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// Other services

Port 4728: CA Port Multiplexer

Port 4728 is assigned by IANA to capmux (CA Port Multiplexer).

4728

How capmux works

Other services
client:4728capmuxresponse client:4728capmuxresponse

The client sends a request to port 4728 and the capmux service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Registered (1024-49151)
Category
Other services
01 Security

Is port 4728 dangerous?

We have no specific security note for port 4728. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 4728

ServicePortTransportDescriptionReferenceUpdated
capmux 4728 TCP, UDP CA Port Multiplexer - 2006-06

// source: IANA service name and port number registry

03 Check it

Which program uses port 4728, and is it reachable?

Linux

sudo ss -ltnup 'sport = :4728'      # process listening on 4728
sudo lsof -nP -i :4728

macOS

sudo lsof -nP -i :4728

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 4728 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 4728
netstat -ano | findstr :4728

From another machine

nmap -p 4728 -sV <host>
sudo nmap -sU -p 4728 <host>
nc -zv <host> 4728

Block it in the firewall

sudo ufw deny 4728/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 4728" -Direction Inbound -LocalPort 4728 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 4728 used for?

Port 4728 is assigned by IANA to capmux (CA Port Multiplexer).

Is port 4728 TCP or UDP?

Port 4728 is registered for TCP and UDP.

Is it safe to open port 4728?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Close the port if nothing on the host needs it.

How do I check if port 4728 is open?

On Linux run "ss -ltnup 'sport = :4728'", on macOS "lsof -nP -i :4728", and on Windows "netstat -ano | findstr :4728" to see the local process. From another machine, "nmap -p 4728 <host>" shows whether it is reachable.

05 Related
:4444 Metasploit default / krb524 tcp · udp · high risk :31337 Back Orifice / elite tcp · udp · high risk :1 TCP Port Service Multiplexer tcp · udp :5 Remote Job Entry tcp · udp :9 Discard tcp · udp · sctp · dccp :11 Active Users tcp · udp :17 Quote of the Day tcp · udp :18 Message Send Protocol (historic) tcp · udp

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