An apparatus and method for detecting intrusions in Voice over Internet Protocol systems with distributed stateful intrusion detection. When a Session Initiation Protocol (SIP) signal is received as an application-layer protocol signal, the received application-layer protocol signal is distributed to a first finite-state machine and to a second finite-state machine. A data processing system detects that the application-layer protocol enters a first application-layer protocol state S1 at a first node at a first time, determines whether the application-layer protocol fails to enter a second application-layer protocol state S2 at a second node within δ seconds, a positive real number, and generates a signal that indicates a potential intrusion in response to the determination.