The apt package in Debian jessie before 1.0.9.8.4, in Debian unstable before 1.4~beta2, in Ubuntu 14.04 LTS before 1.0.1ubuntu2.17, in Ubuntu 16.04 LTS before 1.2.15ubuntu0.2, and in Ubuntu 16.10 before 1.3.2ubuntu0.1 allows man-in-the-middle attackers to bypass a repository-signing protection mechanism by leveraging improper error handling when validating InRelease file signatures.
https://www.debian.org/security/2016/dsa-3733
http://www.ubuntu.com/usn/USN-3156-1
http://packetstormsecurity.com/files/140145/apt-Repository-Signing-Bypass.html