APT before 1.0.4 does not properly validate source packages, which allows man-in-the-middle attackers to download and install Trojan horse packages by removing the Release signature.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2014-0513
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=749795
http://www.ubuntu.com/usn/USN-2246-1
http://www.debian.org/security/2014/dsa-2958