Security for SMS messages through serial port to GMS device

We have a need to send out simple text messages (alarms) from an HMI PC based system's RS232 serial port to a GMS device. When the message is sent out the operator can respond back with a code that is embedded in the message to acknowledge that the alarm was received. The communication is two way (otherwise a data diode would work for security).

First if the system were to fail it would not end up as a security issue. 2nd if someone intercepted the message and changed the data it would also not be a security issue, the loss of the information isn't going to be a security issue either. I'm not worried about an inside the fence security attack, much easier ways to cause damage once inside the fence.

The software isn't being used to telnet or SSH into the PC, it is only doing messaging and only that software has control over the serial RS232 port. If an attacker flooded the system until the buffers overran or became full and the entire PC locked up that would be an issue, if only the application locked up it would not be an issue.

I'm sure I'm missing something but what additional security measures need to be in place. I suppose the domain controllers could limit the serial RS232 port for just the program that sends the messages.
One thing you might consider is using encryption to protect the content of your messages. This would help prevent anyone from intercepting and reading the messages if they were able to access the communication channel. Another option might be to use a virtual private network (VPN) to secure the connection between the HMI PC and the GMS device.
In order to enhance the security of your system while sending text messages (alarms) from an HMI PC-based system's RS232 serial port to a GSM device, there are several additional security measures you can consider:

1. Secure Communication Protocol: Implement a secure communication protocol, such as SSL/TLS, to encrypt the messages being transmitted between the HMI PC and the GSM device. This ensures that the data remains confidential and protected against interception or tampering.

2. Access Control: Restrict access to the HMI PC and the RS232 serial port to authorized personnel only. Implement strong password policies, user authentication mechanisms, and role-based access control to prevent unauthorized access to the system.

3. Firewall and Intrusion Detection/Prevention Systems: Deploy a firewall to monitor and control the incoming and outgoing network traffic to the HMI PC. Additionally, consider implementing intrusion detection/prevention systems (IDS/IPS) to detect and respond to any suspicious or malicious activities.

4. Physical Security: Ensure the physical security of the HMI PC and the RS232 serial port. Limit physical access to authorized personnel and protect the system against theft, tampering, or unauthorized modifications.

5. Logging and Auditing: Enable detailed logging and auditing of all activities related to the messaging system. This includes capturing information such as message transmissions, acknowledgments, and any potential security events. Regularly review and analyze the logs to identify any anomalies or potential security breaches.

6. Software Updates and Patch Management: Keep the HMI PC's operating system, messaging software, and security-related applications updated with the latest patches and security updates. Regularly check for vendor-provided security advisories and apply necessary patches to address any known vulnerabilities.

7. Security Awareness and Training: Conduct security awareness programs and training sessions for the operators and users of the system. Educate them about the importance of security practices, such as avoiding suspicious links or attachments and following secure communication protocols.

8. Backup and Disaster Recovery: Implement a robust backup and disaster recovery plan to ensure that critical data and configurations are regularly backed up and can be restored in case of system failures, data loss, or security incidents.

9. Regular Security Assessments: Conduct periodic security assessments, vulnerability scans, or penetration testing to identify any potential weaknesses or vulnerabilities in the system. Address the identified issues promptly to maintain a secure environment.

By implementing these additional security measures, you can enhance the overall security of your HMI PC-based system and protect the confidentiality, integrity, and availability of your messaging system.