Sentinel Detection Engineering

When to use

sentinel

defender-xdr

Sources of content (use in this order)

1. Content Hub solutions — Microsoft- and partner-published, versioned bundles of data connectors, analytics rules, hunting queries, workbooks, playbooks. Always start here. Install the solution; enable rules in Active.
2. Microsoft Sentinel GitHub — community-contributed queries that haven't (yet) made it into a solution. Vet, then promote into your repo.
3. Custom detections — domain-specific, authored against your own data sources or specific business logic.

Rule of thumb: If a Microsoft solution covers 80% of the use case, install the solution and override the 20% in your repo — don't fork from scratch.

Choose the analytics rule type

Approach

1. Start from a use case, not a query. Phrase the detection as: "Detect <attacker behavior> on <data source> with <expected fidelity>." Map to MITRE ATT&CK technique IDs before opening the KQL editor.
2. Confirm data coverage. Does the connector emit the fields you need? Run a

   take 100

   first, look at the schema, and check ingestion latency for that table.
3. Author the KQL with detection patterns:
   • Filter early, project late.
   • Use

     summarize

     +

     bin(TimeGenerated, 5m)

     for rate-based detections.
   • Use

     materialize()

     once for joined intermediate results.
   • For lookup-rich rules (allowlists, asset criticality), use watchlists via

     _GetWatchlist("name")

     .
   • Avoid

     *

     joins on huge tables — pre-filter both sides.
4. Map entities. Set User, Host, IP, FileHash, Url entities in the rule UI so the incident graph and investigation experience work. Without entity mapping, Sentinel incidents are flat blobs.
5. Set incident creation thoughtfully. Group alerts into a single incident using

   Group all alerts triggered by this rule into a single incident

   for high-volume rules with bursty behavior, or per-entity grouping for "one-incident-per-user."
6. Validate. Run the rule's KQL over a 7-day historical window. Look at:
   • Average alerts/day → must be triageable (target <10 for high severity).
   • False-positive samples — if 50% are FP, tune; don't ship.
   • Coverage — does it actually fire on a known attack scenario? Use Attack Simulation Training logs or replay an exported red-team event.
7. Wire automation.
   • Automation rule (no-code) for assignment, severity tweak, suppression windows, bulk close.
   • Playbook (Logic App) for actions — disable user, isolate device (via Defender for Endpoint), enrich from MDTI, post to Teams/Slack, open ITSM ticket.
   • Use incident-trigger playbooks (run once per incident) over alert-trigger for anything with shared context.
8. Detection-as-code. Connect Sentinel to a GitHub or Azure DevOps repo via the Repositories feature. Source of truth is the repo; deployments are pipelines. Branch protection + PR review = no more "someone tweaked the rule in the portal."
9. Lifecycle.
   • Tag rules with version (in metadata) and owner.
   • Re-validate quarterly: data-source schema changes break detections silently.
   • Retire rules with sustained zero alerts and no FN coverage.

Guardrails

• Don't ship a rule without a 7-day backtest. "Looks right in the editor" → noise storm in production.
• High-volume tables (

  SecurityEvent

  ,

  Syslog

  ,

  CommonSecurityLog

  ) — filter early. Whole-table scans cost LA money and run slow.
• NRT has limits. Don't try to bend a 5-table-join detection into NRT; use scheduled.
• Always map entities. No entities = no investigation graph = analyst pain.
• Don't auto-disable users from a low-confidence rule. SOAR power × low fidelity = outages. Pair with high-fidelity rules only.
• Watchlists are not append-only logs. They have row limits (~10 MB / 1M rows depending). Use Log Analytics tables for big reference data.
• Microsoft-published rules update via solutions. If you fork by editing in-place, you lose updates. Clone/rename or override in repo.
• Defender XDR custom detections vs Sentinel rules — author where the data lives. If the data is XDR-only and triage happens in XDR, do the custom detection in XDR; don't duplicate.

Common anti-patterns

• "Wrote 200 rules, all P2 severity" — meaningless prioritization. Severity reflects business impact + fidelity, not count.
• "Used

  where TimeGenerated > ago(90d)

  in a 5-minute scheduled rule" — destroys cost. Match the lookback to the rule cadence.
• "Authored detections only in the portal, no source control" — change history, rollback, and review are gone.
• "Group-all-alerts on a noisy rule" — single mega-incident with thousands of alerts; triage impossible.
• "Auto-isolate-device playbook on day-one detection" — false-positive outage. Audit-mode SOAR first.
• "Threat-intel match rule against every IOC" — terabytes of indicators × millions of log rows. Filter by indicator type/confidence and scope tables.
• "Skipped MITRE mapping because 'we know what it does'" — blocks coverage analysis and stakeholder reporting.

Example prompts

• Author a Sentinel scheduled rule for impossible-travel using SigninLogs with proper entity mapping.

• Convert a noisy detection (avg 800 alerts/day) into actionable form via tuning and grouping.

• Build the playbook to isolate a Defender for Endpoint device on a Sentinel incident, with an approval step in Teams.

• Set up Sentinel Repositories to deploy analytics rules from GitHub via PR.

• Write an NRT rule for newly-added Global Administrator that fires within 1 minute.

• Map a custom detection to MITRE ATT&CK techniques and validate coverage gaps.

• Build a hunting workbook for living-off-the-land binary usage across endpoints.

• Quarterly content review: identify zero-alert rules, validate they're not silently broken, retire or refactor.

Microsoft Learn

• Detection lifecycle: https://learn.microsoft.com/azure/sentinel/threat-detection
• Analytics rule types: https://learn.microsoft.com/azure/sentinel/detect-threats-built-in
• Custom analytics rules: https://learn.microsoft.com/azure/sentinel/detect-threats-custom
• NRT rules: https://learn.microsoft.com/azure/sentinel/near-real-time-rules
• Entity mapping: https://learn.microsoft.com/azure/sentinel/map-data-fields-to-entities
• Automation rules: https://learn.microsoft.com/azure/sentinel/automate-incident-handling-with-automation-rules
• Playbooks (Logic Apps): https://learn.microsoft.com/azure/sentinel/automate-responses-with-playbooks
• Watchlists: https://learn.microsoft.com/azure/sentinel/watchlists
• Repositories (CI/CD): https://learn.microsoft.com/azure/sentinel/ci-cd
• Content hub: https://learn.microsoft.com/azure/sentinel/sentinel-solutions
• KQL reference: https://learn.microsoft.com/kusto/query/
• MITRE ATT&CK in Sentinel: https://learn.microsoft.com/azure/sentinel/mitre-coverage