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error code from gpgv, which causes apt to treat a repository as valid even when it has been signed with a key that has been revoked or expired, which might allow remote attackers to trick apt into installing malicious repositories.
bugs.debian.org/cgi-bin/bugreport.cgi?bug=433091
secunia.com/advisories/34829
secunia.com/advisories/34832
secunia.com/advisories/34874
www.debian.org/security/2009/dsa-1779
www.securityfocus.com/bid/34630
bugs.launchpad.net/ubuntu/+source/apt/+bug/356012
exchange.xforce.ibmcloud.com/vulnerabilities/50086
usn.ubuntu.com/762-1/