The correct answer is Malicious insider.

Malicious insider refers to a staff member who has legitimate system privileges and intentionally exploits those privileges to perform unauthorized activities or cause harm. The emphasis is on the combination of authorized access and deliberate malicious intent which distinguishes this actor from accidental or negligent users.

Compromised credentials describes credentials that have been stolen or used by an unauthorized party and does not imply a staff member who knowingly abuses valid privileges.

Advanced persistent threat typically denotes a sustained and targeted external attack campaign by organized groups and it does not describe an internal employee misusing their legitimate access.

Grey hat hacker refers to an external actor who may break rules sometimes for perceived benefit and sometimes without malicious intent and it does not capture the concept of a trusted staff member deliberately abusing their privileges.

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