Facebook has been hacked, but the company has found no evidence that user data was affected.

Facebook's systems were "targeted in a sophisticated attack" in January after some of the company's developers visited a mobile-developer website that had been compromised, the company wrote on Friday afternoon.

Malware was installed onto fully-patched Facebook laptops via a Java zero-day vulnerability – a vuln that Oracle patched on February 1.

"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," the company wrote.

Facebook realized that the hack had occurred when its security team found a "suspicious" domain within the company's corporate DNS logs that was tracked back to a company laptop.

"Upon conducting a forensic examination of that laptop, we identified a malicious file, and then searched company-wide and flagged several other compromised employee laptops," Facebook reports.

Other companies were targeted in this attack as well, though Facebook did not disclose who. It has formed a working group to share information among the affected parties.

The company is working with law enforcement and other entities to prevent further attacks, it said, and encouraging people to submit security vulnerabilities affecting Facebook to the company's Bug Bounty Program. ®