Block Command Message

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Block Command Message
Technique
ID T0803
Tactic Inhibit Response Function
Data Sources Alarm History, Network protocol analysis, Packet capture
Asset Field Controller/RTU/PLC/IED

Description

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

In the 2015 attack on the Ukranian power grid, malicious firmware was used to render communication devices inoperable and effectively prevent them from receiving remote command messages.2


Procedure Examples

  • 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

Mitigations

  • 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.
  • 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.
  • Static Network Configuration - Unauthorized connections can be prevented by statically defining the hosts and ports used for automation protocol connections.