For years, police have been in a cat-and-mouse game with an unexpected foe that can frustrate investigations—antivirus software.

Law enforcement's battle against Symantec Corp.'s Norton, Intel Corp.'s McAfee brands and others gained new attention this month after anonymous activists published documents from FinFisher GmbH, a secretive German firm that sells computer code to help governments snoop on targets. Amid customer names and secret price lists, the cache exposed complaints from authorities that antivirus programs had...