The simple coding errors are alarming considering the extent to which social networks have gone to reassure their users that their data will be safe. The problem involved the way those sites handle requests for data from other domains, known as the "cross-domain policy."
Sites such as MySpace and Facebook typically block other domains from requesting and receiving data for privacy reasons, except for their own vetted subdomains.
Facebook disallowed access from other applications on its main domain, but a developer in the Netherlands, Yvo Schaap, found that Facebook would allow data to be given out from one of its subdomains.
Since the subdomain also hosted all of Facebook's data, it would be possible to steal data by luring a victim to a URL with a Flash application rigged to grab the data if the victim had their auto-login enabled, which most people do, according to Schaap's blog.
A "more invasive and hidden exploit could harvest all the user's personal photos, data and messages to a central server without any trace, and there is no reason why this wouldn't be happening already with both Facebook and MySpace data," Schaap wrote on his blog.
He also found the problem on MySpace, which allowed a domain called "farm.sproutbuilder.com" to access data. A Flash application could be uploaded to that site, which would then be allowed access to the data if a victim visited a malicious URL.
MySpace disagreed with the severity of the error, saying it would have only exposed information that was already public. The problem was with the sproutbuilder domain, and it has since been fixed, a spokeswoman said in an emailed statement.
"No public MySpace data was exposed and the vulnerability was never exploited," the statement read.
A look at Facebook's latest crossdomain.xml file shows that the bug appears to have been fixed. MySpace also appears to have taken "farm.sproutbuilder.com" out of its cross-domain list.
In an emailed statement, Facebook said it "worked with the researcher who identified this issue to fix it. We have not received any reports that it was ever exploited."