Microsoft Pushes Deprecation of Some Client Access Rules to September 2024

Extra Year to Give Tenants Time to Overcome Technical Limitations that Prevent Migration

Without much fanfare, the Exchange Online team decided to give tenants that use client access rules an extra year to make the transition to Azure AD conditional access policies. The original deprecation date of September 2023 is now September 2024. The kicker in this statement is that only rules that cannot be migrated to conditional access policies because of a “technical limitation” can continue working for the extra year.

First announced at the Microsoft Ignite 2017 conference, client access rules are only available in Exchange Online and exist to control connections coming into Exchange Online. You can do things like block POP3 and IMAP4 connections or restrict access to these protocols from specific IP addresses.

Client access rules are a solution created by Exchange Online for an environment that was very different to today. In 2017, customers wanted help to control inbound connections to Exchange Online. Azure AD conditional access policies didn’t have the feature set that’s available now and Exchange Online still supported basic authentication across all its connectivity protocols.

The Current State of Microsoft 365 Connection Management

Moving forward to today, Microsoft has largely eliminated basic authentication from Exchange Online to cut off techniques like password sprays that attackers heavily depended on to retrieve user account credentials. Attackers still try, but attempts to use basic authentication to check username/password combinations fail at the first hurdle. Apart from keeping access to SMTP AUTH, there’s no further need for authentication policies.

Deprecating client access rules could be construed as another side effect of modern authentication becoming the norm in Exchange Online. But the more important factor is that Microsoft 365 favors features that function across the entire ecosystem instead of being tied to a single workload. Azure AD is the directory of record and conditional access policies are how Azure AD controls inbound connections. Dropping client access rules (specific to Exchange Online) to embrace conditional access policies is evidence of that trend. We see the same in the compliance and information protection areas where Microsoft 365 policies take prime position.

Microsoft is pouring engineering effort into Azure AD conditional access policies. In the last year alone, Microsoft has added features such as:

• Granular access for external user types.
• App filters.
• Authentication strengths.

Conditional access policies are closely associated with multi-factor authentication and are often configured to ensure that inbound user connections use MFA, especially for administrator accounts.

Migration Away from Workload-Specific Implementations Can be Problematic

Moving from a workload-specific implementation to a platform-wide implementation can be problematic. It’s likely that the workload-specific code covers narrower use cases that only occur within a workload. For example, Exchange Online mailbox retention policies can process items at a folder level and move items to the online archive when their retention period expires. By comparison, Microsoft 365 retention polices process mailboxes as a single unit and don’t go to folder level and can only delete or keep items instead of being able to move them to the archive. Then again, because Microsoft is actively developing Microsoft 365 retention policies, they benefit from recent innovations like adaptive scopes and disposition reviews, neither of which are available for Exchange mailbox retention policies.

In the case of client access rules, Microsoft acknowledges that they “have encountered a few scenarios where it’s not possible to migrate current rules” and say that they will allow the ongoing use of client access rules “until we can support them” (presumably the scenarios that migration is not currently possible).

Microsoft doesn’t describe what scenarios are problematic. They also don’t say how customers can discover if their client access rules fall into the category of those that Microsoft consider to have a technical limitation that prevents migration. Microsoft plans to disable client access rules that they consider can be migrated to conditional access policies in September 2023. Only rules that receive Microsoft approval can continue running until September 2024, at which point the curtain descends for all client access rules (Figure 1).

Apart from not documenting the technical limitations that get in the way of migration, Microsoft also doesn’t say how customers receive approval for the one-year extension for client access rules. The blog says that customers should “open a support ticket so we can investigate and understand your needs,” but doesn’t explain how this process results in a rule being allowed to continue running past September 2023.

The Filtering Issue

Browsing the documentation for client access rules, the obvious issue appears to be in the areas of filtering. Like in other areas (like dynamic distribution lists), Exchange Online supports recipient filters for client access rules in a different manner than operated for conditional access policies. You can assign conditional access policies to specific users and groups (including dynamic Microsoft 365 groups), but that’s not quite the same as using a recipient filter. I’m sure that I am missing something else, but recipient filtering seems like the big obstacle for migration.

I strongly support moving away from client access rules to embrace more secure implementations like conditional access policies that operate across the complete Microsoft 365 ecosystem. Microsoft throws continuous access evaluation (CAE) into the pot. Although CAE is a very good innovation to revoke access from accounts immediately important events like password changes occur, the issue here is all about migration.

Client Access Rules Migration Might Need Updates to Conditional Access Policies

If you’re using client access rules, it’s time to review the state of the migration.

• Remove client access rules that are no longer necessary.
• Migrate the remaining rules to conditional access policies.
• If any rules cannot be migrated, contact Microsoft Support to discover if a technical limitation exists to prevent migration. If it does, the rules can continue working until September 2024. If not, Microsoft Support should be able to tell you how to migrate to conditional access policies before September 2023.

Microsoft doesn’t say how they will address the technical limitations that allow some client access rules to remain operational until September 2024. This might be because the Exchange Online team is waiting for their Azure AD colleagues to implement some features in conditional access policies. At least, I hope that’s what’s happening.

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