The mirror:// method implementation in Advanced Package Tool (APT) 1.6.x before 1.6.4 and 1.7.x before 1.7.0~alpha3 mishandles gpg signature verification for the InRelease file of a fallback mirror, aka mirrorfail.
https://usn.ubuntu.com/3746-1/
https://salsa.debian.org/apt-team/apt/commit/aebd4278bacc728ab00ebe31556983e140f60e47
https://salsa.debian.org/apt-team/apt/commit/29658a3a74af49e2a24e17bdebb20e1612aac3ec