The Linux kernel 2.6.0 through 220.127.116.11, and 2.4.4 through 18.104.22.168, does not initialize all function pointers for socket operations in proto_ops structures, which allows local users to trigger a NULL pointer dereference and gain privileges by using mmap to map page zero, placing arbitrary code on this page, and then invoking an unavailable operation, as demonstrated by the sendpage operation (sock_sendpage function) on a PF_PPPOX socket.
What this amounts to is that someone with physical access to an affected computer could gain root and wreak havoc. Debian and Ubuntu have already released patches and users are urged to update immediately through their distro's update utility.
Proof of concept code for an exploit is already available.
Edited by Amazing Andrew, 21 August 2009 - 12:35 AM.