A security system for a mobile network (1) has a gateway (3) for receiving messages from outside the network and a HLR (10) storing mobile terminal location information. The security system monitors in real time messages entering the network through the gateway (3), and decides according to said monitoring if messages are likely to be unsolicited. The system may block messages which are likely to be unsolicited. The system monitors a source address of a look-up request and a source address of a corresponding message, and decides that the message is likely to be unsolicited if its source address is different from that of the corresponding look-up request. The system further comprises a data store (5) and a timer (6), and stores look-up requests received from the gateway in the data store, and decides that a message is likely to be unsolicited if a corresponding look-up request has not been received within a pre-set time period.