Facebook on Thursday said it has fixed a security issue wherein millions of its users' passwords were stored in plain text and "readable" format for years and according to reports, were searchable by thousands of its employees.

The report by KrebsOnSecurity claimed on Thursday around 200-600 million Facebook users may have had their account passwords stored in plain text and searchable by over 20,000 Facebook employees.

In a blog post later, Facebook said as part of a routine security review in January, it found that some user passwords were being stored in a readable format within our internal data storage systems.

"This caught our attention because our login systems are designed to mask passwords using techniques that make them unreadable.

"We have fixed these issues and as a precaution will be notifying everyone whose passwords we found stored this way," wrote Pedro Canahuati, VP Engineering, Security and Privacy at Facebook.

The company, however, said these passwords were never visible to anyone outside of Facebook.

"We have found no evidence to date that anyone internally abused or improperly accessed them. We estimate that we will notify this to hundreds of millions of Facebook Lite users, tens of millions of other Facebook users, and tens of thousands of Instagram users.

Facebook Lite is a version of Facebook, predominantly used by people in regions with lower connectivity.

"Out of an abundance of caution, we are telling people so that they can change passwords if they choose," Facebook tweeted.

Earlier this month, Facebook came under scrutiny for using phone numbers provided for security reasons -- like two-factor authentication (2FA) -- for things like advertising and making users searchable by their phone numbers across its different platforms.

"Consider enabling a security key or two-factor authentication to protect your Facebook account using codes from a third party authentication app. When you log in with your password, we will ask for a security code or to tap your security key to verify that it is you," Facebook advised.

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