Possible SignIn from Azure Backdoor

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Id	fa00014c-c5f4-4715-8f5b-ba567e19e41e
Rulename	Possible SignIn from Azure Backdoor
Description	Identifies when a user adds an unverified domain as an authentication method, followed by a sign-in from a user the newly added domain. Threat actors may add custom domains to create a backdoor to your tenant. It’s important to monitor whenever custom domains are added to the tenant.
Severity	Medium
Tactics	Persistence
Techniques	T1098
Required data connectors	AzureActiveDirectory
Kind	Scheduled
Query frequency	1h
Query period	1h
Trigger threshold	0
Trigger operator	gt
Source Uri	https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft Entra ID/Analytic Rules/PossibleSignInfromAzureBackdoor.yaml
Version	1.0.0
Arm template	fa00014c-c5f4-4715-8f5b-ba567e19e41e.json

KQL

// Microsoft Entra ID  Backdoors: Identity Federation
//Ref: https://www.inversecos.com/2021/11/how-to-detect-azure-active-directory.html
AuditLogs
| where OperationName == "Add unverified domain"
| where Result == "success"
| extend InitiatedBy = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
| extend DomainName = tostring(TargetResources[0].displayName)
| summarize DomainAddedTime = min(TimeGenerated), ModifiedProperties = make_set(parse_json(TargetResources[0].modifiedProperties),1048576) by InitiatedBy, DomainName
| join kind=inner (
SigninLogs
| where ResultType == "0"
| extend UserDomain = tostring(parse_json(split(UserPrincipalName,"@",1)[0]))
| summarize SignInTime = min(TimeGenerated)  by UserPrincipalName, IPAddress, tostring(LocationDetails),AppDisplayName,ResourceDisplayName,UserDomain
) on $left.DomainName == $right.UserDomain
// Getting UserName and Domain
| extend Name = split(UserPrincipalName,"@",0), Domain = split(UserPrincipalName,"@",1)
| mv-expand Name,Domain

YAML

triggerThreshold: 0
requiredDataConnectors:
- connectorId: AzureActiveDirectory
  dataTypes:
  - SigninLogs
  - AuditLogs
severity: Medium
queryFrequency: 1h
id: fa00014c-c5f4-4715-8f5b-ba567e19e41e
relevantTechniques:
- T1098
queryPeriod: 1h
name: Possible SignIn from Azure Backdoor
customDetails:
  InitiatedBy: InitiatedBy
  AppDisplayName: AppDisplayName
  DomainAdded: DomainName
  ResourceDisplayName: ResourceDisplayName
  SignInTime: SignInTime
  ModifiedProperties: ModifiedProperties
  DomainAddedTime: DomainAddedTime
kind: Scheduled
tactics:
- Persistence
triggerOperator: gt
version: 1.0.0
entityMappings:
- entityType: Account
  fieldMappings:
  - columnName: UserDomain
    identifier: NTDomain
  - columnName: UserPrincipalName
    identifier: FullName
- entityType: IP
  fieldMappings:
  - columnName: IPAddress
    identifier: Address
description: |
    'Identifies when a user adds an unverified domain as an authentication method, followed by a sign-in from a user the newly added domain. Threat actors may add custom domains to create a backdoor to your tenant. It's important to monitor whenever custom domains are added to the tenant.'
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft Entra ID/Analytic Rules/PossibleSignInfromAzureBackdoor.yaml
query: |
  // Microsoft Entra ID  Backdoors: Identity Federation
  //Ref: https://www.inversecos.com/2021/11/how-to-detect-azure-active-directory.html
  AuditLogs
  | where OperationName == "Add unverified domain"
  | where Result == "success"
  | extend InitiatedBy = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
  | extend DomainName = tostring(TargetResources[0].displayName)
  | summarize DomainAddedTime = min(TimeGenerated), ModifiedProperties = make_set(parse_json(TargetResources[0].modifiedProperties),1048576) by InitiatedBy, DomainName
  | join kind=inner (
  SigninLogs
  | where ResultType == "0"
  | extend UserDomain = tostring(parse_json(split(UserPrincipalName,"@",1)[0]))
  | summarize SignInTime = min(TimeGenerated)  by UserPrincipalName, IPAddress, tostring(LocationDetails),AppDisplayName,ResourceDisplayName,UserDomain
  ) on $left.DomainName == $right.UserDomain
  // Getting UserName and Domain
  | extend Name = split(UserPrincipalName,"@",0), Domain = split(UserPrincipalName,"@",1)
  | mv-expand Name,Domain

ARM

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "workspace": {
      "type": "String"
    }
  },
  "resources": [
    {
      "apiVersion": "2024-01-01-preview",
      "id": "[concat(resourceId('Microsoft.OperationalInsights/workspaces/providers', parameters('workspace'), 'Microsoft.SecurityInsights'),'/alertRules/fa00014c-c5f4-4715-8f5b-ba567e19e41e')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/fa00014c-c5f4-4715-8f5b-ba567e19e41e')]",
      "properties": {
        "alertRuleTemplateName": "fa00014c-c5f4-4715-8f5b-ba567e19e41e",
        "customDetails": {
          "AppDisplayName": "AppDisplayName",
          "DomainAdded": "DomainName",
          "DomainAddedTime": "DomainAddedTime",
          "InitiatedBy": "InitiatedBy",
          "ModifiedProperties": "ModifiedProperties",
          "ResourceDisplayName": "ResourceDisplayName",
          "SignInTime": "SignInTime"
        },
        "description": "'Identifies when a user adds an unverified domain as an authentication method, followed by a sign-in from a user the newly added domain. Threat actors may add custom domains to create a backdoor to your tenant. It's important to monitor whenever custom domains are added to the tenant.'\n",
        "displayName": "Possible SignIn from Azure Backdoor",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "UserDomain",
                "identifier": "NTDomain"
              },
              {
                "columnName": "UserPrincipalName",
                "identifier": "FullName"
              }
            ]
          },
          {
            "entityType": "IP",
            "fieldMappings": [
              {
                "columnName": "IPAddress",
                "identifier": "Address"
              }
            ]
          }
        ],
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft Entra ID/Analytic Rules/PossibleSignInfromAzureBackdoor.yaml",
        "query": "// Microsoft Entra ID  Backdoors: Identity Federation\n//Ref: https://www.inversecos.com/2021/11/how-to-detect-azure-active-directory.html\nAuditLogs\n| where OperationName == \"Add unverified domain\"\n| where Result == \"success\"\n| extend InitiatedBy = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)\n| extend DomainName = tostring(TargetResources[0].displayName)\n| summarize DomainAddedTime = min(TimeGenerated), ModifiedProperties = make_set(parse_json(TargetResources[0].modifiedProperties),1048576) by InitiatedBy, DomainName\n| join kind=inner (\nSigninLogs\n| where ResultType == \"0\"\n| extend UserDomain = tostring(parse_json(split(UserPrincipalName,\"@\",1)[0]))\n| summarize SignInTime = min(TimeGenerated)  by UserPrincipalName, IPAddress, tostring(LocationDetails),AppDisplayName,ResourceDisplayName,UserDomain\n) on $left.DomainName == $right.UserDomain\n// Getting UserName and Domain\n| extend Name = split(UserPrincipalName,\"@\",0), Domain = split(UserPrincipalName,\"@\",1)\n| mv-expand Name,Domain\n",
        "queryFrequency": "PT1H",
        "queryPeriod": "PT1H",
        "severity": "Medium",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "Persistence"
        ],
        "techniques": [
          "T1098"
        ],
        "templateVersion": "1.0.0",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}