| name | detection-authoring |
| description | Create, deploy, update, and manage custom detection rules in Microsoft Defender XDR via the Graph API (/beta/security/rules/detectionRules). Covers query adaptation from Sentinel KQL to custom detection format, deployment via PowerShell (Invoke-MgGraphRequest), manifest-driven batch deployment, and lifecycle management (list, enable/disable, delete). Companion script: Deploy-CustomDetections.ps1. |
Custom Detection Authoring — Instructions
Purpose
This skill deploys custom detection rules to Microsoft Defender XDR via the Microsoft Graph API (/beta/security/rules/detectionRules). It handles:
- Query adaptation — Converting Sentinel KQL queries into custom detection format
- Single-rule deployment — Creating one rule via Graph API
- Batch deployment — Deploying multiple rules from a JSON manifest
- Lifecycle management — Listing, updating, enabling/disabling, and deleting rules
- Validation — Dry-run queries in Advanced Hunting before deployment
Entity Type: Custom detection rules (Defender XDR)
📑 TABLE OF CONTENTS
- Prerequisites — Auth, scopes, PowerShell modules
- Critical Rules — Mandatory constraints (includes query adaptation checklist)
- API Reference — Graph API schema and field values
- Frequency & Lookback — Schedule periods, lookback windows, NRT constraints
- Deployment Workflow — Step-by-step process
- Batch Deployment — Manifest-driven multi-rule deployment
- Lifecycle Management — CRUD operations
- Existing Rule Discovery — Search Analytic Rules & Custom Detections by table, EventID, or keyword
- Known Pitfalls — Lessons learned (13 pitfalls documented)
- CD Metadata Contract — Schema for query file ↔ detection skill coordination
Prerequisites
Required PowerShell Module
# Microsoft.Graph.Authentication — provides Invoke-MgGraphRequest
Install-Module Microsoft.Graph.Authentication -Scope CurrentUser
Required Graph API Scopes
| Operation | Scope | Type |
|---|
| List / Get rules | CustomDetection.Read.All | Delegated |
| Create / Update / Delete | CustomDetection.ReadWrite.All | Delegated |
Authentication
# Read-only
Connect-MgGraph -Scopes "CustomDetection.Read.All" -NoWelcome
# Full CRUD
Connect-MgGraph -Scopes "CustomDetection.ReadWrite.All" -NoWelcome
Why Invoke-MgGraphRequest? The Graph MCP server and az rest both return 403 for custom detection endpoints — they lack the CustomDetection.* scopes. Invoke-MgGraphRequest uses interactive delegated auth with consent, which works.
Companion Script
Deploy-CustomDetections.ps1 — PowerShell script for manifest-driven batch deployment. See Batch Deployment.
⚠️ CRITICAL RULES — READ FIRST ⚠️
Mandatory Query Requirements
Custom detection queries have strict requirements that differ from Sentinel analytic rules:
| Requirement | Detail |
|---|
| Timestamp column must be projected as-is | The query MUST project the timestamp column exactly as it appears in the source table — TimeGenerated for Sentinel/LA tables, Timestamp for XDR-native tables. Do not alias one to the other (e.g., Timestamp = TimeGenerated causes 400 Bad Request). See Pitfall 1. |
| Event-unique columns (per table type) | Required columns that uniquely identify the event differ by table family. A bare summarize count() or make_set() loses these columns and fails. summarize with arg_max IS allowed — see Pitfall 3. See table below for per-type requirements. |
| Impacted asset identifier column | The query must project at least one column whose name matches a valid impactedAssets identifier (e.g., AccountUpn, DeviceName, DeviceId). See Impacted Asset Types and Pitfall 9. Queries without project or summarize typically return these columns automatically. |
impactedAssets must be non-empty | The impactedAssets array must contain at least 1 element. An empty array ([]) is rejected with 400 BadRequest: "The field ImpactedAssets must be a string or array type with a minimum length of '1'." Every detection must declare which entity it impacts. See Pitfall 13. |
No let statements (NRT) | NRT rules (schedule: "0") reject let entirely — the API returns a generic 400 Bad Request. This is not documented by Microsoft (empirically discovered Feb 2026) but consistently reproducible. Inline all dynamic arrays/lists directly in where clauses. Non-NRT rules (1H+) tolerate let. |
Unique displayName AND title | Both the rule displayName and the alert title must be unique across all custom detections. Duplicate displayName returns 409 Conflict. Duplicate title returns 400 Bad Request. |
| 150 alerts per run | Each rule generates a maximum of 150 alerts per execution. Tune the query to avoid alerting on normal day-to-day activity. |
| 🔴 No response actions | All rules deployed by this skill MUST use "responseActions": []. Automated response actions (isolate device, disable user, block file, etc.) are PROHIBITED — they must only be configured manually by a human operator in the Defender portal after the rule is validated. Never populate responseActions in manifests or API calls. |
| First run = 30-day backfill | When a new rule is saved, it immediately runs against the past 30 days of data. Expect a burst of initial alerts if the query has broad coverage. |
Required event-unique columns by table type (MS Learn source):
| Table Family | Required Columns (besides timestamp) |
|---|
| MDE tables (Device*) | DeviceId AND ReportId |
| Alert* tables | None (just timestamp) |
| Observation* tables | ObservationId |
| All other XDR tables | ReportId |
| Sentinel/LA tables (AuditLogs, SigninLogs, SecurityEvent, OfficeActivity, etc.) | ReportId recommended (use proxy: CorrelationId, OfficeObjectId, CallerProcessId) but not strictly mandated by the docs |
Query Adaptation Checklist
When converting a Sentinel query to custom detection format:
- ✅ Remove bare
summarize — project raw rows instead. Exception: summarize with arg_max is allowed for threshold-based detections (see Pitfall 3)
- ✅ Project the timestamp column as-is:
TimeGenerated = TimeGenerated for Sentinel/LA tables, Timestamp for XDR tables. Never alias one to the other.
