Adversaries may attempt to leverage Application Program Interfaces (APIs) used for communication between control software and the hardware. Specific functionality is often coded into APIs which can be called by software to engage specific functions on a device or other software.
|A0002||Human-Machine Interface (HMI)|
|A0005||Intelligent Electronic Device (IED)|
|A0003||Programmable Logic Controller (PLC)|
|A0004||Remote Terminal Unit (RTU)|
Access Management technologies can be used to enforce authorization policies and decisions, especially when existing field devices do not provide capabilities to support user identification and authentication.  These technologies typically utilize an in-line network device or gateway system to prevent access to unauthenticated users, while also integrating with an authentication service to first verify user credentials.
All APIs used to perform execution, especially those hosted on embedded controllers (e.g., PLCs), should provide adequate authorization enforcement of user access. Minimize user's access to only required API calls. 
Minimize the exposure of API calls that allow the execution of code.
|M0804||Human User Authentication||
All APIs on remote systems or local processes should require the authentication of users before executing any code or system changes.
|ID||Data Source||Data Component||Detects|
|DS0009||Process||OS API Execution||
Devices that provide user access to the underlying operating system may allow the installation of custom software to monitor OS API execution. Monitoring API calls may generate a significant amount of data and may not be useful for defense unless collected under specific circumstances, since benign use of API functions are common and may be difficult to distinguish from malicious behavior. Correlation of other events with behavior surrounding API function calls using API monitoring will provide additional context to an event that may assist in determining if it is due to malicious behavior.