Hackers broke into British company Piriform’s free software for optimizing computer performance last month potentially allowing them to control the devices of more than two million users, the company and independent researchers said on Monday. The malicious program was slipped into legitimate software called CCleaner, which is downloaded for personal computers and Android phones as often as five million times a week. It cleans up junk programs and advertising cookies to speed up devices. CCleaner is the main product made by London’s Piriform, which was bought in July by Prague-based Avast, one of the world’s largest computer security vendors. At...