This method for detecting an illicit use of a security processor used for the descrambling of different multimedia contents disseminated on several respective channels comprises: the incrementing (153) of a counter Kchi by a predetermined step whenever, after verification, a message ECMi,c is received within a time slot immediately consecutive to a message ECMi,p and, if not, the resetting (158) of the counter Kchi at its initial value, the detection (162) of an illicit use as soon as the counter Kchi reaches a predetermined threshold.