CSS Exfil Protection is a browser extension for Mozilla Firefox and Google Chrome that protects data against CSS Exfil attacks.
Internet users who have a good understanding of online security know that JavaScript is a great technology but also something that can be used in attacks. There are plenty of solutions available to deal with JavaScript-based attacks including using content blockers like uBlock Origin, extensions like NoScript that block JavaScript executions, or disabling JavaScript outright (the latter is not very practical).
An attack, named CSS Exfil (from exfiltrate), uses CSS to steal data. Mike Gualtieri, the researcher who discovered the vulnerability, published several proof of concept attacks designed to steal usernames, passwords, and other data on web pages it is used on.