sh _uid does not drop supplementary groups (incomplete privilege drop)
The _uid option performed an incomplete privilege drop on Linux/Unix-like systems. When sh was run from a process with elevated privileges, such as root, and a command was launched with _uid=<unprivileged user>, the child process changed its UID and primary GID but did not reset its supplementary groups. As a result, the child process could retain the parent process’s supplementary groups, potentially including privileged groups such as root, docker, disk, …