Versions mentioned in the description apply to the upstream
Remediation section below for
Debian:9 relevant versions.
apt 0.8.16, 0.9.7, and possibly other versions does not properly handle InRelease files, which allows man-in-the-middle attackers to modify packages before installation via unknown vectors, possibly related to integrity checking and the use of third-party repositories.
apt to version 0.9.7.8 or higher.