The pam_console PAM module in Linux systems performs a chown on various devices upon a user login, but an open file descriptor for those devices can be maintained after the user logs out, which allows that user to sniff activity on these devices when subsequent users log in.
The web GUI for the Linux Virtual Server (LVS) software in the Red Hat Linux Piranha package has a backdoor password that allows remote attackers to execute arbitrary commands.
IP masquerading in Linux 2.2.x allows remote attackers to route UDP packets through the internal interface by modifying the external source IP address and port number to match those of an established connection.