Sophos has warned computer users that Facebook has accidentally publicly revealed personal information about its members, which could be useful to identity thieves.
Earlier this week, the full dates of birth of many of Facebook's 80m active users were visible to others, even if the individual member had requested that the information remained confidential.
Graham Cluley, senior technology consultant at Sophos, said a security slip-up by the website during the process of a public beta test of its new design for members' profiles, left birth date information exposed.
"I was shocked to see peoples' full date of birth revealed, even though I knew they had their privacy set up correctly to supposedly hide the information," said Cluley.
"It's essential that users of social networks should have confidence that their privacy will be protected, and it's especially important with information like your date of birth, which can be a golden nugget for a committed identity thief."
Cluley says he informed Facebook as soon as he discovered the flaw, which now appears to have been fixed.
He said, "It's good that Facebook fixed the problem, but can people feel confident that this kind of mistake won't happen again in the future?"
"My advice to Facebook users would be, even if your date of birth is set to be non-visible, change it to a made-up date in case this kind of blunder happens again."