The personal details of 100 million Facebook users have been collected and published online in a downloadable file, meaning they will now be unable to make their publicly available information private.
However, Facebook downplayed the issue, saying that no private data had been compromised.
The information was posted by Ron Bowes, an online security consultant, on the Internet site Pirate Bay.
Bowes used code to scan the 500 million Facebook profiles for information not hidden by privacy settings. The resulting file, which allows people to perform searches of various different types, has been downloaded by several thousand people.
This means that if any of those on the list decide to change their privacy settings on Facebook, Bowes and those who have the file will still be able to access information that was public when it was compiled.
Bowes’ actions also mean people who had set their privacy settings so their names did not appear in Facebook’s search system can now be found if they were friends with anyone whose name was searchable.
'Scary privacy issue'
On his website, www.skullsecurity.org, Bowes said the results of his code were "spectacular," giving him 171 million names of which were 100 million unique.
"As I thought more about it and talked to other people, I realized that this is a scary privacy issue. I can find the name of pretty much every person on Facebook," he wrote.
"Facebook helpfully informs you that "[a]nyone can opt out of appearing here by changing their Search privacy settings" — but that doesn't help much anymore considering I already have them all (and you will too, when you download the torrent). Suckers!"
"Once I have the name and URL of a user, I can view, by default, their picture, friends, information about them, and some other details," Bowes added. "If the user has set their privacy higher, at the very least I can view their name and picture. So, if any searchable user has friends that are non-searchable, those friends just opted into being searched, like it or not! Oops :)"
He said he discovered the top first name in the list was Michael, followed by John, David, Chris and Mike. The top surnames were Smith, Johnson, Jones, Williams and Brown.
A privacy expert expressed concern at the implications of Bowes' actions. Simon Davies, of campaign group Privacy International, told the BBC that some Facebook users "did not understand the privacy settings and this is the result."
"Facebook should have anticipated this attack and put measures in place to prevent it," he told the BBC. "It is inconceivable that a firm with hundreds of engineers couldn't have imagined a trawl of this magnitude and there's an argument to be heard that Facebook have acted with negligence."
'A little terrifying'
Some users of Pirate Bay shared his concerns.
"This is awesome and a little terrifying," lusifer69 wrote on the site. And another, Porkster, said: "I don't think this is a hack, but a collection from public domain info that people have shared. The importance of the info is structuring it and allowing someone to search or compute the data."
However, jak322 said: "I've got to say, who cares. All the info here is already in the public domain, is not sensitive and as a developer I already have access to what could be deemed personal and private data through the Facebook API."
In a statement emailed to msnbc.com, Facebook agreed, saying the information on the list was already available online.
"People who use Facebook own their information and have the right to share only what they want, with whom they want, and when they want," it said.
"Our responsibility is to respect their wishes. In this case, information that people have agreed to make public was collected by a single researcher. This information already exists in Google, Bing, other search engines, as well as on Facebook," the statement added.
"No private data is available or has been compromised. Similar to a phone book, this is the information available to enable people to find each other, which is the reason people join Facebook. If someone does not want to be found, we also offer a number of controls to enable people not to appear in search on Facebook, in search engines, or share any information with applications."
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