APT before 1.0.9 does not "invalidate repository data" when moving from an unauthenticated to authenticated state, which allows remote attackers to have unspecified impact via crafted repository data.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2014-0519
http://www.debian.org/security/2014/dsa-3025
http://ubuntu.com/usn/usn-2348-1