The developer, Yvo Schaap, discovered the vulnerability, which works by taking advantage of how the two sites remember users' login information and use that information to activate certain Flash apps. Specifically, if a user checks the "remember me" box in the login modules of either site, and then use a Flash app that makes use of their login information, those actions would make their login information vulnerable to a hacker.
That basic problem could give hackers the power to build malicious Flash apps that could harvest users' other personal information, account numbers, photos, messages and everything else posted on either of the two sites.
Schaap emailed administrators at both sites. MySpace resolved the problem first, while Facebook followed close behind. That's the good news.
The bad news is that this vulnerability has been around for months, which means that any number of users may have had their information harvested.
Facebook has launched an investigation into the origin of the bug.
"The security of our users is a top priority for Facebook and we worked with the researcher who identified the issue to fix it," a representative for Facebook said. "We have not received any reports that it was ever exploited."
Tech analyst Jason Kincaid of TechCrunch.com criticized both sites for their lax security standards, but he saved his harshest words for Facebook
"Facebook is no longer just a platform for learning about your college buddies — it’s a serious business, used for photos and messages that can be very sensitive," he said. "I’ve heard of journalists who regularly use Facebook to reach out to potential sources, when secrecy is of the utmost importance. Apparently that’s not a good idea."
Tech-savvy developers may want to read Schaap's full description of the vulnerability, which apparently takes advantage of an imperfection in the programming of a file called "crossdomain.xml."