Rootkits are programs that hide the existence of malware by intercepting and modifying operating-system API calls that supply system information. Firmware rootkits that affect the operating system yield nearly full control of the system. Stuxnet contains code to monitor and intercept requests to read requests for its own malicious code blocks. Digital signatures may be used to ensure application DLLs are authentic prior to execution. By tampering with device processes, an adversary may inhibit its expected response functions and possibly enable Impact.”]
Source: https://collaborate.mitre.org/attackics/index.php/Technique/T0851