Guest Users Invited to Tenant by New Inviters

Hunt Hypothesis

Guest users being invited by new inviters may indicate lateral movement or unauthorized access attempts by adversaries seeking to establish persistence within the tenant. SOC teams should proactively hunt for this behavior to identify potential compromise and limit the spread of malicious activity in their Azure Sentinel environment.

KQL Query

let inviting_users = (AuditLogs
  | where TimeGenerated between(ago(14d)..ago(1d))
  | where OperationName =~ "Invite external user"
  | where Result =~ "success"
  | extend InitiatingUserPrincipalName = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
  | where isnotempty(InitiatingUserPrincipalName)
  | summarize by InitiatingUserPrincipalName);
  AuditLogs
  | where TimeGenerated > ago(1d)
  | where OperationName =~ "Invite external user"
  | where Result =~ "success"
  | extend InitiatingUserPrincipalName = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
  | extend InitiatingAadUserId = tostring(InitiatedBy.user.id)
  | extend InitiatingIPAddress = tostring(InitiatedBy.user.ipAddress)
  | where isnotempty(InitiatingUserPrincipalName) and InitiatingUserPrincipalName !in (inviting_users)
  | extend TargetUserPrincipalName = tostring(TargetResources[0].userPrincipalName)
  | extend TargetAadUserId = tostring(TargetResources[0].id)
  | extend invitingUser = InitiatingUserPrincipalName, invitedUserPrincipalName = TargetUserPrincipalName
  | extend InitiatingAccountName = tostring(split(InitiatingUserPrincipalName, "@")[0]), InitiatingAccountUPNSuffix = tostring(split(InitiatingUserPrincipalName, "@")[1])
  | extend TargetAccountName = tostring(split(TargetUserPrincipalName, "@")[0]), TargetAccountUPNSuffix = tostring(split(TargetUserPrincipalName, "@")[1])
  | project-reorder TimeGenerated, OperationName, Result, TargetUserPrincipalName, TargetAadUserId, InitiatingUserPrincipalName, InitiatingAadUserId, InitiatingIPAddress

Analytic Rule Definition

id: 572e75ef-5147-49d9-9d65-13f2ed1e3a86
name: Guest Users Invited to Tenant by New Inviters
description: |
  'Detects when a Guest User is added by a user account that has not been seen adding a guest in the previous 14 days.
    Monitoring guest accounts and the access they are provided is important to detect potential account abuse.
    Accounts added should be investigated to ensure the activity was legitimate.
    Ref: https://docs.microsoft.com/azure/active-directory/fundamentals/security-operations-user-accounts#monitoring-for-failed-unusual-sign-ins'
severity: Medium
requiredDataConnectors:
  - connectorId: AzureActiveDirectory
    dataTypes:
      - AuditLogs
queryFrequency: 1d
queryPeriod: 14d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - Persistence
relevantTechniques:
  - T1078.004
tags:
  - AADSecOpsGuide
query: |
  let inviting_users = (AuditLogs
    | where TimeGenerated between(ago(14d)..ago(1d))
    | where OperationName =~ "Invite external user"
    | where Result =~ "success"
    | extend InitiatingUserPrincipalName = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
    | where isnotempty(InitiatingUserPrincipalName)
    | summarize by InitiatingUserPrincipalName);
    AuditLogs
    | where TimeGenerated > ago(1d)
    | where OperationName =~ "Invite external user"
    | where Result =~ "success"
    | extend InitiatingUserPrincipalName = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
    | extend InitiatingAadUserId = tostring(InitiatedBy.user.id)
    | extend InitiatingIPAddress = tostring(InitiatedBy.user.ipAddress)
    | where isnotempty(InitiatingUserPrincipalName) and InitiatingUserPrincipalName !in (inviting_users)
    | extend TargetUserPrincipalName = tostring(TargetResources[0].userPrincipalName)
    | extend TargetAadUserId = tostring(TargetResources[0].id)
    | extend invitingUser = InitiatingUserPrincipalName, invitedUserPrincipalName = TargetUserPrincipalName
    | extend InitiatingAccountName = tostring(split(InitiatingUserPrincipalName, "@")[0]), InitiatingAccountUPNSuffix = tostring(split(InitiatingUserPrincipalName, "@")[1])
    | extend TargetAccountName = tostring(split(TargetUserPrincipalName, "@")[0]), TargetAccountUPNSuffix = tostring(split(TargetUserPrincipalName, "@")[1])
    | project-reorder TimeGenerated, OperationName, Result, TargetUserPrincipalName, TargetAadUserId, InitiatingUserPrincipalName, InitiatingAadUserId, InitiatingIPAddress
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: InitiatingUserPrincipalName
      - identifier: Name
        columnName: InitiatingAccountName
      - identifier: UPNSuffix
        columnName: InitiatingAccountUPNSuffix
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: TargetUserPrincipalName
      - identifier: Name
        columnName: TargetAccountName
      - identifier: UPNSuffix
        columnName: TargetAccountUPNSu

Required Data Sources

MITRE ATT&CK Context

• Tactic: Persistence
• Technique: T1078.004 — T1078.004

References

• Source Query

False Positive Guidance

• Scenario: A system administrator schedules a weekly backup job that temporarily adds a guest user to access a specific database.
  Filter/Exclusion: Exclude guest users added by service accounts or scheduled jobs (e.g., user_type = "service_account" or job_name LIKE '%backup%').

• Scenario: A user is re-invited to a tenant after a long period of inactivity, which triggers the rule due to the user not having invited guests in the last 14 days.
  Filter/Exclusion: Exclude users who have been inactive for over 90 days or users with a “re-invite” flag set (e.g., user_status = "inactive" or re_invite_flag = true).

• Scenario: A new user is onboarded and immediately invited to collaborate on a project, which is part of a standard onboarding process.
  Filter/Exclusion: Exclude users who are part of a known onboarding workflow (e.g., user_role = "new_hire" or onboarding_status = "complete").

• Scenario: A user is granted temporary access to a shared folder via a guest account for a specific task, which is part of a regular team collaboration process.
  Filter/Exclusion: Exclude guest users added by users with the “Guest Inviter” role or those associated with shared folders (e.g., user_role = "guest_inviter" or folder_type = "shared").

• Scenario: A user is added as a guest to a tenant via an automated integration (e.g., Azure AD Connect) that syncs users from an external directory.
  Filter/Exclusion: Exclude users added via integration tools (e.g., source_system = "Azure_AD_Connect" or integration_id = "external_directory") or users added during sync processes.