CVE-2026-54552: 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, 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.