[NEWS] Ambiguities in TCP/IP May Allow Firewall Bypassing
From: support@securiteam.comDate: 10/21/02
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From: support@securiteam.com To: list@securiteam.com Date: 21 Oct 2002 20:46:31 +0200
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Ambiguities in TCP/IP May Allow Firewall Bypassing
------------------------------------------------------------------------
SUMMARY
There are ambiguities in implementations of the TCP/IP suite for various
operating systems. Even if this fact has been used since a long time in
different software for OS fingerprinting, no real attempt has been made to
identify the security impact of the differences in the TCP/IP semantics.
Paul Starzetz has done some research on the TCP/IP connection open
semantics which is of course very important for security of networked
systems. Paul Starzetz believes that the flaws that he detected have a big
impact on design of firewalls and packet filters since an improper
implementation can easily lead to serious security problems.
DETAILS
The TCP/IP protocol stack offers a three way handshake for connection
oriented communication using the TCP protocol. Basically, a connection can
be opened by sending a synchronization packet to a listening service on a
particular host. The host will respond with a synchronization
acknowledgment packet which in turn must be acknowledged by the requesting
host. Then, the connection is considered to be open (at least at the
transport layer) and the two hosts may exchange some data.
The three way handshake is an essential part of the communication using
the TCP protocol. Therefore many packet filter firewalls try to prevent
the three way handshake from completion in order to protect an
internal/corporate network from being accessed from the outside. Of
course, statefull firewalls may have some more sophisticated mechanism.
Paul Starzetz have found a very ambiguous behavior of TCP/IP
implementations while doing some research on the connection request phase.
Below you will find the findings about various OSes, however the list
should not be considered complete. We have used the NemesisTCP tool [1] to
generate the traffic and tcpdump to capture the responses.
* The normal behavior (of all OSes) is like this:
14:18:00.595517 192.168.1.184.12345 > 192.168.1.111.9999: S 420:420(0) win
512 (DF) [tos 0x18]
14:18:00.595731 192.168.1.111.9999 > 192.168.1.184.12345: S
1679763291:1679763291(0) ack 421 win 5840 <mss 1460> (DF)
The first host sends a SYN packet from port 12345 to a service on port
9999 and receives a SYN, ACK
* Linux 2.4.19
The examination of the source code of the TCP engine reveals that a TCP
connection can be opened by any combination of the TCP flags having the
SYN bit set and the ACK bit reset. For example we can open a TCP
connection by sending an obviously bogus SYN, RST packet:
14:25:43.888897 192.168.1.184.12345 > 192.168.1.111.9999: SR 420:420(0)
win 512 (DF) [tos 0x18]
14:25:43.889143 192.168.1.111.9999 > 192.168.1.184.12345: S
2168208394:2168208394(0) ack 421 win 5840 <mss 1460> (DF)
Or something called 'Christmas packet' having mostly every TCP flag set
(except the ACK flag of course):
14:30:46.341732 192.168.1.184.12345 > 192.168.1.111.9999: SFRP 420:420(0)
win 512 urg 8 (DF) [tos 0x18]
14:30:46.342444 192.168.1.111.9999 > 192.168.1.184.12345: S
2492223280:2492223280(0) ack 421 win 5840 <mss 1460> (DF)
Also SYN, FIN packets works well...
* Solaris 5.8
Here Paul Starzetz has success by sending SYN, FIN packets:
14:33:24.549246 192.168.1.184.12345 > 192.168.1.84.9999: SF 420:420(0) win
512 (DF) [tos 0x18]
14:33:24.549757 192.168.1.84.9999 > 192.168.1.184.12345: S
913533039:913533039(0) ack 421 win 24656 <mss 1460> (DF)
Or SYN, FIN, PSH packets with no payload
14:35:14.398346 192.168.1.184.12345 > 192.168.1.84.9999: SFP 420:420(0)
win 512 (DF) [tos 0x18]
14:35:14.398801 192.168.1.84.9999 > 192.168.1.184.12345: S
940377913:940377913(0) ack 421 win 24656 <mss 1460> (DF)
Other combinations don't seem to induce the SynSent state in the TCP/IP
stack
* FreeBSD 4.5
Here Paul Starzetz also have luck with SYN, FIN packets:
14:47:21.558541 192.168.1.184.12345 > 192.168.1.104.9999: SF 420:420(0)
win 512 (DF) [tos 0x18]
14:47:21.558719 192.168.1.104.9999 > 192.168.1.184.12345: S
1333327436:1333327436(0) ack 421 win 65535 <mss 1460>
As well as with other combinations which don't combine the RST and/or ACK
flag with SYN:
14:48:11.678246 192.168.1.184.12345 > 192.168.1.104.9999: SP 420:420(0)
win 512 (DF) [tos 0x18]
14:48:11.678366 192.168.1.104.9999 > 192.168.1.184.12345: S
1714046856:1714046856(0) ack 421 win 65535 <mss 1460>
* Windows NT 4.0
As in the case of BSD we can open connections using any combination of TCP
flags as long as we do not set the RST and/or ACK flag:
14:59:46.315126 192.168.1.184.12345 > 192.168.1.17.9999: SF 420:420(0) win
512 (DF) [tos 0x18]
14:59:46.315566 192.168.1.17.9999 > 192.168.1.184.12345: S
15062452:15062452(0) ack 421 win 8576 <mss 1460> (DF)
Other OSes than those tested above are expected to behave in a similar
manner after obtaining such a discouraging result...
Impact:
The ambiguities can be used to bypass/tunnel firewalls filtering TCP
packets according to the TCP flags set. Especially stateless firewalls
simply comparing the flags field with some expected value(s) to
distinguish between synchronization packets and packet from an already
open connection can be easily bypassed just by sending a bogus
synchronization packet to a listening port. The currently deployed TCP
stacks seem to be highly bogus and lazy implemented.
Administrators of firewall devices should set up some filtering rules to
drop bogus TCP packets as mentioned above. For example on systems using
iptables to filter packets bogus packets can be easily distinguished by
following rules:
iptables -A INPUT -p tcp -d HOST/MASK --tcp-flags SYN,FIN SYN,FIN -j LOG
-m limit --limit 10/m --log-level "LOGLEVEL" --log-prefix="bogus packet"
$IP -A INPUT -p tcp -d HOST/MASK --tcp-flags SYN,FIN SYN,FIN -j DROP
And so on for other flag combinations.
Under Cisco:
ip access-list extended filterout permit tcp 219.80.71.0 0.0.0.255 any
reflect tcp-state
ip access-list extended filterin evaluate tcp-state
ADDITIONAL INFORMATION
References:
[1] NemesisTCP: <http://online.securityfocus.com/tools/1278>
http://online.securityfocus.com/tools/1278
[2] Packet normalization:
<http://www.icir.org/vern/papers/norm-usenix-sec-01-html>
http://www.icir.org/vern/papers/norm-usenix-sec-01-html
The information has been provided by <mailto:paul@starzetz.de> Paul
Starzetz and <mailto:cbrenton@slartibartfast.pa.net> cbrenton.
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