CVE-2021-21284: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

In Docker before versions 9.03.15, 20.10.3 there is a vulnerability involving the –userns-remap option in which access to remapped root allows privilege escalation to real root. When using “–userns-remap”, if the root user in the remapped namespace has access to the host filesystem they can modify files under “/var/lib/docker/” that cause writing files with extended privileges. Versions 20.10.3 and 19.03.15 contain patches that prevent privilege escalation from remapped user.