Chris Evans discovered several buffer overflows in the libungif library. By tricking an user (or automated system) into processing a specially crafted GIF image, this could be exploited to execute arbitrary code with the privileges of the application using libungif.
Solution: The problem can be corrected by upgrading the affected package to version 4.1.0b1-6ubuntu0.1 (for Ubuntu 4.10), 4.1.3-1ubuntu0.1 (for Ubuntu 5.04), or 4.1.3-2ubuntu0.1 (for Ubuntu 5.10). In general, a standard system upgrade is sufficient to effect the necessary changes.