Facebook has moved quickly to patch a security loophole that potentially allowed identity thieves and spammers to gather users' personal email addresses.

According to a report in The New York Times, a 29-year-old former Peace Corps volunteer stumbled across the vulnerability by accident after using the Website's "Find a friend on Facebook" facility.

Mike Sheppard from Holland, Mich., discovered that when users uploaded a file of email addresses to Facebook to find out if their "friends" were members, the Web site would return additional information in the search results, including details of other (often personal and private) email addresses owned by the individuals.

In some cases, Facebook's privacy settings were supposed to have restricted access to the personal email addresses.

Sheppard tested the flaw by assembling a list of more than 10,000 corporate email addresses, including staff at CNN, Microsoft, Google, the Gates Foundation, government organizations, and various newspaper reporters.

Sheppard told The New York Times that Facebook revealed the personal email addresses of more than 30 percent of his list -- without the knowledge of their owners. Facebook helpfully ignored the addresses of people who weren't members of its site, but provided profile pictures, names, and so forth for the others.

This could have provided a goldmine of email addresses for spammers and scammers to exploit.

Let's make this simpler to understand.

Imagine you knew my email address was gc@sophos.com, but didn't know my name, what I looked like, or any other email addresses for me. Simply asking Facebook if I was a member of its site would mean it spat out my picture, my private email address and even which networks I had joined on its site.

That's sloppy security, and it's relieving to hear that Facebook has now fixed this loophole.

As more and more sites collect our personal information, the risk of cybercriminals getting hold of it (through accidental leakage or malicious hacking) inevitably increases. That's why we would all be wise to think more carefully about the personal data we share on social networks.

Graham Cluley is senior technology consultant at Sophos, and has been working in the computer security field since the early 1990s. When he's not updating his other blog on the Sophos Website, then you can find him on Twitter at @gcluley. Special to Dark Reading.