In Docker before versions 19.03.15, 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/<remapping>" that cause writing files with extended privileges. Versions 20.10.3 and 19.03.15 contain patches that prevent privilege escalation from remapped user.
External References: https://github.com/moby/moby/security/advisories/GHSA-7452-xqpj-6rpc
Upstream commits: https://github.com/moby/moby/commit/edb62a3ace8c4303822a391b38231e577f8c2ee8 https://github.com/moby/moby/commit/bfedd2725971303efb7a2fe5d6990317b381622f https://github.com/moby/moby/commit/e908cc39018c015084ffbffbc5703ccba5c2fbb7
Support for user namespaces (--userns-remap) was introduced in Docker-1.10.0: https://docs.docker.com/engine/release-notes/prior-releases/#security-6.