Abstract: A one way threaded security fastener comprises a head with a recess containing a deformable or detachable torque transfer element which engages a tool such as a screwdriver during installation and deforms or shears off at a predetermined torque leaving the recess with a generally smooth inner wall. Preferably the recess is at least partially frustoconical or hemispheroidal with a circular cross section. The torque transfer element may be made from metal or plastics material, and may be integral with the fastener or may be attached to the socket by glue or the like. An installation tool is disclosed for use with the fastener.