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.
https://www.openwall.com/lists/oss-security/2018/01/29/4
https://lists.debian.org/debian-lts-announce/2019/04/msg00022.html
http://lists.opensuse.org/opensuse-updates/2018-02/msg00109.html