Skip to content

T0805 Block Serial COM

Adversaries may block access to serial COM to prevent instructions or configurations from reaching target devices. Serial Communication ports (COM) allow communication with control system devices. Devices can receive command and configuration messages over such serial COM. Devices also use serial COM to send command and reporting messages. Blocking device serial COM may also block command messages and block reporting messages.

A serial to Ethernet converter is often connected to a serial COM to facilitate communication between serial and Ethernet devices. One approach to blocking a serial COM would be to create and hold open a TCP session with the Ethernet side of the converter. A serial to Ethernet converter may have a few ports open to facilitate multiple communications. For example, if there are three serial COM available – 1, 2 and 3 –, the converter might be listening on the corresponding ports 20001, 20002, and 20003. If a TCP/IP connection is opened with one of these ports and held open, then the port will be unavailable for use by another party. One way the adversary could achieve this would be to initiate a TCP session with the serial to Ethernet converter at via Telnet on serial port 1 with the following command: telnet 20001.

Item Value
ID T0805
Tactics TA0107
Platforms Device Configuration/Parameters, Field Controller/RTU/PLC/IED, Input/Output Server
Version 1.1
Created 21 May 2020
Last Modified 20 October 2022

Procedure Examples

ID Name Description
S0604 Industroyer In Industroyer the first COM port from the configuration file is used for the actual communication and the two other COM ports are just opened to prevent other processes accessing them. Thus, the IEC 101 payload component is able to take over and maintain control of the RTU device. 1


ID Mitigation Description
M0807 Network Allowlists Implement network allowlists to minimize serial comm port access to only authorized hosts, such as comm servers and RTUs.
M0930 Network Segmentation Restrict unauthorized devices from accessing serial comm ports.
M0810 Out-of-Band Communications Channel Ensure devices have an alternative method for communicating in the event that a valid COM port is unavailable.


ID Data Source Data Component
DS0015 Application Log Application Log Content
DS0029 Network Traffic Network Traffic Flow
DS0040 Operational Databases Process History/Live Data
DS0009 Process Process Termination