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// IoT & industrial (ICS)

Port 102: Siemens S7 (ISO-TSAP)

Siemens S7 PLC communication and ISO transport.

ISO-TSAPS7comm
102

How Siemens S7 (ISO-TSAP) works

IoT & industrial (ICS)
controllerPLCS7 writeno authentication controllerPLCS7 writeno authentication

A controller sends commands to an industrial device on port 102, which acts on the physical process. It travels in clear text: anyone on the path can read it.

Transport
TCP · UDP
Exposure risk
10/10 critical if reachable from the Internet
Encryption
Clear textreadable on the network
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
IoT & industrial (ICS)
01 Security

Is port 102 dangerous?

Common attacks and weaknesses

  • PLC program upload and download, stop or start commands (Stuxnet era tooling).

How to secure it

  • Isolate OT networks and enable PLC access protection.
02 Registry

IANA registry entries for port 102

ServicePortTransportDescriptionReferenceUpdated
iso-tsap 102 TCP, UDP ISO-TSAP Class 0 - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 102, and is it reachable?

Linux

sudo ss -ltnup 'sport = :102'      # process listening on 102
sudo lsof -nP -i :102

macOS

sudo lsof -nP -i :102

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 102 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 102
netstat -ano | findstr :102

From another machine

nmap -p 102 -sV <host>
sudo nmap -sU -p 102 <host>
nc -zv <host> 102

Block it in the firewall

sudo ufw deny 102/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 102" -Direction Inbound -LocalPort 102 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 102 used for?

Siemens S7 PLC communication and ISO transport.

Is port 102 TCP or UDP?

Port 102 is registered for TCP and UDP. In practice the service mostly uses TCP.

Is it safe to open port 102?

It is rated 10/10 (critical) when exposed to the Internet. PLC program upload and download, stop or start commands (Stuxnet era tooling). Isolate OT networks and enable PLC access protection.

How do I check if port 102 is open?

On Linux run "ss -ltnup 'sport = :102'", on macOS "lsof -nP -i :102", and on Windows "netstat -ano | findstr :102" to see the local process. From another machine, "nmap -p 102 <host>" shows whether it is reachable.

05 Related
:1883 MQTT tcp · udp · high risk :8883 MQTT over TLS tcp · udp · moderate risk :502 Modbus TCP tcp · udp · critical risk :47808 BACnet udp · tcp · critical risk :20000 DNP3 tcp · udp · sctp · critical risk :44818 EtherNet/IP tcp · udp · critical risk :802 Modbus Application Protocol Secure tcp · udp :2221 EtherNet/IP over TLS tcp · udp

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