Detections
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2.1.13 (L1) Ensure the connection filter safe list is off

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Information

In Microsoft 365 organizations with Exchange Online mailboxes or standalone Exchange Online Protection (EOP) organizations without Exchange Online mailboxes, connection filtering and the default connection filter policy identify good or bad source email servers by IP addresses. The key components of the default connection filter policy are IP Allow List, IP Block List and Safe list.

The safe list is a pre-configured allow list that is dynamically updated by Microsoft.

The recommended safe list state is: Off or False

Without additional verification like mail flow rules, email from sources in the IP Allow List skips spam filtering and sender authentication (SPF, DKIM, DMARC) checks. This method creates a high risk of attackers successfully delivering email to the Inbox that would otherwise be filtered. Messages that are determined to be malware or high confidence phishing are filtered.

The safe list is managed dynamically by Microsoft, and administrators do not have visibility into which sender are included. Incoming messages from email servers on the safe list bypass spam filtering.

Solution

To remediate using the UI:

 - Navigate to Microsoft 365 Defender https://security.microsoft.com.
 - Click to expand Email & collaboration select Policies & rules > Threat policies.
 - Under Policies select Anti-spam.
 - Click on the Connection filter policy (Default).
 - Click Edit connection filter policy.
 - Uncheck Turn on safe list.
 - Click Save.

 To remediate using PowerShell:

 - Connect to Exchange Online using Connect-ExchangeOnline.
 - Run the following PowerShell command:
Set-HostedConnectionFilterPolicy -Identity Default -EnableSafeList $false

Impact:

This is the default behavior. IP Allow lists may reduce false positives, however, this benefit is outweighed by the importance of a policy which scans all messages regardless of the origin. This supports the principle of zero trust.

See Also

https://workbench.cisecurity.org/benchmarks/24619

Item Details

Category: SYSTEM AND INFORMATION INTEGRITY

References: 800-53|SI-3, 800-53|SI-8, 800-53|SI-16

Plugin: microsoft_azure

Control ID: c704d75f954243e3482100d34371a88f77ff62362f3ab0b6c884ef508cefed48