Paul A. Watson has published research about a somewhat known vulnerability in the TCP specification (RFC793), which can be exploited by malicious people to cause a DoS (Denial of Service). Cisco has acknowledged that Cisco IOS is affected.
According to the TCP specification, an established TCP connection can be reset by sending a suitable TCP packet with the RST or SYN flag set. Since a source IP address and port can be forged, this may potentially be exploited by a malicious person to reset a connection between other systems.
However, in the past, this has only been thought of as a theoretical security issue, as a valid 32-bit sequence number is required for the so-called "spoofing attack" to be successful, and the probability of guessing a correct sequence number is extremely low.
It has now been proven that exploitation is possible. The problem is that the probability of guessing an acceptable sequence number is much higher than expected, since the receiving TCP implementation accepts any sequence number in a certain range ("window") of the correct sequence number.
This can be exploited to reset an established TCP connection on a vulnerable device by sending a specially crafted TCP packet with an acceptable sequence number and a forged source IP address and port.
Especially long-lived TCP connections with a guessable source port and protocols like BGP and DNS (for zone transfers) are affected.
NOTE: See the original advisory for a list of affected versions.
Solution: See patch matrix in original advisory.
Provided and/or discovered by: Paul A. Watson
Original Advisory: http://www.cisco.com/warp/public/707/cisco-sa-20040420-tcp-ios.shtml
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Subject: Cisco IOS TCP Connection Reset Denial of Service Vulnerability
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