Facebook admitted it had been the target of a sophisticated hacking attack but that it had no evidence any user data had been compromised.
The attack comes two weeks after Twitter asked 250,000 of its users to reset their passwords after it too was hacked. In the same week, The New York Times and The Wall Street Journal also reported being hacked.
Facebook said its systems were targeted last month when a handful of employees visited a mobile developer website that had been compromised. The website allowed malicious code to be installed on the employees' laptops, Facebook said in a blog post. It said the laptops were fully patched and running up-to-date antivirus software.
"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.
The company said it had no evidence that any user data was compromised. It did not say where it thinks the attack originated or who may have conducted it, and a spokesman reached via email said the company had no further comment at this time.
The attack was identified when Facebook's security team spotted a suspicious domain in the company's corporate DNS logs and tracked it back to an employee laptop. After a malicious file was found on that laptop, the search was expanded companywide and several other compromised employee laptops were flagged, Facebook said.
"After analyzing the compromised website where the attack originated, we found it was using a 'zero-day' (previously unseen) exploit to bypass the Java sandbox (built-in protections) to install the malware," Facebook said.
It said it immediately reported the exploit to Oracle, which provided a patch on Feb. 1 to address the vulnerability.
The attack was not limited to Facebook. "It is clear that others were attacked and infiltrated recently as well," it said. As one of the first companies targeted by the malware, it said it "immediately took steps to start sharing details about the infiltration with the other companies and entities that were affected."
Oracle's Java technology has come under fire recently over security. It patched some critical vulnerabilities in the platform in mid-January, but still the U.S. Computer Emergency Readiness Team (US-CERT) urged users to disable browser plug-ins that use Java.
Oracle released another patch at the start of February, but did not mention at the time that the vulnerability had been used to target Facebook. Last week it said it would release yet more patches on 19 February.