Sometimes, it's not curiosity, recognition, or money that motivates a bugfinder. The hacker who exposed the latest MySpace vulnerability did so because he needed a zero-day to qualify for a special hacker team.

Kuza55, who released proof-of-concept code on a zero-day vulnerability on MySpace this week, says he had never before checked the social networking site for holes. "I was prompted to find the MySpace vulnerability because I wanted to join the w4cking.com zero-day team so that I could see other people's research and ideas," he says. "And they had asked for a zero-day to show that I actually had some skills and could contribute to the group." (See Zero Day Flaw Found in MySpace.)

He decided to release the proof-of-concept code to demonstrate that the vulnerability wasn't just there, but that it was real and usable, he says. The POC could be used to steal cookies, impersonate a user, or get people to post comments or add friends to their circle, he says.

MySpace has now fixed the flaw, kuza55 says. But the fix is "rather near-sighted," he says, because it appears to just address only his POC, not related XSS attacks.

As of this story's posting time, security officials at MySpace had not yet responded to inquiries.

Security experts point to blacklists in XSS filters as the problem, but kuza55 disagrees. "I don't think this has anything to do with whitelists and blacklists in XSS filters," he says. "Blacklists are obviously a bad idea because you have to know everything bad that could be done. Whereas with whitelists, you only need to know that a certain amount of things are definitely safe... I don't see how this could have been avoided using whitelisting."

Kuza55's tips for protecting your site from XSS fragmentation attacks: Use an XSS filter that doesn't allow unfinished or unclosed tags. "It's just another path to XSS, and it's something else people who write XSS filters need to keep in mind," he says. "It has the exact same severity as XSS attacks because it is an XSS attack... It's just another way to defeat filters."

— Kelly Jackson Higgins, Senior Editor, Dark Reading