Port 0: The Port That Does Not Exist

Researchers scanned the entire IPv4 address space for services listening on port 0, a port number typically reserved by operating systems for dynamic allocation and considered unusable. They discovered that port 0 is not technically forbidden by TCP specifications, only "reserved," which allowed some systems to inadvertently or intentionally expose services on it—making it an effective hiding place since most security tools and browsers skip it by default. The findings reveal a gap between how the sockets API treats port 0 (as a request for any available port) and how it can function on the wire as an actual destination port.