Some of the recent additions to the Cascading Style Sheets (CSS) web standard are so powerful that a security researcher has abused them to deanonymize visitors to a demo site and reveal their Facebook usernames, avatars, and if they liked a particular web page of Facebook.
Information leaked via this attack could aid some advertisers link IP addresses or advertising profiles to real-life persons, posing a serious threat to a user's online privacy.
The leak isn't specific to Facebook but affects all sites which allow their content to be embedded on other web pages via iframes.
Vulnerability resides in browsers, not websites
The actual vulnerability resides in the browser implementation of a CSS feature named "
mix-blend-mode," added in 2016 in the CSS3 web standard.
The mix-blend-mode feature allows web developers to stack web components on top of each other and add effects for controlling to the way they interact.
As the feature's name hints, these effects are inspired by the blend modes found in photo editing software like Photoshop, Gimp, Paint.net, and others. Example blend modes are Overlay, Darken, Lighten, Color Dodge, Multiply, Inverse, and others.
The CSS3 mix-blend-mode feature supports 16 blend modes and is fully
supported in Chrome (since v49) and Firefox (since v59), and partially supported in Safari (since v11 on macOs and v10.3 on iOS).
