Posted: December 20, 2017 by
Jérôme Segura
Tech support scammers have been relying on fraudulent pop-ups for many years in order to scare potential victims into calling for remote assistance. These so-called browser lockers (or browlocks) typically originate from malicious ads (malvertising) that can appear on any website, including trusted online portals.
The purpose of browser lockers is not only to scare but also to create the illusion that the computer has been locked, which is not quite true. What’s happened is simply that the browser is stuck in between a flurry of alert dialogs that won’t seem to go away, no matter how many times they are clicked on.
Google Chrome is often the most-targeted browser because of its dominant market share, but pop-ups come in as many different flavors as browser types, with landing pages specific to those browsers. For example, a particularly vicious technique abused the history.pushState HTML5 API to literally freeze machines while displaying the fake pop-up.
Historically, browser makers have let users down by not being to handle those tricks cleanly. However, they appear to have taken note, fixing many of the issues that have to do with poor user experience, while also suggesting other ways for (legitimate) webmasters to send notifications, for example via the proper
Notifications API.
Unfortunately, crooks are adapting as well. Despite browser developers’ best intentions, browlocks are still the best bet to scam innocent folks. The following shows a browser locker that went into full screen mode after the user clicked somewhere on the page. Pressing the Escape key to exit full screen (as instructed by the browser) triggered a malicious loop in the code that prevented closing the fraudulent pop-up (without resorting to Task Manager):
This is a similar technique to what we reported on recently with
persistent drive-by mining attacks in that it uses a pop-under as a “helper.” There are actually three different layers in play to make this work:
- a background window in full screen mode
- another window that is superimposed (triggered on click or Escape key)
- the pop-under (triggered on click)
