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 10.0.0.1 via Telnet on serial port 1 with the following command: telnet 10.0.0.1 20001.
|C0028||2015 Ukraine Electric Power Attack||
During the 2015 Ukraine Electric Power Attack, Sandworm Team overwrote the serial-to-ethernet converter firmware, rendering the devices not operational. This meant that communication to the downstream serial devices was either not possible or more difficult. 
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. 
|A0002||Human-Machine Interface (HMI)|
|A0005||Intelligent Electronic Device (IED)|
|A0003||Programmable Logic Controller (PLC)|
|A0004||Remote Terminal Unit (RTU)|
|A0011||Virtual Private Network (VPN) Server|
Implement network allowlists to minimize serial comm port access to only authorized hosts, such as comm servers and RTUs.
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||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 messages over serial COM ports are blocked.
Monitor for the termination of processes or services associated with ICS automation protocols and application software which could help detect blocked communications.