The experimental IP packet queuing feature in Netfilter / IPTables in Linux kernel 2.4 up to 2.4.19 and 2.5 up to 2.5.31, when a privileged process exits and network traffic is not being queued, may allow a later process with the same Process ID (PID) to access certain network traffic that would otherwise be restricted.

ip_conntrack_ftp in the IPTables firewall for Linux 2.4 allows remote attackers to bypass access restrictions for an FTP server via a PORT command that lists an arbitrary IP address and port number, which is added to the RELATED table and allowed by the firewall.