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A set of tools used by an attacker after gaining root-level access to a host to conceal the attacker’s activities on the host and permit the attacker to maintain root-level access to the host through covert means.
NIST SP 800-150 under Rootkit
A collection of files that is installed on a host to alter the standard functionality of the host in a malicious and stealthy way.
NIST SP 800-83 Rev. 1 under Rootkit