SAN FRANCISCO (AP) — Facebook had stored millions of user passwords in plain text for years, the social media company confirmed on Thursday after a security researcher posted about the issue online.

Facebook says there is no evidence that employees had abused access to this data. The company says the passwords were stored on internal company servers, no outsiders could access them. But the incident reveals a huge oversight for the company amid a slew of bruises and stumbles in the last couple of years.

The security blog KrebsOnSecurity says some 600 million Facebook users may have had their passwords stored in plain text. Facebook said in a blog post Thursday it will likely notify "hundreds of millions" of Facebook Lite users, millions of Facebook users and tens of thousands of Instagram users.