Because Tuff Client is a third-party modification, you are trusting the developer with the code that runs inside your browser. Since it’s JavaScript, you can theoretically inspect the source code (press F12 > Sources tab) to look for malicious segments like:
Since "tuff client" can sometimes refer to a specific hacked client or performance-tuned version of Eaglercraft, here’s a to guide you through common problems and solutions. tuff client eaglercraft 1.12 2