Side-Channel Attack Exposes User Accounts on Facebook, XBox, Other Social Sites

Blocking people on social media and other sites is a popular privacy and security option for online users, but security researchers in Japan have discovered a way to abuse that feature and access sensitive information about users who deploy it.

At least a dozen social media and other online sites – including Facebook, Instagram, Tumblr, Google+, Twitter, eBay, PornHub, Medium, Xbox Live, Ashley Madison, Roblox, and Xvideos – were vulnerable to the side-channel attack found by researchers at NTT Secure Platform Laboratories and Waseda University. So far, Twitter and eBay have updated their platforms to prevent the attack, and some browsers, including Microsoft Edge, Microsoft Internet Explorer, and Mozilla Firefox, have added a feature to thwart the attack, according to Takuya Watanabe, who will present his team's findings in December at Black Hat Europe in London.

Watanabe and fellow researchers Eitaro Shioji, Mitsuaki Akiyama, Keito Sasaoka, Takeshi Yagi, and Tatsuya Mori disclosed their initial research earlier this year at the IEEE European Symposium on Security and Privacy. They also provided suggestions to social media, platform, and browser vendors on how to thwart the attack.

"No matter how robust the security algorithm is, attackers will look for ways to bypass it. The side-channel attack we introduce is one of its practical cases," Watanabe said in an email interview. "Service providers need to design a robust service while recognizing such risk."

At Black Hat Europe, Watanabe plans to detail the attack as well as countermeasures recently adopted by vendors. "Furthermore, I will show whether services will be affected by functions other than user blocks," he said.

The Attack
The side-channel attack exploits the user-blocking function of a social media or other platform by taking advantage of its feature that requires certain Web pages to return specific Web content. If the victim is logged into the social media platform, the attacker can wage an attack on that user when he or she visits the attacker's site. The attacker can surreptitiously communicate with the victim's social media platform and steal personal information from his or her account.

"We generalize this property as 'visibility control,' which we consider to be the fundamental assumption of our attack. Building on this primitive, an attacker with a set of controlled accounts can gain a flexible control over the data leaked through the side channel. Using this mechanism, it is possible to design a robust, large-scale user identification attack on social web services," the researchers wrote in their paper on the attack. 

Such an attack basically exposes the social media account of a user – including name, activities, and posts – who visits the attacker's website. "By exploiting the blocking function, an attacker can identify the social web service account of a visitor to an attacker's website. That is, an attacker or a malicious marketer can grasp the visitor's real name and daily posts," Watanabe told Dark Reading.

The user-blocking function allowing the user to control visibility of his or her profile to other users is basically exploited, he said. "I believe that this social nature is the cause," Watanabe said.

An attacker first would need to phish the user via an email or use other means to lure him or her to the malicious website. Or "if an attacker is a malicious marketer who wants to collect the visitor's personal information: just wait for the user's visit" to its website, he said.

[See Takuya Watanabe give his talk at Black Hat Europe on the social media side-channel attack, "I Block You Because I Love You: Social Account Identification Attack Against a Website Visitor."]

The attack, which targets Windows users, employs a combination of cross-site request forgery (CSRF) and timing attacks. This allows the attacker to bypass the same-origin policy on a website to grab information about the target's view of other sites – namely, the social media or other platform. The attack measures via JavaScript code the round-trip times of the website visitors, which allows the attacker to identify social media accounts and determine user-blocking status.

Watanabe says while the underlying security weaknesses exploited in the attack are "deep-rooted," there are ways to defend against it. The SameSite feature that removes cookies from cross-site requests is one defense. Microsoft and Mozilla have adopted SameSite in their browsers, he noted.

"It is a function that can comprehensively prevent CSRF attacks including our attack. Based on our attack demonstration, it was installed in most major browsers," he said.

Social media users can protect themselves either via private browsing mode or by signing out of their social media accounts when browsing elsewhere.

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