APT before 0.8.15.2 does not properly validate inline GPG signatures, which allows man-in-the-middle attackers to install modified packages via vectors involving lack of an initial clearsigned message.
https://launchpad.net/ubuntu/+archive/primary/+sourcepub/1817196/+listing-archive-extra
https://launchpad.net/bugs/784473
https://exchange.xforce.ibmcloud.com/vulnerabilities/68560
http://www.ubuntu.com/usn/USN-1169-1
http://www.securityfocus.com/bid/48671
http://packages.debian.org/changelogs/pool/main/a/apt/current/changelog
http://launchpadlibrarian.net/75126628/apt_0.8.13.2ubuntu2_0.8.13.2ubuntu4.1.diff.gz