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Introduction:
Microsoft Sentinel’s true power emerges when you treat detection rules, workbooks, and analytics as code—versioned, tested, and deployed like any other infrastructure. Wave 3 of Sentinel-As-Code introduces a mandatory PR validation gate powered by 6,000+ Pester assertions, drift detection that turns portal edits into tracked pull requests, and a reusable dependency manifest to keep your KQL and Bicep aligned across Azure DevOps and GitHub Actions.
Learning Objectives:
- Implement branch-protected PR validation with Pester tests, KQL syntax checks, and ARM/Bicep What-If analysis.
- Deploy a drift detector that transforms manual Sentinel changes into automated YAML PRs across all analytics rule buckets.
- Configure Copilot agents with persona-specific instructions to assist with Sentinel detection engineering from any IDE or CLI.
- PR Validation Gate with Pester and Branch Policies
The core promise of Wave 3 is that no broken code reaches main. A combination of ~6,000 Pester assertions and four sidecar jobs runs on every pull request in both Azure DevOps and GitHub Actions.
What this does:
- Validates Bicep/ARM templates using `Test-AzResourceGroupDeployment` and What-If operations.
- Checks KQL syntax against a live Sentinel workspace (dry-run mode).
- Ensures dependency manifests are in sync with the repo content.
Step-by-step implementation (GitHub Actions):
.github/workflows/pr-validation.yml
name: PR Sentinel Gate
on:
pull_request:
branches: [ main ]
permissions:
id-token: write
contents: read
jobs:
pester-tests:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: OIDC login to Azure
uses: azure/login@v2
with:
client-id: ${{ secrets.AZURE_CLIENT_ID }}
tenant-id: ${{ secrets.AZURE_TENANT_ID }}
subscription-id: ${{ secrets.AZURE_SUBSCRIPTION_ID }}
- name: Install Pester and Sentinel module
run: |
Install-Module -Name Pester -Force -SkipPublisherCheck
Install-Module -Name Sentinel.Common -Force
- name: Run Pester tests
run: |
Invoke-Pester -Script @{ Path = "./tests/Sentinel.Tests.ps1" } -OutputFormat Detailed
kql-syntax:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Validate KQL
run: |
Get-ChildItem -Recurse .kql | ForEach-Object {
$content = Get-Content $_.FullName -Raw
Use Sentinel API to dry-run KQL
Invoke-AzRestMethod -Path "/subscriptions/{subId}/resourceGroups/{rg}/providers/Microsoft.OperationalInsights/workspaces/{workspace}/query" -Method POST -Payload (@{query=$content} | ConvertTo-Json)
}
Branch protection requirement:
In GitHub: Settings → Branches → Add rule → Require status checks → Select `pester-tests` and kql-syntax.
In Azure DevOps: Branch policies → Build validation → Add build pipeline → Set “Required”.
2. Drift Detection Across All Three Buckets
Sentinel changes made directly in the portal (“drift”) are now automatically converted into pull requests. Wave 3 covers Custom analytics rules, ContentHub rules, and Orphan analytics – all flowing back to your repo as YAML with the resource GUID reused as id:.
Step-by-step drift detector usage:
Run drift detection manually .\Detect-SentinelDrift.ps1 -Workspace "soc-workspace" -ResourceGroup "rg-sentinel" What it outputs: - ./drafts/custom/myRule.id.yaml - ./drafts/contenthub/identity-threat.yaml - ./drafts/orphan/legacy_rule.yaml Then, from the repo root: git checkout -b drift/portal-edits-$(date +%Y%m%d) git add drafts/ && git commit -m "Portal drift converted to YAML" git push origin drift/portal-edits-$(date +%Y%m%d) gh pr create --title "fix: absorb portal drift" --body "Auto-generated from portal changes"
Key file structure example (YAML drift output):
id: "a1b2c3d4-...-resource-guid" reused from portal name: "Suspicious PowerShell CommandLine" severity: Medium query: | DeviceProcessEvents | where ProcessCommandLine contains "-EncodedCommand" schedule: 24h
3. Shared Module Sentinel.Common and API Bump
Every deploy script previously carried its own helpers. Wave 3 centralizes them into Sentinel.Common PowerShell module, which now uses Sentinel API GA version 2025-09-01.
Installation and usage:
Install from PowerShell Gallery (internal or public) Install-Module -Name Sentinel.Common -Repository PSGallery -Force Example: Deploy custom content using the new API Import-Module Sentinel.Common $analytics = Get-SentinelAnalyticsRule -Workspace "soc-workspace" -ApiVersion "2025-09-01" $analytics | Deploy-SentinelRule -SubscriptionId $subId -ResourceGroup $rg
API version bump note: Replace all older API calls:
Old (2023-02-01) Invoke-AzRestMethod -Path "/subscriptions/.../providers/Microsoft.SecurityInsights/alertRules?api-version=2023-02-01" New (2025-09-01) Invoke-AzRestMethod -Path "/subscriptions/.../providers/Microsoft.SecurityInsights/contentPackages?api-version=2025-09-01"
4. ADO ↔ GitHub Actions Full Parity
Every Azure DevOps pipeline now has a GitHub Actions twin. Two composite actions collapse repetitive OIDC and module-install blocks.
