The invention concerns a method and device for detecting attempts at intruding on a communication link between an aircraft and a ground station, the communication being capable of being configured according to multiple security levels, comprising the following steps: a) analyzing a message received to determine a security level corresponding to said message; b) comparing said security level with a security level associated with the communication; c) if the security levels compared at step b) are different, performing an action concerning the rejection of the received message and the signalling of an attempt at intrusion. The invention also concerns a device for implementing said method.