A locking fastener assembly is disclosed for securing electronic equipment or the like to a mounting bracket. The device includes a stud assembly comprising a threaded shank portion for passing through an opening in the bracket, and being threadingly received in an opening at the equipment. An enlarged head adjoining the shank has a pair of openings for receiving engaging pins from a cooperating tool. An annular freely rotatable collar surrounds the head and projects to at least cover the rim of the head, to prevent grasping of such head by a tool. A tool member is provided having a face opposable to the head, the face including a pair of engaging pins having a spacing and arrangement with respect to the tool face center which is in mirror correspondence to the spacing and arrangement of the openings on the head of the stud assembly, whereby the pins may engage with the openings in the head to permit application of torque for tightening and loosening the stud assembly. The tool member is therefore keyed to engage with the head portion; i.e. a similar tool which is unkeyed to the pattern of head openings will not be operable to engage with the stud assembly.