When starting a new X session, one or more sockets are created in /tmp/.ICE-unix. One of these sockets is named after the PID of the process. This means that it is trivial for a local user of the system to prevent all other users from logging in, causing a local DoS. This can be done with the following command: seq 1 $(cat /proc/sys/kernel/pid_max) | (cd /tmp/.ICE-unix && xargs touch) I've verified it on a variety of Linux distributions, including Slackware 12.0, SLED10 SP2 and openSUSE 11.1. Evil intentions left apart, as sockets created in /tmp/.ICE-unix are not deleted on logout, on systems with many users, logins will start failing eventually after some use time. I did experience this on a system with 3 users after just a few months (this is how I noticed the problem.) I think that at least the sockets should be deleted when logging out. Then the socket names should be made non-predictable to prevent any local DoS attack.
Admins should set perms on /tmp/.ICE-unix and friends such that only authorized local users can create those sockets ... I realize tthat doesn't really help the situation, but it makes me think this isn't that big of an issue.
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