Released: April 2021 Exchange Server Security Updates

Just had a call with microsoft support yesterday and wanted to share my info with you.

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Issue description: 

Per-User Free busy information in a trusted cross-forest topology not visible after security update KB5001779.

Free/Busy lookups based on the PerUser Availability Address Space mechanism do not work between trusted organizations (cross-forest) - Exchange 2013 On-premises organizations.

As we discussed, we are aware of issues with cross-forest Free/Busy requests appearing in some configurations after installation of the KB5001779 Security Update, released: April 2021 

Description of the security update for Microsoft Exchange Server 2019, 2016, and 2013: April 13, 2021 (KB5001779)

https://support.microsoft.com/en-us/topic/description-of-the-security-update-for-microsoft-exchange-server-2019-2016-and-2013-april-13-2021-kb5001779-8e08f3b3-fc7b-466c-bbb7-5d5aa16ef064

The issue appears in configurations using the “per-user free/busy information in a trusted cross-forest topology:

https://docs.microsoft.com/en-us/exchange/architecture/client-access/availability-service-for-cross-forest-topologies?view=exchserver-2016#use-the-exchange-management-shell-to-configure-per-user-freebusy-information-in-a-trusted-cross-forest-topology

The issue has already been reported to Our Product Group (developers of Exchange).

The problem is still under investigation.

Action plan (Workaround):

*Use the “organization-wide free/busy information in an untrusted cross-forest topology”:

*Use the Exchange Management Shell to configure organization-wide free/busy information in an untrusted cross-forest topology

https://docs.microsoft.com/en-us/exchange/architecture/client-access/availability-service-for-cross-forest-topologies?view=exchserver-2016#use-the-exchange-management-shell-to-configure-organization-wide-freebusy-information-in-an-untrusted-cross-forest-topology

*Instead of using the Availability Address Space mechanism, configure federation trusts in both the Exchange organizations and then configure organization relationships between the organizations:

 *Configure a federation trust

https://docs.microsoft.com/en-us/exchange/configure-a-federation-trust-exchange-2013-help

*Create an organization relationship

https://docs.microsoft.com/en-us/exchange/create-an-organization-relationship-exchange-2013-help

*Managing Federated Sharing with the EAC

https://techcommunity.microsoft.com/t5/exchange-team-blog/managing-federated-sharing-with-the-eac/ba-p/598111

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I did not test the workaround yet so i cannot tell if it is working.

Maybe this will help some of you.