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.

NHIMG editorial — based on content published by Prophet: Why Your MDR Won't Cover the Custom Detections You Built

Questions worth separating out

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 identity-heavy custom detections create a coverage gap in outsourced SOC models?

A: Because they often depend on local authentication patterns, privilege states, or service account behaviour that generic content does not understand.

Q: How do security teams know whether a custom detection is actually being handled well?

A: Look for documented ownership, clear response criteria, preserved evidence, and consistent investigation outcomes.

Practitioner guidance

• Document rule ownership and intent Assign each custom detection a named owner, a business rationale, the signals it depends on, and the exact conditions that justify closure or escalation.
• Separate triage from investigation in contracts Define whether the MDR provider only receives custom alerts, performs tier-1 triage, or carries out full investigation.
• Build evidence packs for high-context alerts For each custom detection, predefine the SIEM queries, identity provider checks, and cloud telemetry needed to confirm or dismiss the alert.

What's in the full article

Prophet's full article covers the operational detail this post intentionally leaves for the source:

• How Prophet frames the difference between a standard MDR library and customer-authored detections in day-to-day SOC operations.
• The article's full side-by-side coverage of where custom alerts stay with the internal team versus where MDR triage begins.
• More detail on the AI SOC investigation workflow and the evidence trail used to validate custom detections.
• The vendor's comparison criteria for teams evaluating MDR coverage against in-house detection engineering needs.

👉 Read Prophet's analysis of why custom detections fall outside MDR coverage →

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

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