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New Conditional Access Policy Settings to Exert Granular Control Over External Users
Building on their recent work to improve Azure AD conditional access policies by adding a control to require specific authentication strengths for connections, Microsoft has released another interesting control (in preview). You can now differentiate between the different kinds of external users that connect to your tenant in a feature that Microsoft calls “fine-grained Azure B2B access control.”
Azure AD recognizes different kinds of connections based on the authentication flow, so it’s able to focus on connections such as B2B Collaboration like guests accessing a SharePoint Online site or when an account authenticated in another tenant uses B2B Direct Connect to access a Teams shared channel. The differentiation between connection types allows Azure AD to apply conditional access to impose conditions on specific connections.
Adding Control for External Users
To try the new control out, I created a new conditional access policy. Under the assignments section, I chose to include specific users. This option has always been available, but now you get to pick from the different types of external users supported by Azure AD (Figure 1).
For most Microsoft 365 tenants, the interesting options are B2B Collaboration and B2B Direct Connect. Guest accounts created using Azure B2B Collaboration have been in use since mid-2016 to support external access to resources in Outlook Groups, Teams, SharePoint Online, OneDrive for Business, Yammer, and Planner. The Azure B2B Collaboration policy is available to control the creation of guest accounts using a block list of domains. Even with a policy in place, tenants end up with large numbers of guest accounts and need to do some pruning to remove obsolete guests.
External accounts that use B2B Direct Connect to access Teams shared channels (the only workload currently supported) don’t have a presence in the tenant directory. Instead, these accounts authenticate against their own directory and present the credentials to gain access to the resources in the host tenant. If the cross-tenant access policies configured in both tenants permit access, the accounts can work with the resources.
The external user control includes other account types used in more specific circumstances. The point is that a lot of flexibility exists in the control of inbound connections. For instance, you can restrict the control to specific Microsoft 365 tenants (Figure 2) using either the tenant identifier or a registered domain for the tenant to add it to the policy (if you don’t know the tenant identifier, you can find it using an online service).
The new control works alongside the other controls available in a conditional access policy. In this instance, I configured the policy to apply to Office 365 apps and to require multi-factor authentication to grant access.
Planning Conditional Access Policies
An Azure AD tenant can support up to 195 conditional access policies. It takes planning to make sure that you don’t create a set of conditional access policies that conflict with each other and that each policy serves a well-defined purpose. For instance, the new ability to control external connections from specific tenants might tempt administrators to create to create multiple policies to control external access for external access from specific tenants. This is a bad idea and will probably be a maintenance nightmare. Try to use the one policy to handle external access from all partner tenants as it’s likely that much the same kind of controls will apply to all.
To make sure that other policies didn’t interfere with testing, I put any policy relating to external access into report-only mode.
Testing the Control
To test that the new policy worked as expected, I signed into Teams using an account belonging to the tenant specified in the policy. I then opened a shared channel and was immediately promoted with an MFA challenge. After satisfying the challenge, the client connected to the shared channel and the account could post messages. Figure 3 shows the authentication and conditional access details for a sign-in processed by the conditional access policy.
One More Control for Connections
Conditional access policies are not a universal panacea for keeping a Microsoft 365 tenant safe and secure from attackers. However, correctly configured and deployed, conditional access policies can stop people who shouldn’t access tenant resources from getting in. The new find-grained external access control is helpful in this respect. Remember that conditional access is an Azure AD Premium feature and deploy it alongside Exchange Online authentication policies to gain maximum protection from attacker probes.
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