FireWall-1 FTP Server Vulnerability Background Paper #1 - The basic idea of the described attack is to subvert the security policy implemented by a stateful firewall. This is done by triggering the generation of a TCP packet that, when inspected by the firewall, will change the firewall's internal state such that an attacker is able to establish a TCP connection to a filtered port through the firewall. This packet is the server response to a PASV user request during a FTP session.
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