Threat Group Exploits Chrome Bug to Serve Malicious Ads to iOS Users

In one of the biggest malvertising campaigns in the last 18 months, a previously known threat group called eGobbler is taking advantage of a security bug in Google's Chrome browser to target millions of iOS users. 

Security vendor Confiant, which has been tracking the campaign since it launched April 6, estimates that more than 500 million malicious ads have been served to iOS users already. Users are being redirected to scam "You've won a gift card" landing pages hosted on a top-level domain previously associated with eGobbler.

Google, which makes most of its money from online advertisements, is currently working on a fix for the bug after being notified about the issue April 11, Confiant said in a report Tuesday. The company did not respond immediately to a request for comment.

According to Confiant, the problem exists in the manner in which Chrome for iOS handles pop-ups. Like other browsers, Chrome incorporates ad sandboxing features to ensure that any code used to insert ads into a Web page only has limited ability to interact with other components.

Sandboxing is a method of restricting what actions are available to any advertisement that is served from a different domain than the page hosting it, says Eliya Stein, senior security engineer at Confiant. The goal is to prevent malicious advertisements from hijacking browser sessions via pop-ups and redirects to websites and landing pages the user did not intend to visit.

Normally, an ad sandbox should prevent a pop-up from being launched unless the user takes some direct action to enable it. The Chrome vulnerability allows attackers a way around this protection.

Chrome Sandboxing Fail
According to Stein, the exploit that eGobbler has developed and is using with such success essentially tricks Chrome for iOS into allowing pop-ups without any direct interaction. "The security bug in Chrome is around Chrome’s built-in pop-up blocker," Stein says. "All versions of Chrome on iOS are impacted."

Because the eGobbler exploit allows the attacker to redirect a user with a pop-up, any other standard sandboxing protections that Chrome has against browser redirections, such as disallowing JavaScript, are moot, he says. "We believe that this exploit was key in magnifying the impact of this attack," Stein says.

He adds that Confiant wants to give Google's Chrome team a reasonable amount of time to fix the bug before releasing more details on how it works. Confiant will release a full analysis of the bug at a later date.

So far, eGobbler has launched eight individual malvertising campaigns mostly targeting iOS users in the US over a six-day period starting April 6. Each individual campaign has lasted between one to two days, Confiant said.

The threat group managed to place over 30 malicious advertisements on legitimate but previously compromised ad servers and used cloaked intermediate CDN domains as part of their ad delivery.

"The CDN domains are used to host the payload that performs the actual redirect and/or the pop-up," Stein says. These are intermediate domains in the ad-serving process that attackers often use for loading ad-serving code. "Attackers rotate these kinds of domains often in attempt to fly under the radar," Stein noted.

The original campaign targeting iOS users has now pivoted to another platform and is ongoing, though Stein declined to name the targeted platform. It is not clear whether eGobbler is exploiting the same Chrome bug, but chances are high they are, he says. "This requires some follow-up on our end," Stein says.

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