An Intrusion Detection System (IDS) is a passive monitoring tool that detects unauthorized or malicious activity in networked systems. It does not block traffic (like an IPS), but rather alerts administrators to potential breaches.

“An IDS monitors network or system activities for malicious actions or policy violations and produces alerts or logs for analysis.”

— ISA/IEC 62443-3-3:2013, SR 3.2 – Detection of Security Events

It’s a core component of security monitoring and response — often paired with an Incident Response Plan (IRP) as defined in ISA/IEC 62443-2-1.

Clarification of Options:

Option A is metaphorical and not technically accurate.

Option B is false; IDS does not protect against all vulnerabilities.

Option C is incorrect; IDS does not block, only detects.

Option D is correct — it detects unauthorized access or misuse.