D-Bus 1.4.x through 1.6.x before 1.6.30, 1.8.x before 1.8.16, and 1.9.x before 1.9.10 does not validate the source of ActivationFailure signals, which allows local users to cause a denial of service (activation failure error returned) by leveraging a race condition involving sending an ActivationFailure signal before systemd responds.