Versions mentioned in the description apply to the upstream
Remediation section below for
Debian:9 relevant versions.
The apt package in Debian jessie before 188.8.131.52.4, 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.
apt to version 1.4~beta2 or higher.