Browser Blocking Port 6000? Cross-Protocol Scripting Vulnerability Strikes!

While working on a Docker and Flask project, the author encountered an issue where port 6000 was inaccessible, while port 8000 functioned correctly. Investigation revealed that browsers actively block certain ports, including 6000, as a security measure against Cross-Protocol Scripting (XSS) vulnerabilities. Chrome displays an ERR_UNSAFE_PORT error, Safari shows a blank page, and Firefox provides a more informative "This address is restricted" message. The browser cancels requests to these ports before they are even sent. The author confirmed the server was functioning correctly using curl, isolating the issue to browser security policies. The article lists Firefox's blocked ports and their services, explaining how this protection works.