On May 14th an attack in the wild began circling which enables non-root users to become root for kernels 2.6.37–3.8.8 (inclusive) compiled with PERF_EVENTS, in addition to cirtain earlier kernels containing the bug as a backport. This only affects 64-bit operating systems. This is the best technical writeup I have seen on the subject: CVE-2013-2094 Perf Events Exploit Explained
Ubuntu 10.04 is not affected.
RHEL 5 are not affected.
Debian Squeeze is not affected.
Known Vulnerable Distributions and Kernel Versions
NOTE: You are extra-vulnerable if you have untrusted non-root users on your server!
CentOS/RHEL kernels earlier than 2.6.32-358.6.2
If you can’t reboot, try this fix: https://access.redhat.com/site/solutions/373743
Ubuntu 12.04 3.2 kernels earlier than 3.2.0-43.68
Ubuntu 12.04 3.5 kernels earlier than 3.5.0-30.51~precise1
Ubuntu 12.10 3.5 kernels earlier than 3.5.0-30.51
Ubuntu 13.04 3.8 kernels earlier than 3.8.0-21.32
Debian Wheezy 3.2 kernels earlier than 3.2.41-2+deb7u2
Debian Jessie 3.2 kernels earlier than 3.2.41-2+deb7u2
Debian unstable 3.8 kernels earlier than 3.8.11-1