A method of detecting pirated software includes receiving a request for a software update by a client computer and providing to the client computer a test to be performed. The test is performed on the client computer against the client software application. The client computer may be denied a software update as a result of the test finding an illegitimate copy of the client software. The invention may be practiced in a network environment where a server transfers a test program for a client to execute upon request of a software update. The test performs an integrity check and denies the request for a software update if the client software is found to be illegitimate.