Unauthorized Guest User Invitation Attempt in Azure

This alert detects instances where a user attempts to invite an external guest user to an Azure environment but fails due to insufficient permissions. This activity can signify several potential security risks, including unauthorized privilege escalation attempts by internal users or malicious insiders attempting to expand access without proper authorization. While legitimate failed attempts may occur, repeated or targeted failures should be investigated. The activity is logged within the Azure Audit Logs. Detecting this activity is crucial for maintaining control over user access and preventing potential data breaches. The relevant log data resides within Azure’s audit logs.

Attack Chain

1. An internal user (either compromised or malicious) attempts to invite an external guest user via the Azure portal or API.
2. The Azure Active Directory service checks the inviter’s permissions against the organization’s guest invitation policies.
3. The system determines the user lacks the necessary permissions to invite guest users.
4. Azure Audit Logs record the “Invite external user” event with a “failure” status.
5. The failed invitation attempt is blocked, preventing the external user from gaining access.
6. The attacker may retry the invitation with different accounts or methods, attempting to bypass access controls.
7. If successful through other means (not detected by this rule), the guest user could be used for lateral movement or data exfiltration.

Impact

A successful privilege escalation could grant unauthorized access to sensitive data and resources within the Azure environment. While this specific detection focuses on failed attempts, repeated failures may indicate a concerted effort to bypass security controls. If successful, unauthorized guest users could be used for lateral movement, data exfiltration, or other malicious activities. The number of affected resources depends on the permissions granted to the guest user if the invitation had been successful.

Recommendation

• Deploy the Sigma rule “Guest User Invited By Non Approved Inviters” to your SIEM to detect unauthorized invitation attempts within Azure Audit Logs.
• Investigate any alerts generated by the Sigma rule to determine the legitimacy of the invitation attempt and the intent of the user.
• Review and enforce the principle of least privilege for user roles and permissions within Azure Active Directory.
• Monitor for repeated failed invitation attempts from the same user account (correlate with the Azure Audit Logs data).