systemd up to version 237 does not perform any check on the content of the PIDFile file of a service, which may result in systemd killing a process owned by other services when the service the PIDFile belongs to is stopped. In case the PIDFile is owned by unprivileged users (e.g. a service run with a different User field), systemd may kill privileged processes. A local attacker who is able to tamper with the PIDFile of a service may trick systemd into killing processes he would not have permissions to kill, in the moment a privileged user tries to kill the mentioned service.
When PIDFile is specified, systemd uses the content of the file to find out the pid of the main process of a service. A vulnerable/malicious service (or one executed by unprivileged user with the User/Group fields) may allow non-root users to write into this file, tricking systemd into killing the wrong process. This is particularly important when services are run by unprivileged users, because systemd kills the process with root permission, even if the service was started by a different user (e.g. User=unprivileged-user) and can be used by a local attacker to cause a Denial of Service.