SearchEnterpriseLinux.com has an article describing a serious issue with the 2.4 Linux kernel, whereby a naughty user could escalate his/her privileges so that they could run naughty code.

Naughty users.

Anyways, if you're too busy/lazy to read the article above, you can download the patch here

The bad news is that this may also be an issue with the 2.6 Linux kernel. An indeterminate number of penguins are probably looking at that right now.