Facebook password security lapse probed by privacy regulator; Hundreds of millions of user passwords were inadvertently stored in a readable text format earlier this year.
Facebook's lead regulator in the European Union has launched an inquiry into whether the company violated EU data rules by saving user passwords in plain text format on internal servers.
Last month, the social network announced it had resolved the glitch which meant company employees could have seen the passwords of users which are usually stored in an unreadable form.
But the Irish Data Protection Commission, the lead supervising authority for Facebook in the EU, confirmed it had been notified of the incident and has started an inquiry to determine whether the social network breached GDPR (General Data Protection Regulation) laws designed to protect people's data.
"The Data Protection Commission was notified by Facebook that it had discovered that hundreds of millions of user passwords, relating to users of Facebook, Facebook Lite and Instagram, were stored by Facebook in plain text format in its internal servers," the authority said in a statement.
"We have this week commenced a statutory inquiry in relation to this issue to determine whether Facebook has complied with its obligations under relevant provisions of the GDPR."
The social network warned that the incident could have affected hundreds of millions of Facebook Lite users, a downscaled version of the app for people with older phones or slow internet connections, as well as millions of main Facebook and Instagram users.
Facebook fixed the flaw after uncovering it January.
Its own investigation found no evidence that anyone outside Facebook got hold of the passwords, or that were they abused by staff internally.
The development is the latest in a string of headaches for Facebook chief executive Mark Zuckerberg in recent years, including rampant misinformation spread on the network, breaches of user data and allegations of political manipulation.
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A Facebook spokesman said: "We are working with the IDPC on their inquiry.
"There is no evidence that these internally stored passwords were abused or improperly accessed."