first response must not contain superfluous
data beyond the end of the first response (i.e. a second response).
Requirement #2: Since from #1 it follows that the end of the first
response coincides with an end of a packet, and end of
transmission, such packet should contain a PSH bit set (see above).
Requirement #1 is obvious, yet insufficient, as shown in the
successful HTTP Response Splitting attacks against Squid and
NetCache (see "Divide and Conquer - HTTP Response Splitting, Web
Cache Poisoning Attacks, and Related Topics",
pp. 15-19), although both products impose packet
boundary requirements (making sure that the first response
terminates at a packet boundary isn't so hard if the packet length
is known to the attacker). But if we are to add requirement #2, the
attack can be thwarted in many cases, because the attacker has no
way of forcing the web server to send the PSH flag (remember that
the from the web server's perspective, it is in the middle of the
first response, and will send the PSH bit only at the end of the
first response). Of course, it may be possible that the web server
always sends the PSH bit (for example, the TCP stack of IBM TPF 4.1
apparently does so by default - see the description of TCP_PSH_LAST
ioctl option in http://www-306.ibm.com/software/htp/tpf/serv/GTPMLB19.pdf),
or sends it after each HTTP response header, or in the middle of
the response, but that is not the case with many servers (although
I have seen servers that on occasion will send the PSH flag in the
middle of HTTP responses).
So, while it may not be perfect, it seems that this technique has a
high detection probability (much higher than enforcing requirement
#1 alone), yet very low likelihood for false positives.
1. The technique fails if there are TCP/HTTP aware devices between
the web server and the proxy server, as these may alter the TCP
2. The technique can be applied to HTTP Request Smuggling as
following: the proxy server forwards the requests to the web
server. The web server (or the receiving entity) needs to verify
that the request ends on a packet boundary, and that the PSH flag
3. While the technique covers detection of attacks, since such
detection is carried out in real-time, it is possible to terminate
the TCP connection (or perform other actions) and thereby to block
4. This technique lends itself nicely to detection/prevention by
network IDS/IPS, as it only requires sniffing the TCP/IP traffic
and flagging HTTP requests/responses that do not terminate on a
packet boundary, with PSH bit set.
Other, complementary methods (for prevention) that are known to
1. The web site can use SSL connections (HTTPS) only. This will
only eliminate 3rd party proxy servers. It does not eliminate the
browser cache issue, and it may not handle the site's own cache
server (or any other on-site HTTP aware devices). Of course,
migrating to SSL only has many real-life drawbacks.
2. A proxy cache server can be configured not to use persistent
HTTP connections with the *server*, in which case, its cache will
not be poisoned. This prevention technique has significant
Observing that an end of HTTP request/response message is aligned
with a TCP segment (packet) boundary, *and* that that segment has
the PSH bit set, can be an effective way to detect and prevent HTTP
Response Splitting and HTTP Request Smuggling. This technique can
be easily employed when the TCP layer is directly accessible (as
opposed to the sockets model).
Off topic personal notice/clarification
I am often asked whether I work for Watchfire, or did so in the
past. So let me state the following:
1. I am not employed by Watchfire, and I am not part of any
2. In fact, I have never been employed by watchfire (I was employed
by Sanctum for many years, but I quit Sanctum slightly before it
was acquired by Watchfire).
Having said that, I have nothing against Watchfire, and I wish them
best of luck.