From an Ubuntu security advisory:
After a standard system upgrade you need to restart emacs to effect the necessary changes. Details follow: Hendrik Tews discovered that emacs21 did not correctly handle certain GIF images. By tricking a user into opening a specially crafted GIF, a remote attacker could cause emacs21 to crash, resulting in a denial of service.
Gosh, they make it sound as if Emacs is a daemon, run from an init file, running all the time and… oh, wait. Right.