apt-get in apt before 0.7.21 does not check for the correct error code from gpgv, which causes apt to treat a repository as valid even when it has been signed with a key that has been revoked or expired, which might allow remote attackers to trick apt into installing malicious repositories.
https://exchange.xforce.ibmcloud.com/vulnerabilities/50086
https://bugs.launchpad.net/ubuntu/+source/apt/+bug/356012
http://www.securityfocus.com/bid/34630
http://www.debian.org/security/2009/dsa-1779
http://secunia.com/advisories/34874
http://secunia.com/advisories/34832