Hundreds of millions of Facebook users had their account passwords stored in plain text and searchable by thousands of Facebook employees — in some cases going back to 2012, KrebsOnSecurity has learned. Facebook says an ongoing investigation has so far found no indication that employees have abused access to this data.

Facebook is probing the causes of a series of security failures in which employees built applications that logged unencrypted password data for Facebook users and stored it in plain text on internal company servers. That’s according to a senior Facebook employee who is familiar with the investigation and who spoke on condition of anonymity because they were not authorized to speak to the press.

The Facebook source said the investigation so far indicates between 200 million and 600 million Facebook users may have had their account passwords stored in plain text and searchable by more than 20,000 Facebook employees. The source said Facebook is still trying to determine how many passwords were exposed and for how long, but so far the inquiry has uncovered archives with plain text user passwords in them dating back to 2012.

My Facebook insider said access logs showed some 2,000 engineers or developers made approximately nine million internal queries for data elements that contained plain text user passwords.

“The longer we go into this analysis the more comfortable the legal people [at Facebook] are going with the lower bounds” of affected users, the source said. “Right now they’re working on an effort to reduce that number even more by only counting things we have currently in our data warehouse.”

In an interview with KrebsOnSecurity, Facebook software engineer Scott Renfro said the company wasn’t ready to talk about specific numbers — such as the number of Facebook employees who could have accessed the data.

Renfro said the company planned to alert Facebook users today, but that no password resets would be required.

“We’ve not found any cases so far in our investigations where someone was looking intentionally for passwords, nor have we found signs of misuse of this data,” Renfro said. “In this situation what we’ve found is these passwords were inadvertently logged but that there was no actual risk that’s come from this. We want to make sure we’re reserving those steps and only force a password change in cases where there’s definitely been signs of abuse.”

A written statement from Facebook provided to KrebsOnSecurity says the company expects to notify “hundreds of millions of Facebook light users, tens of millions of other Facebook users, and tens of thousands of Instagram users.” Facebook Lite is a version of Facebook designed for low speed connections and low-spec phones.

Both Github and Twitter were forced to admit similar stumbles in recent months, but in both of those cases the plain text user passwords were available to a relatively small number of people within those organizations, and for far shorter periods of time.

Renfro said the issue first came to light in January 2019 when security engineers reviewing some new code noticed passwords were being inadvertently logged in plain text.

“This prompted the team to set up a small task force to make sure we did a broad-based review of anywhere this might be happening,” Renfro said. “We have a bunch of controls in place to try to mitigate these problems, and we’re in the process of investigating long-term infrastructure changes to prevent this going forward. We’re now reviewing any logs we have to see if there has been abuse or other access to that data.”

Facebook’s password woes come amid a tough month for the social network. Last week, The New York Times reported that federal prosecutors are conducting a criminal investigation into data deals Facebook struck with some of the world’s largest tech companies.

Earlier in March, Facebook came under fire from security and privacy experts for using phone numbers provided for security reasons — like two-factor authentication — for other things (like marketing, advertising and making users searchable by their phone numbers across the social network’s different platforms).