By Joseph Menn
SAN FRANCISCO (Reuters) - Hackers broke into British company Piriform's free software for optimizing computer performance last month and installed tools that could have allowed them to take control of tens of millions of devices, 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 5 million times a week. It cleans up junk programmes 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 the time of the acquisition, the company said 130 million people used CCleaner.
A version of CCleaner downloaded in August included remote administration tools that tried to connect to several unregistered web pages, presumably to download additional unauthorized programs, security researchers at Cisco's Talos unit said.
Talos researcher Craig Williams said it was a sophisticated attack since it penetrated an established and trusted supplier in a manner similar to June's “NotPetya” attack on companies that downloaded infected Ukrainian accounting software.
"There is nothing a user could have noticed," Williams said, noting that the optimization software had a proper digital certificate, which means other computers automatically trust the program.
In a blog post, Piriform confirmed that two programmes released in August were compromised. It advised users of CCleaner v5.33.6162 and CCleaner Cloud v1.07.3191 to download new versions. It did not disclose how many users were affected.
Piriform said Avast, its new parent company, had uncovered the attacks on September 12. A new, uncompromised version of CCleaner was released the same day and a clean version of CCleaner Cloud was released on September 15, it said.
The nature of the attack code suggests that the hacker won access to a machine used to create CCleaner, Williams said.
CCleaner does not update automatically, so each person who has installed the problematic version will need to delete it and install a fresh version, he said.
Williams said 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.
It said the server was closed down on Sept. 15 "before any known harm was done".
(Additional reporting by Eric Auchard in Frankfurt; editing by Jason Neely)