April 2021 Exchange Server Security Updates

Microsoft released several Security Updates (SUs) for Microsoft Exchange Server to address vulnerabilities. Due to the critical nature of these vulnerabilities, we recommend that customers apply the updates to affected systems immediately to protect the environment.

Note: These vulnerabilities affect Microsoft Exchange Server. Exchange Online is not affected.

Exchange Server Security Updates

Microsoft has released a set of out of band Security Updates for vulnerabilities for the following versions of Exchange Server:

• Exchange Server 2013
• Exchange Server 2016
• Exchange Server 2019

Security Updates are available for the following specific versions of Exchange:

• Exchange Server 2013 (CU23)
• Exchange Server 2016 (CU19, CU20)
• Exchange Server 2019 (CU8, CU9)

Read more on how to Install Exchange Security Update.

If you are not at these Exchange Server CU versions, please update right now and apply the above patch.

Read more on how to Install Exchange Cumulative Update.

Vulnerabilities addressed in the April 2021 Security Updates were responsibly reported to Microsoft by a security partner. Although we are not aware of any active exploits in the wild, our recommendation is to install these updates immediately to protect your environment.

We recommend prioritizing installing updates on Exchange Servers that are externally facing. All affected Exchange Servers should ultimately be updated.

FAQs

The last SU that we installed is (a few months old). Do we need to install all SUs in order, to install the latest one?
The Exchange Server Security Updates are cumulative. If you are running the CU that the SU can be installed on, you do not need to install all the SUs in sequential order but can install the latest SU only.

My organization is in Hybrid mode with Exchange Online. Do I need to do anything?
While Exchange Online customers are already protected, the April 2021 security updates do need to be applied to your on-premises Exchange Servers, even if they are used only for management purposes. You do not need to re-run the Hybrid Configuration Wizard (HCW) after applying updates.

Do I need to install the updates on “Exchange Management Tools only” workstations?
Servers or workstations running only Microsoft Exchange Management Tools (no Exchange services) do not need to apply these updates.

Further information

• Microsoft Security Response Center (MSRC)
• Exchange Team Blog
• CVE-2021-28480
• CVE-2021-28481
• CVE-2021-28482
• CVE-2021-28483