systemd-tmpfiles in systemd before 237 attempts to support ownership/permission changes on hardlinked files even if the fs.protected_hardlinks sysctl is turned off, which allows local users to bypass intended access restrictions via vectors involving a hard link to a file for which the user lacks write access, as demonstrated by changing the ownership of the /etc/passwd file.

Mitigation

This flaw, and many others like it, is mitigated by enabling hardlink and symlink protections. These protections are enabled by default in Red Hat Enterprise Linux and this vulnerability will only be exploitable if disabled.

To ensure your system is protected, check that fs.protected_hardlinks is enabled as in the following example:

sysctl fs.protected_hardlinks

fs.protected_hardlinks = 1