Until now, the global security community believed that TCP ports end at 65535. This assumption was widely accepted, documented, and implemented in virtually every scanner on the market. It is rooted in a simple math problem involving only 0 and 1. The calculation appeared sound, was easy to implement, and therefore became an unquestioned industry standard. For decades, scanners, specifications, and compliance frameworks have relied on it.
After extensive research and multiple confirmed sightings during manual security assessments, I can finally disclose what many suspected but could never prove:
TCP port 65536 does exist.
Port 65536 appears precisely at the moment when automated tooling confidently reports “no findings” and moves on. In many cases, the most critical issues were discovered shortly after traffic on this port was “observed”.
Administrators attempting to block 65536/tcp via firewall rules have reported no success, as firewalls consistently claim that the port is out of range.
Further research is ongoing.
