Common ports, such as TCP port 80 (HTTP), may be locked down — but other ports may get overlooked and be vulnerable to hackers. In your security tests, be sure to check these commonly hacked TCP and UDP ports:
TCP port 21 – FTP (File Transfer Protocol)
TCP port 22 – SSH (Secure Shell)
TCP port 23 – telnet
TCP port 25 – SMTP (Simple Mail Transfer Protocol)
TCP and UDP port 53 – DNS (Domain Name System)
TCP port 443 – HTTP (Hypertext Transport Protocol) and HTTPS (HTTP over SSL)
TCP port 110 – POP3 (Post Office Protocol version 3)
TCP and UDP port 135 – Windows RPC
TCP and UDP ports 137–139 – Windows NetBIOS over TCP/IP
TCP port 1433 and UDP port 1434 – Microsoft SQL Serve