"Our goal was to retrieve Web content anonymously," says Matthew Flick, principal with FYRM Associates, who, along with fellow researcher Jeff Yestrumskas, demonstrated the XSS Anonymous Browser (XAB) framework at Black Hat DC yesterday. "We [said], 'Why don't we volunteer people for our network?'...Cross-site scripting can make people do things we want."
The framework uses the victim as a cover for an attack. "It's basically an agentless botnet...there's no trace of our code on their system," says Flick, who adds such an attack would likely have legal ramifications. "It's a decent way of hiding your tracks."
The researchers demonstrated their proof of concept, but did not release any code. They acknowledged that XSS and anonymization make an unlikely couple. "Putting anonymity and cross-site scripting together is unusual," Flick said during the pair's demo.
In a nutshell, the attack turns an unsuspecting user's browser into an anonymous browsing tool for the attacker, who then can silently abuse the browser to access Web content he doesn't want traced to him, such as porn or a site for espionage or theft purposes.
The attack works like this: The XAB attacker first exploits an XSS-vulnerable Website and injects his initial malicious payload. When the victim visits the site, he's hit with that malware, which then infects the user's browser with another piece of malware. That second infection includes a link to the XAB proxy tool and instructions for the unsuspecting victim's browser, such as which URLs to visit. The proxy then requests the URL and content from the targeted Website. The victim's browser sends the data back to the attacker's site.
The researchers admit their XAB framework has some inherent weaknesses, however, that could render it ineffective. Among its shortcomings: The attack doesn't properly access images due to the way browsers treat nontext data, they said.
Among the next features the researchers plan to add to XAB are the ability to commission simultaneous requests of the victim's browser, binary data transfer, and data encryption.
"XAB [uses] the browser as an attack vector," said Yestrumskas, who is senior manager of information security at Cvent.
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