Block Command Message

Adversaries may block a command message from reaching its intended target to prevent command execution. In OT networks, command messages are sent to provide instructions to control system devices. A blocked command message can inhibit response functions from correcting a disruption or unsafe condition. [1] [2]

ID: T0803
Sub-techniques:  No sub-techniques
Platforms: Device Configuration/Parameters, Field Controller/RTU/PLC/IED
Version: 1.0
Created: 21 May 2020
Last Modified: 24 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. [3]

G0034 Sandworm Team

In the Ukraine 2015 Incident, Sandworm Team blocked command messages by using malicious firmware to render communication devices inoperable. [2]


ID Mitigation Description
M0807 Network Allowlists

Utilize network allowlists to restrict unnecessary connections to network devices (e.g., comm servers, serial to ethernet converters) and services, especially in cases when devices have limits on the number of simultaneous sessions they support.

M0810 Out-of-Band Communications Channel

Provide an alternative method for sending critical commands message to outstations, this could include using radio/cell communication to send messages to a field technician that physically performs the control function.

M0814 Static Network Configuration

Unauthorized connections can be prevented by statically defining the hosts and ports used for automation protocol connections.


ID Data Source Data Component Detects
DS0015 Application Log Application Log Content

Monitor application logs for changes to settings and other events associated with network protocols that may be used to block communications.

DS0029 Network Traffic Network Traffic Flow

Monitor for a loss of network communications, which may indicate this technique is being used.

DS0040 Operational Databases Process History/Live Data

Monitor for lack of operational process data which may help identify a loss of communications. This will not directly detect the technique’s execution, but instead may provide additional evidence that the technique has been used and may complement other detections.

Process/Event Alarm

Monitor asset alarms which may help identify a loss of communications. Consider correlating alarms with other data sources that indicate traffic has been blocked, such as network traffic. In cases where alternative methods of communicating with outstations exist alarms may still be visible even if command messages are blocked.

DS0009 Process Process Termination

Monitor for the termination of processes or services associated with ICS automation protocols and application software which could help detect blocked communications.