- ✅ Project the impacted asset identifier column — the column name must match a valid identifier from Impacted Asset Types. Examples:
DeviceName = Computer for device-focused detections, AccountUpn = UserId for user-focused. See Pitfall 9.
- ✅ Project event-unique columns per table type —
DeviceId + ReportId for MDE tables; ReportId for other XDR tables; recommended proxy ReportId for Sentinel tables (e.g., ReportId = CorrelationId). Caveat: proxy columns may contain empty strings for some events — acceptable but means those rows won't be individually identifiable in alert details.
- ✅ Add a time filter as the first
where clause — prefer ingestion_time() > ago(1h) over TimeGenerated > ago(1h) (see tip below). NRT exception: For NRT rules (schedule: "0"), omit the time filter entirely — events are processed as they stream in, and the platform pre-filters automatically.
- ✅ Remove
let variables for NRT rules — NRT rejects let entirely (generic 400 error, undocumented). Inline all dynamic arrays directly in where clauses. Non-NRT rules tolerate let.
- ✅ Validate via Advanced Hunting dry-run before deployment
- ✅ For NRT rules: avoid
tostring() on dynamic columns — use native string columns instead (e.g., Properties instead of tostring(Properties_d)). See Pitfall 11.
- ✅ For NRT rules: verify the table's ingestion lag justifies NRT. See Pitfall 12.
- ✅ Count unique
{{Column}} references across title AND description combined — max 3 unique columns total (shared across both fields, not per-field). Exceeding this returns 400 Bad Request: "Dynamic properties in alertTitle and alertDescription must not exceed 3 fields". See Pitfall 14.
Performance tip (from MS Learn): "Avoid filtering custom detections by using the Timestamp column. The data used for custom detections is prefiltered based on the detection frequency." Use ingestion_time() instead — it aligns with the platform's pre-filtering for better performance. For scheduled rules, match the time filter to the run frequency (ingestion_time() > ago(1h) for 1H rules). For NRT rules, no time filter is needed.
Example Adaptation
Before (Sentinel KQL — uses summarize):
let _Lookback = 7d;
SecurityEvent
| where TimeGenerated > ago(_Lookback)
| where EventID == 4799
| where TargetSid == "S-1-5-32-544"
| where SubjectUserSid != "S-1-5-18"
| where AccountType != "Machine"
| where not(SubjectUserSid endswith "-500")
| project TimeGenerated, Computer, Actor = SubjectUserName, ...
| summarize EnumerationCount = count(), Processes = make_set(CallerProcess)
by Actor, ActorDomain, ActorSID
After (Custom Detection — row-level, mandatory columns):
SecurityEvent
| where TimeGenerated > ago(1h)
| where EventID == 4799
| where TargetSid == "S-1-5-32-544"
| where SubjectUserSid != "S-1-5-18"
| where AccountType != "Machine"
| where not(SubjectUserSid endswith "-500")
| project
TimeGenerated = TimeGenerated,
DeviceName = Computer,
AccountName = SubjectUserName,
AccountDomain = SubjectDomainName,
AccountSid = SubjectUserSid,
CallerProcess = CallerProcessName,
ReportId = CallerProcessId
Key changes:
- Removed
let _Lookback → hardcoded ago(1h)
- Removed
summarize → raw project
- Added
TimeGenerated = TimeGenerated (identity projection — mandatory)
- Added
DeviceName = Computer (impacted asset identifier — device-focused detection)
- Added
ReportId = CallerProcessId (proxy ReportId — event-unique identifier)
API Reference
Endpoint
POST /beta/security/rules/detectionRules — Create
GET /beta/security/rules/detectionRules — List all
GET /beta/security/rules/detectionRules/{id} — Get by ID
PATCH /beta/security/rules/detectionRules/{id} — Update
DELETE /beta/security/rules/detectionRules/{id} — Delete
Schedule Periods
| Value | Meaning | Notes |
|---|
"0" | NRT (Near Real-Time / Continuous) | Runs continuously. See NRT Constraints. |
"1H" | Every 1 hour | Most common for custom detections |
"3H" | Every 3 hours | |
"12H" | Every 12 hours | |
"24H" | Every 24 hours | Daily |
Alert Severity Values
| Value | Use Case |
|---|
"informational" | Baseline queries, low-noise canaries |
"low" | Suspicious but may be benign |
"medium" | Likely malicious, needs investigation |
"high" | High-confidence detection, immediate response |
Alert Category Values
Valid MITRE ATT&CK tactic names (title case):
InitialAccess, Execution, Persistence, PrivilegeEscalation, DefenseEvasion, CredentialAccess, Discovery, LateralMovement, Collection, Exfiltration, CommandAndControl, Impact, Reconnaissance, ResourceDevelopment
Impacted Asset Types
Device asset:
{
"@odata.type": "#microsoft.graph.security.impactedDeviceAsset",
"identifier": "<identifier>"
}
Valid device identifiers: deviceId, deviceName, remoteDeviceName, targetDeviceName, destinationDeviceName
User asset:
{
"@odata.type": "#microsoft.graph.security.impactedUserAsset",
"identifier": "<identifier>"
}
Valid user identifiers: accountObjectId, accountSid, accountUpn, accountName, accountDomain, accountId, requestAccountSid, requestAccountName, requestAccountDomain, recipientObjectId, processAccountObjectId, initiatingAccountSid, initiatingProcessAccountUpn, initiatingAccountName, initiatingAccountDomain, servicePrincipalId, servicePrincipalName, targetAccountUpn
Mailbox asset:
{
"@odata.type": "#microsoft.graph.security.impactedMailboxAsset",
"identifier": "<identifier>"
}
Valid mailbox identifiers: recipientEmailAddress, senderFromAddress, senderMailFromAddress, senderEmailAddress, senderObjectId
Minimal Valid POST Body
{
"displayName": "Rule Name",
"isEnabled": true,
"queryCondition": {
"queryText": "SecurityEvent\r\n| where TimeGenerated > ago(1h)\r\n| ..."
