Custom detections and MDR: what coverage gap do teams miss?

TL;DR: Custom detections often remain the internal team’s responsibility because MDR services investigate the detections they ship, not the rules customers write for their own environment, according to Prophet. That makes custom-alert ownership, tuning, and 24/7 investigation a structural SOC gap rather than a tooling preference.

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: 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.

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.

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.

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