The National Security Agency and its British counterpart, Government Communications Headquarters, have worked to subvert anti-virus and other security software in order to track users and infiltrate networks, according to documents from NSA whistleblower Edward Snowden. The spy agencies have reverse engineered software products, sometimes under questionable legal authority, and monitored web and email traffic in order to discreetly thwart anti-virus software and obtain intelligence from companies about security software and users of such software. One security software maker repeatedly singled out in the documents is Moscow-based Kaspersky Lab, which has a holding registered in the U.K., claims more than...