Over 500 Million Chrome iOS Sessions Hijacked in Massive Malvertising Campaign

Confiant has spotted the known threat actor eGobbler back in action. This group is well-known for attempting its campaigns around holidays, which is when it assumes there will be increased consumer traffic.

But this go-around the group is exploiting a currently unpatched flaw in the Chrome for iOS browser, to bypass sandboxing and hijack user sessions. This means that iPhone and iPad users are directly targeted.

eGobbler has historically used the ".world" TLD for their landing media like pop-ups, but this campaign seems to be using ".site" TLDs for web-based content.

eGobbler's main session hijacking mechanism in this case, however, was pop-up based. This is a change in strategy since when a browser's pop-up blocker is working it will stop the pop-up from appearing. Session hijacking happens when a user is redirected to another site or landing page. Alternatively, a pop-up can appear that one can't exit out of. The served pages look like ads from well-known brands, but they aren't. If clicked, a malicious payload is then deployed to the user.

Confiant has said that they "discovered techniques that took advantage of iOS Chrome's detection around user activated pop-up detection, resulting in the circumvention of pop-up blocking." Since Chrome remains unpatched at the time of this writing, Confiant is not revealing the exact code details of how this is done. That the exploit is able to bypass that need for user interaction should be impossible according to the same-origin policy (as it pertains to cross-origin iframes).

Additionally, this exploit also completely circumvents the browser's anti-redirect functionality. The attacker no longer needs to even spawn a redirect in order to hijack the user session. This latest hijacking lasted six days and started on Saturday, April 6. Confiant say that it was composed of eight individual campaigns and over 30 fake creatives. While the targeting was primarily US based, European publishers saw significant impact as well.

Confiant thinks over 500 million user sessions were exposed to the hijacking. They also said that "With half a billion user sessions impacted, this is among the top three massive malvertising campaigns that we have seen in the last 18 months."

eGobble will typically use cloaked intermediate CDN domains as part of their ad delivery. The domains may sit behind at least a single layer of client-side fingerprinting.

There remains a question about whether some of the hijacking tricks that are being used on Chrome can also be applied to Apple's Safari. Until explicit details of the exploit are known, no solid answer can be made.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.