Information
The Common Attachment Types Filter lets a user block known and custom malicious file types from being attached to emails.
Blocking known malicious file types can help prevent malware-infested files from infecting a host.
Solution
To remediate using the UI:
- Navigate to Microsoft 365 Defender
https://security.microsoft.com
.
- Click to expand Email & collaboration select Policies & rules
- On the Policies & rules page select Threat policies
- Under polices select Anti-malware and click on the Default (Default) policy.
- On the Policy page that appears on the right hand pane scroll to the bottom and click on Edit protection settings check the Enable the common attachments filter
- Click Save.
To remediate using PowerShell:
- Connect to Exchange Online using Connect-ExchangeOnline
- Run the following Exchange Online PowerShell command:
Set-MalwareFilterPolicy -Identity Default -EnableFileFilter $true
Note: Audit and Remediation guidance may focus on the Default policy however, if a Custom Policy exists in the organization's tenant, then ensure the setting is set as outlined in the highest priority policy listed.
Impact:
Blocking common malicious file types should not cause an impact in modern computing environments.