},
"schedule": {
"period": "1H"
},
"detectionAction": {
"alertTemplate": {
"title": "Alert Title",
"description": "Alert description text.",
"severity": "medium",
"category": "Discovery",
"recommendedActions": null,
"mitreTechniques": ["T1069.001"],
"impactedAssets": [
{
"@odata.type": "#microsoft.graph.security.impactedDeviceAsset",
"identifier": "deviceName"
}
]
},
"responseActions": []
}
}
impactedAssets: Must contain at least 1 element — an empty array causes 400 BadRequest. Every detection must map to at least one impacted entity (device, user, or mailbox). See Pitfall 13.
recommendedActions: Can be null or a string. The portal sets it to null by default.
responseActions: Must always be [] — response actions are prohibited in LLM-authored detections (see Critical Rules). Must be [], not null — sending null causes 400 Bad Request. See Pitfall 10.
organizationalScope: Omit this field entirely for tenant-wide rules (the API default). Including "organizationalScope": null explicitly may cause 400 Bad Request in some API versions.
Custom details (not shown above): The API also supports a customDetails array of key-value pairs surfaced in the alert side panel. Each rule supports up to 20 KVPs with a combined 4KB size limit. Keys are display labels; values are query column names. See MS Learn.
Related evidence (not shown above): Beyond impactedAssets, the entity mapping also supports linking related evidence entities (Process, File, Registry value, IP, OAuth application, DNS, Security group, URL, Mail cluster, Mail message). These provide correlation context but are not impacted assets. See MS Learn.
Dynamic Alert Titles and Descriptions
Alert titles and descriptions can reference query result columns using {{ColumnName}} syntax, making alerts self-descriptive:
{
"title": "Admin Group Enumeration by {{AccountName}} on {{DeviceName}}",
"description": "User {{AccountName}} enumerated group {{TargetGroupName}} on the device."
}
| Constraint | Limit |
|---|
| Max unique dynamic columns | 3 unique {{Column}} references TOTAL across title AND description combined — NOT per field. E.g., the example above uses AccountName + DeviceName in title and AccountName + TargetGroupName in description = 3 unique columns (AccountName is reused). Exceeding this returns 400 Bad Request with "Dynamic properties in alertTitle and alertDescription must not exceed 3 fields". |
| ⚠️ Discrepancy with MS Learn docs: The official documentation states "The number of columns you can reference in each field is limited to three" (i.e., 3 per field). However, the Graph API empirically enforces 3 unique columns total across both fields combined (confirmed Feb 2026). The portal UI may enforce the per-field limit differently than the API. Use 3 unique total as the safe limit for Graph API deployments. |
| Format | {{ExactColumnName}} — must match a column in query output |
| Markup | Plain text only — HTML, Markdown, and code are sanitized |
| URLs | Must use percent-encoding format |
Frequency & Lookback
Lookback Windows by Frequency
Each frequency has a built-in lookback window. Results outside this window are ignored even if the query requests them:
| Frequency | Lookback Period | Query Filter Recommendation |
|---|
| NRT (Continuous) | Streaming | No time filter needed — events processed as collected |
| Every 1 hour | Past 4 hours | ago(4h) or ago(1h) |
| Every 3 hours | Past 12 hours | ago(12h) or ago(3h) |
| Every 12 hours | Past 48 hours | ago(48h) or ago(12h) |
| Every 24 hours | Past 30 days | ago(30d) or ago(24h) |
| Custom (Sentinel only) | 4× frequency (<daily) or 30d (≥daily) | Match lookback |
Tip: Match the query time filter to the run frequency (ago(1h) for 1H rules), not the full lookback window. The lookback ensures late-arriving data is caught, but your filter should target the detection window.
NRT Constraints
NRT (Continuous, period: "0") rules have stricter requirements than scheduled rules:
| Constraint | Detail |
|---|
| Single table only | Query must reference exactly one table — no joins or unions |
No let statements | let variables are silently rejected — the API returns a generic 400 Bad Request with no useful error message. Always inline dynamic arrays/lists directly in where clauses. This constraint is not listed in the official NRT docs (which list only 4 constraints) but is consistently reproducible via Graph API (empirically confirmed Feb 2026). |
No externaldata | Cannot use the externaldata operator |
| No comments | Query text must not contain any comment lines (//) |
| Supported operators only | Limited to supported KQL features. tostring() on dynamic columns is rejected — use native string columns instead (e.g., Properties instead of tostring(Properties_d)). See Pitfall 11. |
| No time filter needed | NRT processes events as they stream in. The platform pre-filters automatically. Adding a time filter (e.g., TimeGenerated > ago(1h)) is unnecessary but harmless. |
NRT-Supported Tables
Not all tables support NRT frequency. Use NRT only with these tables:
Defender XDR tables:
AlertEvidence, CloudAppEvents, DeviceEvents, DeviceFileCertificateInfo, DeviceFileEvents, DeviceImageLoadEvents, DeviceLogonEvents, DeviceNetworkEvents, DeviceNetworkInfo, DeviceInfo, DeviceProcessEvents, DeviceRegistryEvents, EmailAttachmentInfo, EmailEvents*, EmailPostDeliveryEvents, EmailUrlInfo, IdentityDirectoryEvents, IdentityLogonEvents, IdentityQueryEvents, UrlClickEvents
* EmailEvents: LatestDeliveryLocation and LatestDeliveryAction columns are excluded from NRT.
