Poisoning attacks by spoofing location beacons in a WLAN are detected using silence periods. A location beacon identifier is received from a mobile device allegedly within range of a location device transmitting location beacons, along with a timestamp of transmission for each of the location beacons. Also silence periods associated with the location device, during which transmissions of location beacons are temporarily discontinued, and which are unknown to the public, are determined or retrieved. The location beacon transmission time is compared to the silence periods. Responsive to the location beacon transmission time corresponding to at least one of the silence periods, the location device flagged as poisoned.