Facebook, the operator of the largest social network with more than 1 billion members, is working with the Federal Bureau of Investigation in the United States to probe a malware attack, people with knowledge of the matter said.
Facebook said on Friday that it was subjected to a "sophisticated attack" by hackers last month, without saying where the attack originated. Investigators haven't found any evidence that user data was compromised, Menlo Park, California-based Facebook said on its website. The people who discussed the FBI's involvement asked not to be identified, citing the sensitive nature of the probe.
Facebook, which stores information on users, often grapples with attempted breaches.
It has a team dedicated to detecting and responding to attack attempts, pays bounties to outside programmers who help identify malfeasance and said it successfully prevents attacks the "vast majority of the time".
"As soon as we discovered the presence of the malware, we remediated all infected machines, informed law enforcement and began a significant investigation that continues to this day," Facebook said.
Malware infected laptops used by Facebook employees when they visited a mobile developer's website, the company said.
Facebook fell less than 1 per cent to $28.32 at Friday's close in New York, and it has risen 6.4 per cent this year.
The attack also affected other companies, Facebook said, without identifying them. It's collaborating on the incident through an informal working group.
"We are working continuously and closely with our own internal engineering teams, with security teams at other companies, and with law-enforcement authorities to learn everything we can about the attack, and how to prevent similar incidents in the future," Facebook said.
Facebook, as part of its monitoring efforts, flagged a suspicious domain in its corporate logs and tracked it back to an employee computer.
After examining the laptop, the company found a malicious file and then uncovered several other compromised laptops after a companywide search.
* Bloomberg News