Sentinel tables (Preview):
ABAPAuditLog_CL, AuditLogs, AWSCloudTrail, AWSGuardDuty, AzureActivity, Cisco_Umbrella_dns_CL, Cisco_Umbrella_proxy_CL, CommonSecurityLog, GCPAuditLogs, MicrosoftGraphActivityLogs, OfficeActivity, Okta_CL, OktaV2_CL, ProofpointPOD, ProofPointTAPClicksPermitted_CL, ProofPointTAPMessagesDelivered_CL, SecurityAlert, SecurityEvent, SigninLogs
Important: SecurityEvent and SigninLogs support NRT — our Event ID 4799/4702 queries can run as NRT if they meet the single-table/no-joins constraint.
Ingestion Lag Consideration — NRT Suitability
A table being NRT-supported means the API accepts NRT rules — not that NRT is the right choice. If a table's ingestion lag exceeds the detection frequency benefit, NRT adds overhead with no detection speed improvement. See Pitfall 12 for a per-table ingestion lag assessment and recommendation matrix. Rule of thumb: if ingestion lag > 30 min, use 1H scheduled instead.
Custom Frequency (Sentinel Data Only)
For rules based entirely on Sentinel-ingested data, a custom frequency is available (Preview):
- Range: 5 minutes to 14 days
- Lookback: Automatically calculated — 4× frequency for sub-daily, 30 days for daily or longer
- Requirement: Data must be available in Microsoft Sentinel (not XDR-only tables)
Deployment Workflow
Single Rule Deployment
Step 1: Validate the query in Advanced Hunting
Run the adapted query with a 1h lookback to validate schema:
Use RunAdvancedHuntingQuery with the adapted KQL query.
Confirm: 0 or more results, correct column schema (TimeGenerated, DeviceName, AccountName, etc.)
Then run with 30d lookback to confirm it returns real data:
Change ago(1h) to ago(30d) for the validation run.
Verify results contain expected columns and realistic data.
Step 2: Check for duplicates, then build and POST the rule
Connect-MgGraph -Scopes "CustomDetection.ReadWrite.All" -NoWelcome
# Pre-flight: check if rule name already exists
$ruleName = "Rule Name"
$existing = (Invoke-MgGraphRequest -Method GET `
-Uri "/beta/security/rules/detectionRules" -OutputType PSObject).value `
| Where-Object { $_.displayName -eq $ruleName }
if ($existing) {
Write-Host "Rule '$ruleName' already exists (ID: $($existing.id)). Skipping POST."
return
}
$body = @{
displayName = $ruleName
isEnabled = $true
queryCondition = @{
queryText = "SecurityEvent`r`n| where TimeGenerated > ago(1h)`r`n| ..."
}
schedule = @{ period = "1H" }
detectionAction = @{
alertTemplate = @{
title = "Alert Title"
description = "Description"
severity = "medium"
category = "Discovery"
recommendedActions = $null
mitreTechniques = @("T1069.001")
impactedAssets = @(
@{
"@odata.type" = "#microsoft.graph.security.impactedDeviceAsset"
identifier = "deviceName"
}
)
}
responseActions = @()
}
} | ConvertTo-Json -Depth 10
$result = Invoke-MgGraphRequest -Method POST `
-Uri "/beta/security/rules/detectionRules" `
-Body $body -ContentType "application/json" -OutputType PSObject
Step 3: Verify creation
$rules = Invoke-MgGraphRequest -Method GET `
-Uri "/beta/security/rules/detectionRules" -OutputType PSObject
$rules.value | Select-Object id, displayName, isEnabled,
@{N='Schedule';E={$_.schedule.period}},
@{N='Status';E={$_.lastRunDetails.status}} | Format-Table -AutoSize
Batch Deployment
Use the companion script Deploy-CustomDetections.ps1 for manifest-driven batch deployment.
Manifest storage: Save manifest JSON files in the temp/ folder (gitignored). Manifests are deployment artifacts, not versioned query definitions.
Manifest Format
See example-manifest.json for a complete 2-rule reference covering NRT and scheduled (with summarize/arg_max) patterns.
