Abstract: A method of detecting intrusion in a computer program which has number of defined libraries and includes cross border instructions which cause execution to branch from a source library to a target library. The method comprises the step of determining whether execution of the program is in an area consistent with normal execution of the program, by checking whether the source library of a cross border instruction is the expected current execution library of the program. Each cross border instruction has a code stub identifying the source library, and when a legal cross border instruction is executed the target library becomes the current execution library. The method also checks that the target address of a cross border instruction is a legal address. In another arrangement, areas of the program are set so that a cross border instruction will generate page protection fault which is intercepted by the intrusion detection system so that the cross border instruction can be checked.