XSS flaw in popular video-sharing site enabled DDoS attack through visitors' browsers

Attackers exploited the vulnerability to hijack 22,000 browsers and launch a large-scale DDoS attack, researchers from Incapsula said

Attackers exploited a vulnerability in a popular video-sharing site to hijack users' browsers for use in a large-scale distributed denial-of-service attack, according to researchers from Web security firm Incapsula.

The attack happened Wednesday and was the result of a persistent cross-site scripting (XSS) vulnerability in a website that Incapsula declined to name, but said is among the top 50 websites in the world by traffic based on statistics from Amazon-owned firm Alexa.

XSS flaws are the result of improper filtering of user input and can allow attackers to inject unauthorized script code into Web pages. If the code is stored permanently by the server and delivered to all users who view the affected page, the attack is considered persistent.

Users of the unnamed video-sharing site can create profiles and leave comments and the XSS flaw allowed attackers to create a new account with rogue JavaScript code injected into the img tag corresponding to its profile picture.

"As a result, every time the image was used on one of the the site's pages (e.g., in the comment section), the malicious code was also embedded inside, waiting to be executed by every future visitor to that page," the Incapsula researchers said Thursday in a blog post.

The rogue code generated an iframe that loaded a DDoS script into visitors' browsers from a third-party command-and-control (C&C) server, effectively hijacking the browsers and forcing them to send requests in the background to a third-party site.

The resulting attack against the targeted site consisted of 20 million GET requests received from 22,000 browsers at a rate of around 20,000 requests per second, according to the Incapsula researchers.

"Most websites can not sustain 10 percent of that volume," said Marc Gaffan, co-founder of Incapsula, Friday via email. "Furthermore since the requests are coming from real user's browsers, it's very difficult to detect and block them."

The hijacked browsers stop sending requests once the infected page is closed, so the attackers strategically posted comments on popular videos that were 10, 20 and 30 minutes in length. This "effectively created a self-sustaining botnet comprising tens of thousands of hijacked browsers, operated by unsuspecting human visitors who were only there to watch a few funny cat videos," the Incapsula researchers said.

Exploiting XSS vulnerabilities to launch DDoS attacks is not something new. The technique itself has been known for years, but hasn't been used frequently because it requires vulnerabilities in highly trafficked websites to be truly effective.

The Incapsula researchers believe the attack Wednesday might have only been a test run, because the attack script on the C&C server was further improved and updated with tracking capabilities, possibly for future billing purposes. This could indicate that the attackers are building a DDoS-for-hire service around the technique.

"This could be the start of a new trend in which sites that allow user generated content could be systematically exploited," Gaffan said. "Hence the investment in new attack technology."

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