Aptdaemon 0.43 in Ubuntu 11.10 and 12.04 LTS uses short IDs when importing PPA GPG keys from a keyserver, which allows remote attackers to install arbitrary package repository GPG keys via a man-in-the-middle (MITM) attack.
https://bugs.launchpad.net/software-center-agent/%2Bbug/1052789
http://www.ubuntu.com/usn/USN-1666-1
http://www.securitytracker.com/id?1027891