Ubuntu Security Notice USN-762-1 – Alexandre Martani discovered that the APT daily cron script did not check the return code of the date command. If a machine is configured for automatic updates and is in a time zone where DST occurs at midnight, under certain circumstances automatic updates might not be applied and could become permanently disabled. Michael Casadevall discovered that APT did not properly verify repositories signed with a revoked or expired key. If a repository were signed with only an expired or revoked key and the signature was otherwise valid, APT would consider the repository valid.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/76854/USN-762-1.txt
Source: https://packetstormsecurity.com/files/76854/Ubuntu-Security-Notice-762-1.html