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RAT provides attackers with full remote access to a compromised system — controlling the desktop, activating camera/microphone, keylogging, file access, and spreading to other systems.
RATs are the tool of APT groups for long-term access. Famous RATs: AsyncRAT, QuasarRAT, Cobalt Strike. Detection: unusual outbound connections, EDR behavioral alerts. Cover cameras on laptops — RATs can activate them silently.