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

requiredDataConnectors:
- dataTypes:
  - SigninLogs
  - AuditLogs
  connectorId: AzureActiveDirectory
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.'
relevantTechniques:
- T1098
tactics:
- Persistence
triggerThreshold: 0
triggerOperator: gt
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft Entra ID/Analytic Rules/PossibleSignInfromAzureBackdoor.yaml
version: 1.0.0
queryPeriod: 1h
customDetails:
  DomainAddedTime: DomainAddedTime
  AppDisplayName: AppDisplayName
  InitiatedBy: InitiatedBy
  ModifiedProperties: ModifiedProperties
  ResourceDisplayName: ResourceDisplayName
  DomainAdded: DomainName
  SignInTime: SignInTime
id: fa00014c-c5f4-4715-8f5b-ba567e19e41e
severity: Medium
kind: Scheduled
entityMappings:
- entityType: Account
  fieldMappings:
  - columnName: UserDomain
    identifier: NTDomain
  - columnName: UserPrincipalName
    identifier: FullName
- entityType: IP
  fieldMappings:
  - columnName: IPAddress
    identifier: Address
name: Possible SignIn from Azure Backdoor
queryFrequency: 1h
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