Facebook locked some users out of their accounts after determining that their log-in credentials were exposed as a result of a security breach at Adobe. The company is asking users who used the same log-in credentials for Adobe's online services and Facebook to verify their identity and change their password.
When trying to log in, Facebook users who fall into that category will see a security prompt informing them that "there was a security incident on another website unrelated to Facebook" and that their accounts are at risk because they were using the same password in both places.
"To secure your account, you'll need to answer a few questions and change your password," the Facebook security message says. "For your protection, no one can see you on Facebook until you finish."
"We're proactive about finding sources of compromised passwords on the Internet," Facebook security engineer Chris Long said in a comment to a blog post about this action by security reporter Brian Krebs. "Through practice, we've become more efficient and effective at protecting accounts with credentials that have been leaked, and we use an automated process for securing those accounts."
A Facebook spokesman in the UK confirmed Long's comment is legitimate, but declined to share additional information like how many Facebook users were affected by this security measure or what questions those users need to answer in order to regain access to their accounts.
At the beginning of October Adobe announced that hackers broke into its internal computer network and stole source code for several products, as well as information on 2.9 million customers including their log-in IDs and encrypted passwords for Adobe's online services.
After a file containing 150 million Adobe user names and encrypted passwords appeared online late last month, the company revised its estimation and said that the breach affected approximately 38 million active users.
Researchers who looked at the leaked file concluded that the encryption used by Adobe combined with various user-supplied hints accompanying the passwords made it possible for attackers to guess many of them.
"We used the plaintext passwords that had already been worked out by researchers," Long said. "We took those recovered plaintext passwords and ran them through the same code that we use to check your [Facebook] password at login time."
Facebook might not be the only company taking proactive measures as a result of the Adobe breach.
According to Krebs, some customers of Diapers.com and Soap.com, two websites operated by Amazon-owned online retailer Quidsi, received emails over the weekend informing them that their account passwords have been reset because they matched those on a list of leaked credentials that appeared online, but didn't originate from the two websites.