The script reads a JSON file containing an array of rule definitions:
[
{
"displayName": "Admin Group Enumeration by Non-Admin User",
"title": "Admin Group Enumeration by {{AccountName}} on {{DeviceName}}",
"queryText": "SecurityEvent\r\n| where TimeGenerated > ago(1h)\r\n| ...",
"schedule": "0",
"severity": "medium",
"category": "Discovery",
"mitreTechniques": ["T1069.001", "T1087.001"],
"description": "User {{AccountName}} enumerated the local Administrators group.",
"recommendedActions": "Verify whether the user has a legitimate reason to enumerate admin group membership.",
"impactedAssets": [
{ "type": "device", "identifier": "deviceName" },
{ "type": "user", "identifier": "accountSid" }
],
"responseActions": []
}
]
Usage
# Dry-run — validate all queries in Advanced Hunting without creating rules
.\Deploy-CustomDetections.ps1 -ManifestPath .\temp\4799_4702.json -DryRun
# Deploy all rules from manifest (skips existing rules by default)
.\Deploy-CustomDetections.ps1 -ManifestPath .\temp\4799_4702.json
# Deploy and overwrite — attempt POST even if rule name exists (may cause 409)
.\Deploy-CustomDetections.ps1 -ManifestPath .\temp\4799_4702.json -Force
Lifecycle Management
List All Rules
$rules = Invoke-MgGraphRequest -Method GET `
-Uri "/beta/security/rules/detectionRules" -OutputType PSObject
$rules.value | Select-Object id, displayName, isEnabled,
@{N='Schedule';E={$_.schedule.period}},
@{N='LastRun';E={$_.lastRunDetails.status}},
@{N='Created';E={$_.createdDateTime}} | Format-Table -AutoSize
Get Rule by ID
$rule = Invoke-MgGraphRequest -Method GET `
-Uri "/beta/security/rules/detectionRules/5632" -OutputType PSObject
$rule | ConvertTo-Json -Depth 10
Update Rule (PATCH)
$update = @{
isEnabled = $false
schedule = @{ period = "24H" }
} | ConvertTo-Json -Depth 10
Invoke-MgGraphRequest -Method PATCH `
-Uri "/beta/security/rules/detectionRules/5632" `
-Body $update -ContentType "application/json"
Delete Rule
Invoke-MgGraphRequest -Method DELETE `
-Uri "/beta/security/rules/detectionRules/5632"
⚠️ Deletion propagation delay: After deleting a rule, the name remains reserved for ~30-60 seconds. Creating a rule with the same displayName during this window returns 409 Conflict — but the rule may still be created despite the error. Always verify with a GET after creation.
Enable/Disable Without Deleting
# Disable
Invoke-MgGraphRequest -Method PATCH `
-Uri "/beta/security/rules/detectionRules/5632" `
-Body '{"isEnabled": false}' -ContentType "application/json"
# Enable
Invoke-MgGraphRequest -Method PATCH `
-Uri "/beta/security/rules/detectionRules/5632" `
-Body '{"isEnabled": true}' -ContentType "application/json"
Existing Rule Discovery
Before authoring new custom detections, check what Analytic Rules (Sentinel) and Custom Detection rules (Defender XDR) already exist for the same table, EventID, or keyword. This avoids duplicating coverage and helps identify gaps.
Step 0: Construct the Analytic Rules URL (once per session)
$cfg = Get-Content config.json | ConvertFrom-Json
$sub = $cfg.subscription_id
$rg = $cfg.azure_mcp.resource_group
$ws = $cfg.azure_mcp.workspace_name
$arUrl = "https://management.azure.com/subscriptions/$sub/resourceGroups/$rg/providers/Microsoft.OperationalInsights/workspaces/$ws/providers/Microsoft.SecurityInsights/alertRules?api-version=2024-09-01"
# Verify (should return rule count)
az rest --method get --url $arUrl --query "length(value)" -o tsv 2>$null
All patterns below reuse $arUrl. The Sentinel REST API returns the full KQL query text for every rule — there is no server-side content filtering, so we pull all rules in one call and filter client-side with JMESPath contains().
Search Analytic Rules by Table Name or Keyword
# Which rules reference a specific table? (e.g., SecurityEvent)
az rest --method get --url $arUrl `
--query "value[?properties.query && contains(properties.query, 'SecurityEvent')].{name: properties.displayName, severity: properties.severity, enabled: properties.enabled}" `
-o table 2>$null
Search Analytic Rules by EventID
# Which rules reference a specific EventID?
az rest --method get --url $arUrl `
--query "value[?properties.query && contains(properties.query, '<EventID>')].{name: properties.displayName, severity: properties.severity, enabled: properties.enabled}" `
-o table 2>$null
To see the surrounding KQL context of a match:
az rest --method get --url $arUrl `
--query "value[?properties.query && contains(properties.query, '<EventID>')].properties.query" `
-o tsv 2>$null | Select-String -Pattern '<EventID>' -Context 1,1
Search Analytic Rules for ASIM Parser Dependencies
$rules = az rest --method get --url $arUrl `
--query "value[?properties.enabled==``true`` && properties.query].{displayName: properties.displayName, query: properties.query}" `
-o json 2>$null | ConvertFrom-Json
$asimRules = $rules | Where-Object { $_.query -match '_Im_|_ASim_' }
$asimRules | ForEach-Object {
$schemas = [regex]::Matches($_.query, '_Im_(\w+)') | ForEach-Object { $_.Groups[1].Value } | Sort-Object -Unique
Write-Host "$($_.displayName): $($schemas -join ', ')"
}
Dump All Enabled Rule Queries for Local Search
az rest --method get --url $arUrl `
--query "value[?properties.enabled==``true`` && properties.query].{name: properties.displayName, query: properties.query}" `
-o json > temp/analytic_rule_queries.json
# Then search locally for any pattern
Get-Content temp/analytic_rule_queries.json | Select-String -Pattern 'EventID\s*(==|in\s*\(|has|contains)' -AllMatches
Search Custom Detection Rules (Graph API)
⚠️ Important: The Graph MCP server returns 403 for the Custom Detection endpoint. Always use Invoke-MgGraphRequest via the terminal.
