For four days, between May 18 to 22, Facebook tested a new feature that inadvertently switched the default settings for 14 million users from private to public allowing anyone on the Internet view status updates that were intended only for private audiences.
"We recently found a bug that automatically suggested posting publicly when some people were creating their Facebook posts", said Erin Egan, Facebook's chief privacy officer.
Facebook estimates that 14 million users are affected by the glitch, and is notifying them of it.
"We'd like to apologize for this mistake", Egan said.
Users who were affected by the bug will start receiving a notification on Facebook starting Thursday.
Over the past few days, Facebook has been under scrutiny for giving Chinese company Huawei access to users' data, even though the US government has held that company under suspicion in the past over its ties to the country's communist government.
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The mistake happened when Facebook was building a new way for people to share "featured items" on their profiles, it says.
The company said the problem occurred between May 18 and May 27 and has since been fixed.
A message will prompt them to "Please Review Your Posts", while a link will direct the user to view a list of what they shared during the 10 days that the bug was active.
Huawei, a company flagged by USA intelligence officials as a national security threat, was the latest device maker at the center of a fresh wave of allegations over Facebook's handling of private data. In the meantime, it has reverted the audience for any affected posts to whatever setting the user had selected previously. People can also make new posts only visible to friends or sub-groups of friends, for example, by altering this default through their privacy settings.
Facebook tells TechCrunch that it hears loud and clear that it must be more transparent about its product and privacy settings, especially when it messes up.
The mishap comes on the tail of several privacy scandals involving the social network in recent months.