[20Feb] Added information regarding issues reported.

The Exchange product group released February updates for Exchange Server 2013, 2016 and 2019.

The vulnerabilities addressed in these Security Updates are:

Vulnerability	Category	Severity	Rating
CVE-2023-21529	Remote Code Execution	Important	CVSS:3.1 8.8 / 7.7
CVE-2023-21706	Remote Code Execution	Important	CVSS:3.1 8.8 / 7.7
CVE-2023-21707	Remote Code Execution	Important	CVSS:3.1 8.8 / 7.7
CVE-2023-21710	Remote Code Execution	Important	CVSS:3.1 7.2 / 6.3

The Security Updates for each supported Exchange Server build are linked below:

Exchange	Download	Build	KB	Supersedes
Exchange 2019 CU12	Download	15.2.1118.25	KB5023038	KB5022193
Exchange 2019 CU11	Download	15.2.986.41	KB5023038	KB5022193
Exchange 2016 CU23	Download	15.1.2507.21	KB5023038	KB5022143
Exchange 2013 CU23	Download	15.0.1497.47	KB5023038	KB5022188

Note: As mentioned last month, be advised that Exchange Server 2013 support will end in April, 2023. This means: Exchange 2013 will stop to receive security updates. Recommendation is to upgrade to a more recent version, or move to Exchange Online.

Other Issues
Apart from security fixes, these SUs also fix the following:

Issue	Exchange 2013	Exchange 2016	Exchange 2019
Export-UMPrompt fails with InvalidResponseException	Yes	Yes	N/A
Edge Transport service returns an “EseNtOutOfSessions” Exception	Yes	Yes	Yes
Exchange services in automatic startup mode do not start automatically	Yes	Yes	Yes
Data source returns incorrect checkpoint depth	Yes	Yes	Yes
Serialization fails while tried accessing Mailbox Searches in ECP	Yes	Yes	Yes
Transport delivery service mishandles iCAL events	Yes	Yes	Yes

Notes:

• Security updates are Cumulative Update level specific. You cannot apply the update for Exchange 2019 CU12 to Exchange 2019 CU11. When downloading, the security update will carry the same name for different Cumulative Updates, and I would suggest tagging the file name with the CU level when archiving it, e.g. Exchange2019-CU12-KBXXXXXX-x64-en.msp.
• Similar to Cumulative Updates, Security Updates are cumulative and you only need to install the latest SU for your CU.
• Exchange servers running as part of hybrid deployment are managed through PowerShell, and thus need to be receive this patch and eventually be enabled for payload signing.
• If you have installed the Exchange Management Tools separately for managing your on-premises Exchange Servers, or installed it after removal of the Last Exchange Server for recipient management, it is recommended to apply the Security Update.

On a final note, as with any patch or update, I’d recommend to apply this in a test environment first, prior to implementing it in production. However, it is not recommended to wait for regular maintenance cycles when it concerns security updates, and follow a more agile approach; the ratings are an indication of the urgency.

[20Feb] Shortly after release, people reported through the comments that EWS started having issues after deploying the security update. Symptoms reported were problems with (server side) searches, add-ins not loading, and calendar operations such as scheduling or sharing taking a long time to load. Since it’s EWS having problems, applications depending on this protocol also may stop to work, such as Teams.

Meanwhile, Microsoft acknowledged an issue with the initial publication, and published workaround. If experience issues and see the event 4999 in your Eventlog:

E12IIS, c-RTL-AMD64, 15.01.2507.021, w3wp#MSExchangeServicesAppPool, M.Exchange.Diagnostics, M.E.D.ChainedSerializationBinder.EnforceBlockReason, M.E.Diagnostics.BlockedDeserializeTypeException, 437c-dumptidset, 15.01.2507.021.

follow the instructions in the following KB article link:

1. On each Exchange server, create a registry key
   New-ItemProperty -Path HKLM:\SOFTWARE\Microsoft\ExchangeServer\v15\Diagnostics -Name 'DisableBaseTypeCheckForDeserialization' -Value 1 -Type String
2. Create a global override setting
   New-SettingOverride -Name 'Adding learning location ClientExtensionCollectionFormatter' -Component Data -Section DeserializationBinderSettings -Parameters @('LearningLocations=ClientExtensionCollectionFormatter') -Reason 'Deserialization failed'
3. If you cannot wait until the override configuration kicks in (may take an one hour), refresh it manually:
   • Get-ExchangeDiagnosticInfo -Process Microsoft.Exchange.Directory.TopologyService -Component VariantConfiguration -Argument Refresh
   • Restart IIS and the Windows Activation Proces on each server
     Restart-Service -Name W3SVC, WAS -Force

Be advised that event 4999 might still show up in your Eventlog, and it has been reported that this might not completely does away with the issues reported. Keep an eye on the original post and EHLO blog for any future updates.