Import-Module Microsoft.Graph.Authentication -ErrorAction Stop
$ctx = Get-MgContext
if (-not $ctx -or $ctx.Scopes -notcontains 'CustomDetection.Read.All') {
Connect-MgGraph -Scopes 'CustomDetection.Read.All' -NoWelcome
}
$response = Invoke-MgGraphRequest -Method GET `
-Uri '/beta/security/rules/detectionRules?$select=id,displayName,isEnabled,queryCondition,schedule,lastRunDetails,createdDateTime,lastModifiedDateTime' `
-OutputType PSObject
Then filter by table name or keyword:
# Which CD rules reference SecurityEvent?
$response.value | Where-Object { $_.queryCondition.queryText -match 'SecurityEvent' } |
Select-Object displayName, isEnabled, @{N='Query';E={$_.queryCondition.queryText}}
# Which CD rules reference a specific EventID?
$response.value | Where-Object { $_.queryCondition.queryText -match '4688|ProcessCreate' } |
Select-Object displayName, isEnabled, @{N='Query';E={$_.queryCondition.queryText}}
Identify stale rules (no run in 90 days):
$cutoff = (Get-Date).AddDays(-90).ToString('yyyy-MM-ddTHH:mm:ssZ')
$response.value | Where-Object {
$_.lastRunDetails.lastRunDateTime -and $_.lastRunDetails.lastRunDateTime -lt $cutoff
} | Select-Object displayName, isEnabled,
@{N='LastRun';E={$_.lastRunDetails.lastRunDateTime}},
@{N='Status';E={$_.lastRunDetails.status}}
Key API Fields for Rule Discovery
| Source | Field Path | Content |
|---|
| Analytic Rules (REST) | properties.displayName | Rule name |
| properties.query | Full KQL query text |
| properties.severity | High / Medium / Low / Informational |
| properties.enabled | true / false |
| Custom Detections (Graph) | displayName | Rule name |
| queryCondition.queryText | Full KQL query (AH syntax) |
| schedule.period | PT1H, PT24H, PT0S (continuous) |
| lastRunDetails.lastRunDateTime | Last execution timestamp |
| lastRunDetails.status | completed, failed, running |
| isEnabled | true / false |
Tip: JMESPath contains() (used in az rest --query) is case-sensitive. For case-insensitive search, dump to JSON and use PowerShell -match instead.
Known Pitfalls
Pitfall 1: Timestamp vs TimeGenerated — Project As-Is
The query must project the timestamp column exactly as it appears in the source table. Do NOT alias one to the other.
| Source Table Type | Correct | Wrong |
|---|
| Sentinel/LA tables (SecurityEvent, SigninLogs, AuditLogs, etc.) | TimeGenerated = TimeGenerated | Timestamp = TimeGenerated |
| XDR-native tables (DeviceEvents, DeviceProcessEvents, etc.) | Timestamp (native) | TimeGenerated = Timestamp |
The MS Learn docs confirm: "Timestamp or TimeGenerated — This column sets the timestamp for generated alerts. The query shouldn't manipulate this column and should return it exactly as it appears in the raw event." Aliasing across types causes 400 Bad Request.
Pitfall 2: Silent Rule Creation on Error Responses (400 AND 409)
The API can silently create a rule even when it returns an error. This applies to both 400 Bad Request and 409 Conflict responses.
Cause A — 400 with partial validation: A POST may pass structural validation (creating the rule) but fail a secondary check (e.g., let variable in NRT query, >3 dynamic fields). The API returns 400 Bad Request — but the rule was already created. A subsequent retry with a fixed query then hits 409 Conflict because the rule exists.
Cause B — Deletion propagation delay: Deleting a rule leaves a name reservation for ~30-60 seconds. POSTing a rule with the same displayName in this window returns 409 Conflict — but the API may still create the rule.
Cause C — Silent success + accidental retry: When running Invoke-MgGraphRequest in a terminal, the POST may succeed but the output buffer splits across calls, making it look like nothing happened. Re-running the same POST produces a 409 because the rule was already created seconds earlier.
Prevention:
- Always run a GET before POST to check if the rule name already exists (see Step 2)
- Always verify with GET after ANY error response (400 or 409) — the rule may have been created despite the error
- Never re-run a POST without first checking via GET whether the previous attempt succeeded
- If a rule was silently created with a bad query, use PATCH to update the
queryCondition.queryText rather than deleting and re-creating
Pitfall 3: summarize — Allowed Only With Row-Level Output
Custom detection queries must return row-level results with required columns (TimeGenerated, DeviceName, ReportId). A bare summarize count() or make_set() as the final operator fails validation because the output lacks these columns.
However, summarize with arg_max IS allowed when used to return the required columns alongside aggregation:
// ✅ ALLOWED — uses arg_max to preserve row-level columns
DeviceEvents
| where ingestion_time() > ago(1d)
| where ActionType == "AntivirusDetection"
| summarize (Timestamp, ReportId)=arg_max(Timestamp, ReportId), count() by DeviceId
| where count_ > 5
This pattern counts by entity but still returns Timestamp, ReportId, and DeviceId per row — satisfying the requirement. Use this for threshold-based detections ("alert when count > N").
Pitfall 4: Graph MCP and az rest Cannot Access This API
Both the Graph MCP server and az rest lack the CustomDetection.ReadWrite.All scope. Only Invoke-MgGraphRequest with interactive delegated auth works.
Pitfall 5: recommendedActions Type
The recommendedActions field is a String (not an array). Set to null if not needed. The portal always sets it to null.
Pitfall 6: Query Newlines
Use \r\n (CRLF) for line breaks in the queryText field. In PowerShell here-strings or backtick-escaped strings, use `r`n. The portal uses \r\n format.
Pitfall 7: Duplicate Name AND Title Check
The API enforces unique displayName AND unique title (alert title) across all custom detections. Duplicate displayName returns 409 Conflict. Duplicate title returns 400 Bad Request. The batch deployment script checks for displayName duplicates by default — use -Force to override. The MS Learn docs state both should be unique: "Detection name... make it unique" and "Alert title... make it unique".
