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Re: Ambiguities in TCP/IP - firewall bypassing
From: Florian Weimer <Weimer () CERT Uni-Stuttgart DE>
Date: Sat, 19 Oct 2002 01:03:47 +0200

Paul Starzetz <paul () starzetz de> writes:

* 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)

As a result of this bug, it's quite complicated (if not impossible in
some configurations) to properly filter connection attempts to Linux
hosts on Cisco IOS routers.

If your access list is a whitelist with a "permit tcp any any
established" statement somewhere, it's very likely that you can bypass
the filter just by setting the RST in the initial SYN packet, as
described above.  The router will forward the packet, and the Linux
host will happily initiate the three-way handshake.

"established" in Cisco parlance does not mean "SYN unset", but "ACK or
RST set".  This means that the impact for non-Linux hosts (which do
not react to SYN-RST packets according to Paul's survey) is less
severe if your filters run IOS.

-- 
Florian Weimer                    Weimer () CERT Uni-Stuttgart DE
University of Stuttgart           http://CERT.Uni-Stuttgart.DE/people/fw/
RUS-CERT                          fax +49-711-685-5898


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