In a previous blog, we introduced Continuous Access Evaluation (CAE) – a product that brings Zero Trust principles to session management. Today we would like to discuss securing cross-tenant access with a focus on preventing data exfiltration.
It’s impossible to imagine a successful modern organization that doesn’t collaborate with partners across organizational boundaries. While cross-company collaboration empowers employees and enables partnerships, it also lowers barriers for both accidental and malicious data exfiltration. Microsoft Cross-Tenant Access Settings is designed to address security of cross-company exchange.
Outbound and Inbound Cross-Tenant Access Settings offer fine grain security controls for cross-company collaboration using user’s home identity, while Tenant Restriction v2 (TRv2) can be used to prevent data exfiltration using foreign identity.
Some of the hardest-to-prevent data leaks happen when users inside your organization use foreign identities to connect to external tenants. Let’s consider one such attack. A malicious insider creates a Microsoft Entra tenant. Then they authenticate to their malicious tenant from your organization’s device. Now the attacker can leak your files via email using the Exchange Online account of the malicious tenant. These types of attacks can be described as creating a “USB dongle in a cloud.” Regular security methods do not work against such attacks. Your tenant’s policies do not apply to external identities that attackers use. Blocking Microsoft Entra ID or Exchange Online URIs in the firewall would block your legitimate users along with the attacker. These types of attack need special defenses that TRv2 provides.
TRv2 works by sending special signals to Entra ID, Microsoft Account and other Microsoft resources. These signals point to Cross-Tenant Access Settings’ TRv2 policy that you created. Microsoft resources evaluate the policy and block unsanctioned access. We have two major flavors of TRv2.
Auth Plane TRv2 can block logins with external identities based on policy. To configure it you need to deploy a network proxy in your organization and configure that proxy to set TRv2 signals on all traffic to Entra ID and Microsoft Account. In the above example of a malicious insider leaking data over external email, the attacker will not be able to login to their malicious tenant and therefore will not be able to send email. Auth Plane TRv2 is now generally available.
Universal TRv2 as part of Microsoft Entra Global Secure Access goes one step further to protect against more sophisticated attacks where an attacker bypasses authentication by allowing anonymous access to the malicious tenant’s apps, such as anonymous meeting join in Teams. Or the attacker can import to your organizational device an access token lifted from a device in the malicious tenant. All these attack vectors bypass login to Entra ID. Since Universal TRv2 sends TRv2 signals on authentication plane (Entra ID and Microsoft Account) and data plane (Microsoft cloud applications), these attacks will be prevented. Universal TRv2 is currently in public preview.
We have another flavor of TRv2 in public preview – TRv2 on Windows. It’s a partial solution that protects the authentication and data planes but only for some scenarios. It only works on managed Windows devices and does not protect .NET stack, Chrome, or Firefox. We have heard from customers that it is difficult to deploy and does not provide adequate security. The Windows solution was meant to provide temporary protection until Universal TRv2 is released and we’re planning to retire it after Universal TRv2 is generally available.
Principal Product Manager, Identity Division
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