USN-5745-2: shadow regression

USN-5745-2: shadow regression

USN-5745-1 fixed vulnerabilities in shadow. Unfortunately that update
introduced a regression that caused useradd to behave incorrectly in Ubuntu
14.04 ESM, Ubuntu 16.04 ESM, Ubuntu 18.04 LTS and Ubuntu 20.04 LTS. This
update reverts the security fix pending further investigation.

We apologize for the inconvenience.

Original advisory details:

Florian Weimer discovered that shadow was not properly copying and removing
user directory trees, which could lead to a race condition. A local attacker
could possibly use this issue to setup a symlink attack and alter or remove
directories without authorization.
Source: USN-5745-2: shadow regression