Are custom detections outside MDR coverage by default?

TL;DR: Custom detections often stay with the internal team, Prophet reports, because MDR services cover the detections they ship, not the rules customers write for their own environment. That leaves ownership, tuning and after-hours investigation with the team running the SIEM.

Editorial analysis by NHI Mgmt Group, based on content published by Prophet: “Why Your MDR Won't Cover the Custom Detections You Built”.

Key questions

Q: Why do custom detections create a governance problem for SOC teams?

A: Because the rule often depends on business logic, identity patterns or application behaviour that only internal teams understand.

Q: How should teams decide which detections belong inside MDR coverage?

A: Use the level of context required to judge the alert.

Q: What breaks when custom detections are handed to MDR without local context?

A: The rule may still be visible, but the provider often lacks the environment-specific context needed to judge it correctly.

Practitioner guidance

• Define ownership for every custom detection Assign a named internal owner for each customer-written rule, including who tunes it, who interprets it and who approves changes to its logic.
• Separate vendor content from custom rules in triage Route MDR vendor detections and custom SIEM detections through different runbooks so the latter receive context-rich investigation rather than generic closure paths.
• Measure the alert share from custom logic Track how much of alert volume comes from custom detections and identify which of those alerts require specialised context to resolve.

Bottom line: Custom detections often fall outside standard MDR coverage because the alerts require local context that a shared service does not own.

Glossary terms: Custom Detection, Managed Detection And Response, Detection Engineering, Triage Ownership

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