Composite action example (`azure-login-oidc/action.yml`):
name: 'Azure Login OIDC'
description: 'Reusable OIDC login for Azure'
inputs:
environment:
description: 'Azure environment name'
required: true
runs:
using: 'composite'
steps:
- name: Login via OIDC
uses: azure/login@v2
with:
client-id: ${{ env.AZURE_CLIENT_ID }}
tenant-id: ${{ env.AZURE_TENANT_ID }}
subscription-id: ${{ env.AZURE_SUBSCRIPTION_ID }}
Usage in a workflow:
jobs: deploy-sentinel: steps: - uses: ./.github/actions/azure-login-oidc - uses: ./.github/actions/setup-pwsh-modules - run: ./deploy/Deploy-Sentinel.ps1
5. Dependency Manifest Writes Itself
`Build-DependencyManifest.ps1` walks your repo, parses embedded KQL in markdown or YAML files, and emits dependencies.json. Three modes: Generate, Verify, Update.
Step-by-step to implement:
Generate initial manifest .\Build-DependencyManifest.ps1 -Mode Generate -OutputPath ./dependencies.json Verify during PR gate (exit code non-zero if mismatch) .\Build-DependencyManifest.ps1 -Mode Verify -ManifestPath ./dependencies.json Daily cron job to auto-update if drift found .\Build-DependencyManifest.ps1 -Mode Update -ManifestPath ./dependencies.json -CreatePR
Sample dependencies.json output:
{
"kql_queries": [
{ "file": "rules/malware_alert.kql", "tables": ["DeviceProcessEvents", "IdentityLogonEvents"] },
{ "file": "hunts/lateral_movement.kql", "functions": ["union", "join"] }
],
"bicep_modules": ["sentinel.bicep", "analytics.bicep"]
}
The PR gate runs Verify; a daily GitHub Action runs Update and opens a “chore” PR automatically.
6. Copilot Agent Set for Detection Engineering
Thirteen cross-platform agents – five general persona agents and eight engineering specialists – each with path-scoped instruction files and reusable prompts.
Agent configuration example (`.github/copilot/agents/sentinel-specialist.md`):
name: Sentinel Detection Engineer description: Expert in KQL, MITRE ATT&CK mapping, and rule optimization pathScope: ["rules/", "hunts/"] You are a senior SecOps engineer. When given a suspicious behavior, output: 1. A KQL query using the Sentinel schema (IdentityLogonEvents, DeviceProcessEvents) 2. MITRE ATT&CK technique mapping 3. A YAML analytics rule template with recommended severity and schedule
How to use across environments:
- VS Code / JetBrains: Install GitHub Copilot Chat, reference `@sentinel-specialist`
- Copilot CLI: `gh copilot explain “Write KQL for anomalous logon” –agent sentinel-specialist`
- GitHub.com Chat: Type `@workspace /rules/detection.kql` and the agent auto-injects instructions
Reusable prompt example:
`/generate-rule “Suspicious use of rundll32.exe with no command line arguments”` – the agent returns a complete rule YAML.
7. First External Contribution (Hat-tip to `eggleness`)
Wave 3 includes the first external pull request, demonstrating how to safely onboard community contributions. The pattern is:
- Fork the repo and create a feature branch.
2. Run `Build-DependencyManifest.ps1 -Mode Verify` locally.
- Submit PR – the gate automatically runs all Pester tests and drift checks.
4. Maintainer merges after approval.
To enable external contributions:
- Document the `Sentinel.Common` module dependency.
- Provide a local test script that uses
Invoke-Pester -EnableExit. - Use dependabot to keep GitHub Action versions pinned.
External contributor workflow git clone https://github.com/org/sentinel-as-code cd sentinel-as-code ./tools/Setup-LocalDev.ps1 Write new KQL rule in ./rules/custom/ ./Build-DependencyManifest.ps1 -Mode Generate updates manifest git add . && git commit -m "Add anomaly detection rule" gh pr create --title "feat: anomaly rule for T1059" --body "Closes 42"
What Undercode Say
- PR validation is non-negotiable: 6,000 Pester assertions plus ARM What-If and KQL syntax checks ensure that only production‑ready Sentinel content reaches
main. This eliminates the “works on my machine” problem for detection rules. - Drift is now a feature, not a headache: By turning portal edits into YAML PRs with GUID reuse, Wave 3 bridges the gap between UI convenience and Git governance. Your Sentinel workspace becomes fully immutable and auditable.
- Copilot agents transform detection engineering: With persona‑specific instructions and path scoping, these agents generate KQL, map MITRE ATT&CK, and even suggest scheduling – saving senior analysts hours of manual rule writing.
Prediction:
Within 12 months, Sentinel-As-Code will become the default deployment model for enterprise SOC teams. Expect to see drift detection extended to workbooks, watchlists, and automation rules – with AI‑generated PR descriptions that explain why a portal edit was needed. Microsoft will likely absorb this pattern into Sentinel’s native Git integration, making Wave 3’s architecture a blueprint for the entire security industry’s shift toward true GitOps for SIEM.
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Extra Hub: Undercode MoN
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