Adversaries may block or prevent a reporting message from reaching its intended target. In control systems, reporting messages contain telemetry data (e.g., I/O values) pertaining to the current state of equipment and the industrial process. By blocking these reporting messages, an adversary can potentially hide their actions from an operator.
Blocking reporting messages in control systems that manage physical processes may contribute to system impact, causing inhibition of a response function. A control system may not be able to respond in a proper or timely manner to an event, such as a dangerous fault, if its corresponding reporting message is blocked.  
Industroyer uses the first COM port from the configuration file for the communication and the other two COM ports are opened to prevent other processes accessing them. This may block processes or operators from getting reporting messages from a device. 
In the Ukraine 2015 Incident, Sandworm Team blocked reporting messages by using malicious firmware to render communication devices inoperable. 
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 report messages to operators, this could include using radio/cell communication to obtain messages from field technicians that can locally obtain telemetry and status data.
|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.
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 reporting messages are blocked.
Monitor for the termination of processes or services associated with ICS automation protocols and application software which could help detect blocked communications.