NetSecWallFree trialDownload
Port 23 · TCP

Port 23: Telnet

Telnet, an unencrypted remote command line from the 1970s. Everything, passwords included, crosses the network in plain text.

Protocol
TCP
On Windows
Not part of Windows; installed software uses it
Risk if open to the internet
High

Seeing it on your PC

Almost nothing on a modern PC uses it. Some old network gear is still managed over Telnet. Botnets that infect routers and cameras scan the internet for port 23.

Should you block it?

Block it in both directions unless you manage a device that needs it. An unknown program connecting out to port 23 deserves a close look.

Check it on your PC

Run this in PowerShell to see what is using port 23 right now, and which program owns it:

Get-NetTCPConnection -LocalPort 23 -ErrorAction SilentlyContinue | Select-Object LocalAddress,LocalPort,RemoteAddress,RemotePort,State,@{n='Process';e={(Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue).ProcessName}}

No output means nothing is using the port at the moment. For every connection at once, paste netstat -ano into the netstat explainer.

Block it in Windows Firewall

In PowerShell run as administrator, this stops other computers from connecting to this port on your PC. It does not stop your own programs connecting out.

New-NetFirewallRule -DisplayName "Block inbound TCP 23" -Direction Inbound -Protocol TCP -LocalPort 23 -Action Block

Undo it with Remove-NetFirewallRule -DisplayName "Block inbound …", using the same name. If you depend on the service on your local network, this rule stops that too.