A quick blog on an updated security publication for Exchange Server 2016 and 2019. This publication addresses the following vulnerability:

CVE-2021-1730: Microsoft Exchange Server Spoofing Vulnerability

A spoofing vulnerability exists in Microsoft Exchange Server which could result in an attack that would allow a malicious actor to impersonate the user.

As mentioned in the CVE report, this vulnerability can be mitigated in Exchange 2016 and Exchange 2019 by implementing a separate namespace for inline images. These images are served when using Outlook Web Access. Since I never see customers implementing this option, I will repeat these steps below to bring this to your attention.

First, pick a namespace to serve these images from, e.g. img.mail.contoso.com. Create a CNAME for this entry in the DNS, and point it to your OWA namespace, for example img.mail.contoso.com. Add this namespace to your existing SSL certificate (SAN) unless you are using a wildcard certificate and the chosen namespace is covered by it.

Next, configure the InternalDownloadHostName and ExternalDownloadHostName properties from OWAVirtualDirectory configuration, e.g.

Get-OwaVirtualDirectory | Set-OwaVirtualDirectory -ExternalDownloadHostName img.mail.contoso.com -InternalDownloadHostName img.mail.contoso.com

Configure the Exchange organization to use download domains:

Set-OrganizationConfig -EnableDownloadDomains $true

Finally, restart IIS or recycle the OWA application pool using Restart-WebAppPool MSExchangeOWAAppPool.

A quick blog on security updates for Exchange Server 2016 and 2019. These fixes address the following vulnerability:

CVE-2021-24085: Microsoft Exchange Server Spoofing Vulnerability

The exploit can be fixed by single security update, which you can find in the table below per current Exchange version.

Be advised that these security updates are Cumulative Update level specific. You cannot apply the update for Exchange 2016 CU17 to Exchange 2016 CU16. Also, the security update download has the same name for different Cumulative Updates, and I would suggest tagging the file name with the CU level, e.g. Exchange2019-CU6-KB4588741-x64-en.msp.

Also, run the Security Update from an elevated command prompt, to prevent issues during installation. And on a final note, as with any patch or update, I’d recommend to apply this in a acceptance environment first, prior to implementing it in production.