A hole in the IPComp protocol implementation of certain operating systems can be exploited to compromise a server. IPComp is used for compressing individual IP datagrams mainly in conjunction with IPSec and other VPN technologies. According to Tavis Ormandy, certain embedded datagrams can cause a recursion after they have been unpacked, which results in a kernel stack overflow.
This reportedly allows attackers to inject arbitrary code into a system and, in all probability, execute it there. An attack could trigger a system crash even in the simplest of scenarios. Ormandy says that no previous authentication is required, and that attacks can also be launched using a forged sender address.

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