23
Obsolete unencrypted remote terminal protocol
Telnet
What is port 23?
Telnet was the first widely used remote access protocol, but it transmits everything — passwords, commands, and data — as unencrypted plaintext, making it trivially vulnerable to network sniffing. It has been fully superseded by SSH (port 22). Today it is used only for low-level network debugging (e.g., testing TCP connectivity to a specific port). An open port 23 on a public-facing server is a serious security red flag that should be addressed immediately.
Quick checks
$
nc -zv example.com 23
$
nmap -p 23 example.com
$
telnet example.com 23