A method performed by a network node, for detecting a false base station in a communications network. The network node operates in the communication network and is adapted to serve a network device via a serving Radio Access, RA, node. The network node sends a message to a network device, which message comprises configuration data configuring the network device to perform measurements in order to collect information transmitted by network nodes in a surrounding area of the network device. The network node further receives a message comprising measurement reports from the network device according to the configuration. The network node further provides an indication that a false base station is present when a difference between the received information in the measurement report and a predetermined target information is detected.