Security researchers have discovered a critical flaw in the OpenBSD kernel.
The rare vulnerability - only the second severe kernel bug in the history of the development of OpenBSD - involves a flaw in OpenBSD's IPv6 stack that potentially lends itself to remote exploitation by hackers. Fortunately, source code patches are available for OpenBSD 3.9 and 4.0.
Applying the patches involves recompiling the kernel and rebooting affected machines. If applying these updates isn't immediately convenient, workarounds involve disabling IPv6 traffic (as explained here).
Credit for discovering the flaw goes to security researchers at Core Security, whose well-documented advisory, which includes proof of concept code, can be found here). ®