An attacker is able to trick an IDS into ignoring malicious traffic through obfuscation of the packet payload. What type of IDS error has occurred?

A true negative means no attack occurred and the IDS correctly produced no alert, which is the normal benign-traffic scenario.

A true positive means an attack occurred and the IDS correctly detected and alerted on it, the opposite of what happened here.

A false positive means the IDS incorrectly flagged benign traffic as malicious, whereas here the IDS incorrectly cleared actual malicious traffic.

A false negative occurs when malicious activity is present but the detection system does not raise an alert. By obfuscating the payload, the attacker caused the IDS to treat the malicious traffic as benign, meaning the system returned a 'not malicious' verdict on traffic that was, in fact, malicious - the classic definition of a false negative.