3389
Windows Remote Desktop graphical remote access (RDP)
Remote Desktop Protocol
What is port 3389?
RDP (Remote Desktop Protocol) on port 3389 provides graphical remote access to Windows systems. It is widely used for remote work, server management, and IT support. It is one of the most heavily attacked ports on the internet, subject to brute-force attacks, the BlueKeep wormable exploit (CVE-2019-0708), and as a ransomware delivery mechanism. Never expose RDP directly to the internet — place it behind a VPN or RD Gateway, and always enable NLA (Network Level Authentication).
Quick checks
$
nc -zv example.com 3389
$
nmap -p 3389 example.com
$
telnet example.com 3389