Dark Reading

A researcher has published proof-of-concept code on a zero-day vulnerability he found on MySpace.com -- and another variation on the cross-site scripting (XSS) theme.

Called XSS fragmentation, the vulnerability consists of multiple chunks, or fragments, of JavaScript malware that can slip by a filter or firewall because individually they don't constitute a security risk. But when they are combined after hitting the site, they can then be dangerous.

XSS fragmentation is rare, but a potentially powerful vulnerability that could be used against community-based sites such as MySpace or Web-based mail systems, security experts say. MySpace in particular is vulnerable because it takes user-supplied content and stores it without adequate filtering, says Jeremiah Grossman, CTO of White Hat Security. An e-commerce site would not be at risk to this type of attack, he says.

XSS in general has become one of the most prevalent targets of online hackers, with many major Websites sporting XSS vulnerabilities. (See Cross-Site Scripting: Attackers' New Favorite Flaw and Hackers Reveal Vulnerable Websites.)

With XSS fragmentation, an attacker could inject the script fragments onto the MySpace user's interests section, such as music and film, according to the proof-of-concept posting by kuza55, the hacker who discovered the vulnerability.

Once the JavaScript fragments get on the site, they assemble and do their dirty work -- dropping a worm, stealing browser history, port-scanning a victim's intranet, or shooting off phishing emails purportedly from MySpace to steal logins and passwords.

"I personally have only seen XSS fragmentation a few times," says hacker Rsnake, founder of the sla.ckers.org site where kuza55 posted the proof-of-concept code. "This isn't that common of an attack, as it generally requires that there be two or more places to inject code on the page."

Interestingly, scanning alone won't detect XSS fragmentation, nor will blacklisting eliminate the threat. White Hat Security's Grossman, whose company runs a vulnerability assessment and management service for Websites, says it takes human intervention to detect such a vulnerability or attack because it's typically targeted at a specific site or organization. Whitelisting -- specifying what data is allowed -- would be a better way to prevent such an attack, he says.

"This attack is pretty involved. Scanning is not going to find it, but human assessment would," he says. "In Web attacks, there's going to be someone sitting behind a browser... You have to match their skillset, and that's where the human assessment part" comes in.

MySpace had not responded to inquiries for this article as of presstime.

— Kelly Jackson Higgins, Senior Editor, Dark Reading