Konstantin V. Gavrilenko has reported a vulnerability in Cisco PIX, which can be exploited by malicious people to cause a DoS (Denial of Service).
The vulnerability is caused due to the firewall failing to verify the checksum of a TCP SYN packet before it is allowed through the firewall and a connection state is setup to track the half-open connection. Packets with incorrect checksum values will be silently discarded by the destination host without a RST reply. This causes the connection state to be held up to two minutes before it is cleared. In the meantime, legitimate SYN packets with the same protocol, IP addresses, and ports are discarded by the firewall.
Successful exploitation allows an attacker to prevent a host from establishing connections to another host through the firewall.
The vulnerability has been reported in PIX 6.3 and PIX/ASA 7.0.
Note: The vulnerability can also be exploited in the following ways:
1) By sending packets with specially-crafted TTL values that cause the router on the trusted side of the network, between the firewall and the destination host, to time-out the packet.
2) By sending certain packets that contains data. E.g. A SYN packet containing one byte of data.
Solution: The vendor recommends the following workaround.
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Subject: Cisco PIX Spoofed TCP SYN Packets Denial of Service
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