Flip the “days since last Facebook security incident” back to zero.

Facebook confirmed Thursday in a blog post, prompted by a report by cybersecurity reporter Brian Krebs, that it stored “hundreds of millions” of account passwords in plaintext for years.

The discovery was made in January, said Facebook’s Pedro Canahuati, as part of a routine security review. None of the passwords were visible to anyone outside Facebook, he said. Facebook admitted the security lapse months later, after Krebs said logs were accessible to some 2,000 engineers and developers.

Krebs said the bug dated back to 2012.

“This caught our attention because our login systems are designed to mask passwords using techniques that make them unreadable,” said Canahuati. “We have found no evidence to date that anyone internally abused or improperly accessed them,” but did not say how the company made that conclusion.

Facebook said it will notify “hundreds of millions of Facebook Lite users,” a lighter version of Facebook for users where internet speeds are slow and bandwidth is expensive, and “tens of millions of other Facebook users.” The company also said “tens of thousands of Instagram users” will be notified of the exposure.

Krebs said as many as 600 million users could be affected — about one-fifth of the company’s 2.7 billion users, but Facebook has yet to confirm the figure.

Facebook also didn’t say how the bug came to be. Storing passwords in readable plaintext is an insecure way of storing passwords. Companies, like Facebook, hash and salt passwords — two ways of further scrambling passwords — to store passwords securely. That allows companies to verify a user’s password without knowing what it is.

Twitter and GitHub were hit by similar but independent bugs last year. Both companies said passwords were stored in plaintext and not scrambled.

It’s the latest in a string of embarrassing security issues at the company, prompting congressional inquiries and government investigations. It was reported last week that Facebook’s deals that allowed other tech companies to access account data without consent was under criminal investigation.

It’s not known why Facebook took months to confirm the incident, or if the company informed state or international regulators per U.S. breach notification and European data protection laws. We asked Facebook but a spokesperson did not immediately comment beyond the blog post.

The Irish data protection office, which covers Facebook’s European operations, said the company “informed us of this issue” and the regulator is “currently seeking further information.”