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.

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.

“There is nothing a user could have noticed,” Williams said, noting that the optimization software had a proper digital certificate, which means that other computers automatically trust the program.

Williams said that Talos detected the issue at an early stage, when the Hackers appeared to be collecting information from infected machines, rather than forcing them to install new programs.

Piriform said it had worked with U.S. law enforcement to shut down a server located in the United States to which traffic was set to be directed.