Rootkits are programs that hide the existence of malware by intercepting (i.e., Hooking) and modifying operating system API calls that supply system information.  Rootkits or rootkit enabling functionality may reside at the user or kernel level in the operating system or lower, to include a Hypervisor, Master Boot Record, or the System Firmware. 
Adversaries may use rootkits to hide the presence of programs, files, network connections, services, drivers, and other system components. Rootkits have been seen for Windows, Linux, and Mac OS X systems.  
|Hacking Team UEFI Rootkit|
Umbreon hides from defenders by hooking libc function calls, hiding artifacts that would reveal its presence, such as the user account it creates to provide access and undermining strace, a tool often used to identify malware.
This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.
Some rootkit protections may be built into anti-virus or operating system software. There are dedicated rootkit detection tools that look for specific types of rootkit behavior. Monitor for the existence of unrecognized DLLs, devices, services, and changes to the MBR. 
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