When instructing cloud-init to set a random password for a new user account, versions before 21.2 would write that password to the world-readable log file /var/log/cloud-init-output.log. This could allow a local user to log in as another user.
{ "availability": "No subscription required", "ubuntu_priority": "medium", "binaries": [ { "binary_version": "21.1-19-gbad84ad4-0ubuntu1~16.04.1", "binary_name": "cloud-init" }, { "binary_version": "21.1-19-gbad84ad4-0ubuntu1~16.04.1", "binary_name": "ec2-init" }, { "binary_version": "21.1-19-gbad84ad4-0ubuntu1~16.04.1", "binary_name": "grub-legacy-ec2" } ] }