Pitfall 8: Alert Deduplication
Custom detections automatically deduplicate alerts. If a detection fires twice on events with the same entities, custom details, and dynamic details, only one alert is created. This can happen when the lookback period is longer than the run frequency (e.g., 1H frequency with 4H lookback means 3 hours of overlap). Different events on the same entity produce separate alert entries under the same alert.
Pitfall 9: impactedAssets Identifier Must Be a Predefined API Value
The identifier field in impactedAssets must use one of the predefined values from the Impacted Asset Types section — NOT arbitrary query column names. Using a custom column name (e.g., "identifier": "TargetComputer" or "identifier": "Actor") causes a silent 400 InvalidInput with an empty error message.
This aligns with the MS Learn docs which list specific "strong identifier" columns for impacted assets. The portal wizard enforces this via a dropdown; the Graph API rejects non-matching values silently.
Identifier values must use camelCase as listed in the Impacted Asset Types section (e.g., recipientEmailAddress, not RecipientEmailAddress). The API treats identifier values as case-sensitive when matching to the predefined list.
Additionally, the query MUST project a column whose name matches the chosen identifier. If you use "identifier": "accountUpn", the query must project an AccountUpn column (alias if needed: AccountUpn = UserId). The column name match is case-insensitive — AccountUpn in the query matches accountUpn in the identifier.
| Wrong | Correct |
|---|
"identifier": "UserId" | "identifier": "accountUpn" + project AccountUpn = UserId |
"identifier": "Actor" | "identifier": "accountUpn" + rename Actor → AccountUpn |
"identifier": "TargetComputer" | "identifier": "deviceName" + project DeviceName = Computer |
"identifier": "TargetUPN" | "identifier": "accountUpn" + rename TargetUPN → AccountUpn |
DeviceId requirement: For XDR-native tables (Device*, Email*, CloudAppEvents) with a device-type impactedAsset, the query must project DeviceId (not just DeviceName). Sentinel/LA tables (SecurityEvent, AuditLogs) do not require DeviceId.
Pitfall 10: PowerShell Empty Array Swallowing & organizationalScope
Root cause (Feb 2026): When using PowerShell if/else expressions to assign empty arrays, PowerShell swallows @() and produces $null instead:
# ❌ BUG — $x becomes $null, NOT an empty array
$x = if ($false) { @($items) } else { @() }
# Result: $null
# ✅ CORRECT — assign first, then overwrite conditionally
$x = @()
if ($condition) { $x = @($items) }
# Result: empty Object[] (serializes to [])
This caused array fields like responseActions and mitreTechniques to serialize as null instead of [], which the API rejects with 400 Bad Request.
Combined with organizationalScope: null — including this field explicitly (even as null) was also rejected. The fix: omit organizationalScope entirely and use direct assignment for array fields.
Symptoms: All rules in a batch return 400 Bad Request, but some may be silently created (see Pitfall 2). Manual deployment of the same rule body (without the null fields) succeeds.
Fixed in: Deploy-CustomDetections.ps1 — array fields now use direct assignment, organizationalScope removed from body.
Pitfall 11: tostring() on Dynamic Columns Rejected in NRT Mode
Root cause (Feb 2026): NRT rules (schedule: "0") reject tostring() wrapping dynamic-typed columns. The API returns a generic 400 Bad Request with no useful error message — similar to the let rejection described in NRT Constraints. The same query deploys successfully as a scheduled rule (1H+).
Example — AzureActivity table:
// ❌ FAILS in NRT mode — tostring() on dynamic column
AzureActivity
| where OperationNameValue =~ "MICROSOFT.SECURITY/PRICINGS/WRITE"
| where tostring(Properties_d.pricings_pricingTier) == "Free"
// ✅ WORKS — use the native string column instead
AzureActivity
| where OperationNameValue =~ "MICROSOFT.SECURITY/PRICINGS/WRITE"
| where Properties has '"pricingTier":"Free"'
Workarounds:
- Prefer native string columns — many Sentinel tables have both a dynamic column (e.g.,
Properties_d) and a string column (e.g., Properties). Use the string column with has or contains for NRT.
- Switch to 1H schedule — if
tostring() is required for precise extraction, use a scheduled rule where it works reliably.
Ingestion lag consideration: Even when a table is NRT-supported, check whether ingestion lag makes NRT impractical — see Ingestion Lag Consideration.
Pitfall 12: NRT-Supported ≠ NRT-Practical — Check Ingestion Lag
A table appearing in the NRT-Supported Tables list means the API accepts NRT rules for that table — it does NOT mean NRT adds value. Tables with significant ingestion lag negate the benefit of continuous detection.
| Table | Typical Ingestion Lag | NRT Practical? | Recommendation |
|---|
DeviceEvents, DeviceProcessEvents | < 5 min | ✅ Yes | NRT is effective |
SigninLogs, AuditLogs | 5-15 min | ⚠️ Marginal | 1H is usually sufficient |
AzureActivity | 3-20 min (docs) | ⚠️ Marginal | Evaluate per use case |
SecurityEvent | < 5 min | ✅ Yes | NRT is effective |
OfficeActivity | 15-60 min | ⚠️ Marginal | Evaluate per use case |
Rule of thumb: If the table's ingestion lag exceeds 30 minutes, use a 1H scheduled rule instead of NRT. The detection latency is dominated by ingestion lag, not rule frequency.
Pitfall 13: impactedAssets Must Be Non-Empty
Root cause (Feb 2026): The Graph API requires impactedAssets to contain at least 1 element. Sending an empty array ("impactedAssets": []) returns 400 BadRequest with InvalidInput code and the message: "The field ImpactedAssets must be a string or array type with a minimum length of '1'."
