A spoofing vulnerability exists in Microsoft Exchange Server which could result in an attack that would allow a malicious actor to impersonate the user (CVE-2021-1730).

To prevent these types of attacks, Microsoft recommends customers to download inline images from different URL than the rest of OWA. 

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-1730

Change Image Download URL

Basically, you need to change both the external & internal download hostname to a different domain name.

Launch the Exchange Management Shell, and execute the following commands

Set-OwaVirtualDirectory -Identity "owa (default Web site)"  -ExternalDownloadHostName “Images.DeepnetID.com" 
Set-OwaVirtualDirectory -Identity "owa (default Web site)"  -InternalDownloadHostName "Images.DeepnetID.com" 
Set-OrganizationConfig -EnableDownloadDomains $true

If MFA is enabled on OWA, then you must take the following steps

Add URL Binding

On the Exchange server, launch the IIS Management

Select the OWA node, and launch the DualShield IIS Agent console

Click "URL Bindings"

Add the image download URL in to the URL Bindings

Apply the change

Add HTTP Filter

If you enable the proxy option and use the IIS reverse proxy to proxy your DualShield SSO, then you also need to add the HTTP Filter below. Otherwise, your users will be asked to re-authenticate when they attempt to download a file attachment. 

Click "HTTP Filter"

Add the following HTTP filter

Apply the change.

Add URL Filter

Click "URL Filter"

Add the following URL filter

Apply the change.