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.
lists.opensuse.org/opensuse-updates/2018-02/msg00109.html
packetstormsecurity.com/files/146184/systemd-Local-Privilege-Escalation.html
www.openwall.com/lists/oss-security/2018/01/29/3
github.com/systemd/systemd/issues/7736
lists.apache.org/thread.html/r58af02e294bd07f487e2c64ffc0a29b837db5600e33b6e698b9d696b%40%3Cissues.bookkeeper.apache.org%3E
lists.apache.org/thread.html/rf4c02775860db415b4955778a131c2795223f61cb8c6a450893651e4%40%3Cissues.bookkeeper.apache.org%3E
lists.debian.org/debian-lts-announce/2019/04/msg00022.html
www.exploit-db.com/exploits/43935/
www.openwall.com/lists/oss-security/2018/01/29/4