A malvertising operation observed last year by advertising cybersecurity company Confiant exploited what turned out to be a zero-day vulnerability in the WebKit browser engine.
Confiant researchers discovered the security hole while analyzing a campaign carried out by a threat actor they call ScamClub. The group has been around for several years, launching malvertising attacks designed to redirect users to a wide range of scam websites promising prizes.
ScamClub specializes in high-volume operations — even if most of their payloads are blocked, a large number still reach users.
“Over the last 90 days, ScamClub has delivered over 50MM malicious [ad] impressions, maintaining a low baseline of activity augmented by frequent manic bursts — with as many as 16MM impacted ads being served in a single day,” Confiant said in a blog post on Tuesday.
The “allow-top-navigation-by-user-activation” attribute in WebKit’s iframe sandboxing feature is designed to prevent malicious redirections by only allowing a redirection to occur when it’s triggered by user actions (e.g. a click or a tap inside the frame).
However, Confiant discovered that the ScamClub threat actor managed to bypass this iframe sandboxing mechanism by using an event listener for a “message” event. If the event listener picks up a message, it would trigger the redirect, which increases the chances of users being redirected to their scam websites without actually clicking inside their iframe to directly trigger the redirect.
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