Microsoft’s new Azure Active Directory Cross-Tenant Synchronization (CTS) feature, introduced in June 2023, has raised concerns about potential security risks. The feature allows an administrator to synchronize users and groups across multiple Azure tenants, facilitating seamless collaboration and automating B2B project management.
However, cybersecurity researchers have identified that improper configuration of CTS could create a new attack surface, enabling threat actors to spread laterally to other connected tenants. Attackers who have already compromised a privileged account within a tenant may exploit this feature to move laterally and gain unauthorized access to other linked tenants, establishing persistence on their networks.
Cybersecurity firm Vectra elaborates on how threat actors could potentially abuse the CTS feature for lateral movement and persistence. In their report, Vectra outlines two techniques. The first involves reviewing CTS configurations to identify target tenants with ‘Outbound Sync’ enabled, allowing syncing to other tenants. Attackers modify the CTS app’s configuration to include the compromised user in its sync scope, providing access to the other tenant’s network and enabling lateral movement.
The second technique focuses on deploying a rogue CTS configuration for persistent access to target tenants. This method requires a threat actor to compromise a privileged account in the tenant and deploy a new CTS policy with ‘Inbound Sync’ and ‘Automatic User Consent’ enabled, granting access to the target tenant anytime via an external account.
While no known attacks have exploited this feature yet, Vectra advises organizations to take preventive measures. Target tenants should avoid implementing default or overly inclusive inbound CTS configurations and set limits on which users and groups can access their cloud environments. Source tenants, which act as the initial breach points, should monitor all privileged users for suspicious activity.
Understanding and monitoring CTS activity logs can help administrators detect malicious behavior and prevent potential abuse. The report serves as a warning to Azure users to proactively address the configuration of the CTS feature to mitigate potential security risks and protect their cloud environments from lateral movement attacks.
