Facebook said 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, Calif.- 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 percent to $28.32 at Friday's close in New York, and it has risen 6.4 percent this year.
The attack also affected other companies, Facebook said, without identifying them. It's collaborating on the incident through an informal working group.