First of all, don’t panic—this browser vulnerability has been fixed, so the headline is completely out of proportion to the reality. But my goodness, this was a clever technique!
The technique relies on luring users to a malicious site where the attacker embeds iframes to other sites. In their example, the two embedded iframes for one of Facebook’s social widgets, but other sites are also susceptible to this issue.
The attack consists of overlaying a huge stack of DIV layers with different blend modes on top of the iframe. These layers are all 1x1 pixel-sized, meaning they cover just one pixel of the iframe.
Habalov and Weißer say that depending on the time needed to render the entire stack of DIVs, an attacker can determine the color of that pixel shown on the user’s screen.
The researchers say that by gradually moving this DIV “scan” stack across the iframe, “it is possible to determine the iframe’s content.”