Suspicious linking of existing user to external User

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Id	22a320c2-e1e5-4c74-a35b-39fc9cdcf859
Rulename	Suspicious linking of existing user to external User
Description	This query will detect when an attempt is made to update an existing user and link it to an guest or external identity. These activities are unusual and such linking of external identities should be investigated. In some cases you may see internal Entra ID sync accounts (Sync_) do this which may be benign
Severity	Medium
Tactics	PrivilegeEscalation
Techniques	T1078.004
Required data connectors	AzureActiveDirectory
Kind	Scheduled
Query frequency	1d
Query period	1d
Trigger threshold	0
Trigger operator	gt
Source Uri	https://github.com/Azure/Azure-Sentinel/blob/master/Detections/AuditLogs/SuspiciousLinkingofExternalIdtoExistingUsers.yaml
Version	1.1.0
Arm template	22a320c2-e1e5-4c74-a35b-39fc9cdcf859.json

KQL

AuditLogs
| where OperationName=~ "Update user" 
| where Result =~ "success" 
| mv-expand TargetResources 
| mv-expand TargetResources.modifiedProperties 
| extend displayName = tostring(TargetResources_modifiedProperties.displayName), 
TargetUPN_oldValue = tostring(parse_json(tostring(TargetResources_modifiedProperties.oldValue))[0]), 
TargetUPN_newValue = tostring(parse_json(tostring(TargetResources_modifiedProperties.newValue))[0])
| where displayName == "UserPrincipalName" and TargetUPN_oldValue !has "#EXT" and TargetUPN_newValue has "#EXT"
| extend InitiatingAppName = tostring(InitiatedBy.app.displayName)
| extend InitiatingAppServicePrincipalId = tostring(InitiatedBy.app.servicePrincipalId)
| extend InitiatingUserPrincipalName = tostring(InitiatedBy.user.userPrincipalName)
| extend InitiatingAadUserId = tostring(InitiatedBy.user.id)
| extend InitiatingIPAddress = tostring(InitiatedBy.user.ipAddress)
| extend InitiatedBy = tostring(iff(isnotempty(InitiatingUserPrincipalName),InitiatingUserPrincipalName, InitiatingAppName))
| summarize arg_max(TimeGenerated, *) by CorrelationId
| project-reorder TimeGenerated, InitiatedBy, InitiatingAppName, InitiatingAppServicePrincipalId, InitiatingUserPrincipalName, InitiatingAadUserId, InitiatingIPAddress, TargetUPN_oldValue, TargetUPN_newValue
| extend InitiatingAccountName = tostring(split(InitiatingUserPrincipalName, "@")[0]), InitiatingAccountUPNSuffix = tostring(split(InitiatingUserPrincipalName, "@")[1])
| extend TargetAccountName = tostring(split(TargetUPN_oldValue, "@")[0]), TargetUPNSuffix = tostring(split(TargetUPN_oldValue, "@")[1])

YAML

OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Detections/AuditLogs/SuspiciousLinkingofExternalIdtoExistingUsers.yaml
id: 22a320c2-e1e5-4c74-a35b-39fc9cdcf859
triggerThreshold: 0
triggerOperator: gt
query: |
  AuditLogs
  | where OperationName=~ "Update user" 
  | where Result =~ "success" 
  | mv-expand TargetResources 
  | mv-expand TargetResources.modifiedProperties 
  | extend displayName = tostring(TargetResources_modifiedProperties.displayName), 
  TargetUPN_oldValue = tostring(parse_json(tostring(TargetResources_modifiedProperties.oldValue))[0]), 
  TargetUPN_newValue = tostring(parse_json(tostring(TargetResources_modifiedProperties.newValue))[0])
  | where displayName == "UserPrincipalName" and TargetUPN_oldValue !has "#EXT" and TargetUPN_newValue has "#EXT"
  | extend InitiatingAppName = tostring(InitiatedBy.app.displayName)
  | extend InitiatingAppServicePrincipalId = tostring(InitiatedBy.app.servicePrincipalId)
  | extend InitiatingUserPrincipalName = tostring(InitiatedBy.user.userPrincipalName)
  | extend InitiatingAadUserId = tostring(InitiatedBy.user.id)
  | extend InitiatingIPAddress = tostring(InitiatedBy.user.ipAddress)
  | extend InitiatedBy = tostring(iff(isnotempty(InitiatingUserPrincipalName),InitiatingUserPrincipalName, InitiatingAppName))
  | summarize arg_max(TimeGenerated, *) by CorrelationId
  | project-reorder TimeGenerated, InitiatedBy, InitiatingAppName, InitiatingAppServicePrincipalId, InitiatingUserPrincipalName, InitiatingAadUserId, InitiatingIPAddress, TargetUPN_oldValue, TargetUPN_newValue
  | extend InitiatingAccountName = tostring(split(InitiatingUserPrincipalName, "@")[0]), InitiatingAccountUPNSuffix = tostring(split(InitiatingUserPrincipalName, "@")[1])
  | extend TargetAccountName = tostring(split(TargetUPN_oldValue, "@")[0]), TargetUPNSuffix = tostring(split(TargetUPN_oldValue, "@")[1])  
name: Suspicious linking of existing user to external User
kind: Scheduled
tactics:
- PrivilegeEscalation
requiredDataConnectors:
- connectorId: AzureActiveDirectory
  dataTypes:
  - AuditLogs
relevantTechniques:
- T1078.004
queryFrequency: 1d
entityMappings:
- entityType: Account
  fieldMappings:
  - identifier: Name
    columnName: InitiatingAppName
  - identifier: AadUserId
    columnName: InitiatingAppServicePrincipalId
- entityType: Account
  fieldMappings:
  - identifier: FullName
    columnName: InitiatingUserPrincipalName
  - identifier: Name
    columnName: InitiatingAccountName
  - identifier: UPNSuffix
    columnName: InitiatingAccountUPNSuffix
- entityType: Account
  fieldMappings:
  - identifier: AadUserId
    columnName: InitiatingAadUserId
- entityType: Account
  fieldMappings:
  - identifier: FullName
    columnName: TargetUPN_oldValue
  - identifier: Name
    columnName: TargetAccountName
  - identifier: UPNSuffix
    columnName: TargetUPNSuffix
- entityType: IP
  fieldMappings:
  - identifier: Address
    columnName: InitiatingIPAddress
metadata:
  source:
    kind: Community
  categories:
    domains:
    - Security - Others
    - Identity
  support:
    tier: Community
  author:
    name: Microsoft Security Research
description: |
  ' This query will detect when an attempt is made to update an existing user and link it to an guest or external identity. These activities are unusual and such linking of external 
  identities should be investigated. In some cases you may see internal Entra ID sync accounts (Sync_) do this which may be benign'  
queryPeriod: 1d
version: 1.1.0
tags:
- GuestorExternalIdentities
severity: Medium

ARM