This error is particularly difficult to diagnose because:
- The error message only appears in some response formats — when using
Invoke-MgGraphRequest with raw JSON strings, the "message" field is often empty ("")
- The actual error text only surfaced when using
ConvertTo-Json on a PowerShell hashtable body
- All other fields in the payload may be valid, making it seem like a server-side issue
Every custom detection must declare at least one impacted entity. Choose the most relevant asset type for the detection:
| Detection Focus | Asset Type | Example Identifier |
|---|
| Email-based threats | impactedMailboxAsset | recipientEmailAddress, senderFromAddress |
| User activity | impactedUserAsset | accountUpn, accountObjectId |
| Endpoint/device | impactedDeviceAsset | deviceId, deviceName |
Prevention:
- Always include at least one
impactedAssets entry in manifests and API payloads
- The companion script Deploy-CustomDetections.ps1 validates this at manifest load time and rejects rules with empty
impactedAssets before calling the API
- Review the Impacted Asset Types section for the full list of valid identifiers per asset type
Pitfall 14: Max 3 Unique Dynamic Columns Across Title + Description
The Graph API enforces 3 unique {{Column}} references across title and description combined (not per field). Exceeding this returns 400 Bad Request — often with an empty error message via Invoke-MgGraphRequest.
⚠️ MS Learn discrepancy: Docs say 3 per field; the API empirically enforces 3 unique total across both fields (confirmed Mar 2026).
| Scenario | Unique Columns | Result |
|---|
title: {{A}} {{B}}, description: {{A}} {{C}} | A, B, C = 3 | ✅ Accepted |
title: {{A}} {{B}}, description: {{C}} {{D}} | A, B, C, D = 4 | ❌ 400 Bad Request |
Counting: Reuse across fields is free ({{A}} in both = 1). Count distinct names, not occurrences.
Workaround: Replace excess {{Column}} refs with static text, or use customDetails (up to 20 KVPs) to surface extra columns in the alert side panel. Deploy-CustomDetections.ps1 validates this at manifest load time.
CD Metadata Contract
Query files in queries/ can include per-query cd-metadata blocks that provide structured data for the detection authoring skill. This is the producer/consumer contract between the KQL Query Authoring skill (producer) and the Detection Authoring skill (consumer).
When cd-metadata is present
When a query in queries/ includes a cd-metadata block, the detection authoring skill uses it to:
- Pre-populate manifest fields (
schedule, severity, category, title, impactedAssets, etc.)
- Skip manual CD-readiness assessment — the block declares readiness explicitly
- Generate the adapted CD query by applying the Query Adaptation Checklist to the Sentinel query in the same section
Schema
The cd-metadata block is an HTML comment with YAML content, placed immediately after the per-query metadata fields (Severity, MITRE, Tuning Notes) and before the KQL code block:
### Query N: [Title]
**Purpose:** ...
**Severity:** High
**MITRE:** T1053.005, T1059.001
<!-- cd-metadata
cd_ready: true
schedule: "1H"
category: "Persistence"
title: "Encoded PowerShell in Scheduled Task on {{DeviceName}}"
impactedAssets:
- type: device
identifier: DeviceName
recommendedActions: "Investigate the scheduled task XML. Decode the base64 payload and check for malicious content."
adaptation_notes: "Straightforward — already row-level, add mandatory columns"
-->
```kql
// Query code...
### Field Reference
| Field | Required | Type | Description |
|-------|----------|------|-------------|
| `cd_ready` | Yes | `true` / `false` | Whether this query can be adapted for custom detection deployment |
| `schedule` | If cd_ready | `"0"` / `"1H"` / `"3H"` / `"12H"` / `"24H"` | Detection frequency. `"0"` = NRT (single-table, no joins/unions) |
| `category` | If cd_ready | string | Alert category (see [API Reference](#api-reference) for valid values) |
| `title` | No | string | Dynamic alert title with `{{ColumnName}}` placeholders. Falls back to query heading if omitted. **Limit: max 3 unique `{{Column}}` references across `title` AND `description` combined** (see [Pitfall 14](#pitfall-14-max-3-unique-dynamic-columns-across-title--description)) |
| `impactedAssets` | If cd_ready | array | Asset entities to extract. Each entry: `type` (`device`/`user`/`mailbox`) + `identifier` (predefined API value, e.g., `accountUpn`, `deviceName` — see [Impacted Asset Types](#impacted-asset-types)) |
| `recommendedActions` | No | string | Triage guidance shown in the alert. Omit if not needed |
| `responseActions` | No | array | **PROHIBITED** — must always be omitted or empty `[]`. Response actions must only be configured manually in the Defender portal |
| `adaptation_notes` | No | string | Human-readable notes on what adaptation is needed (for the summary table) |
### Queries NOT suitable for CD
For queries that cannot be adapted (baseline queries, statistical aggregations), use:
```markdown
<!-- cd-metadata
cd_ready: false
adaptation_notes: "Statistical baseline query — requires summarize with dcount, not suitable for CD"
-->
This explicitly documents the assessment so the detection skill doesn't re-evaluate it each time.
How the detection skill consumes cd-metadata
- User says "deploy query 8 as a custom detection" → Skill reads the query file, finds the cd-metadata block for Query 8
- Pre-populates manifest entry from cd-metadata fields (schedule, category, severity, title, impactedAssets)
- Applies Query Adaptation Checklist to the Sentinel KQL query in that section
- Deploys via Graph API or generates manifest JSON for batch deployment
If a query file has no cd-metadata blocks, the skill assesses CD-readiness manually based on the query structure and the Query Adaptation Checklist.
