Improper Check for Dropped Privileges

Summary

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, shadow, or sudo. This could allow a subprocess that was expected to run with reduced privileges to access files or resources available to the original process's supplementary groups. Users are impacted if they rely on _uid as a privilege boundary when launching commands from a privileged parent process. This issue affects versions prior to 2.2.4.