Safe Attachments detections

Hunt Hypothesis

Adversaries may use Safe Attachments to bypass email security controls and exfiltrate data, leveraging this technique to evade detection. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential data exfiltration attempts and mitigate advanced persistent threats.

KQL Query

EmailEvents
| where DetectionMethods != "" 
| extend detection= tostring(parse_json(DetectionMethods).Phish) 
| where detection has "File detonation reputation" or detection has "File detonation"
| summarize total=count() by bin(Timestamp, 1d) 
| order by Timestamp asc

Analytic Rule Definition

id: 16eda414-1550-4cdc-8512-0769901d3f05
name: Safe Attachments detections
description: |
  This query provides insights on the detections done by Safe Attachment detections
description-detailed: |
  This query provides insights on the detections done by Safe Attachment detections.
  Reference - https://learn.microsoft.com/en-us/defender-office-365/safe-attachments-about
requiredDataConnectors:
  - connectorId: MicrosoftThreatProtection
    dataTypes:
      - EmailEvents
tactics:
  - InitialAccess
relevantTechniques:
  - T1566
query: |
  EmailEvents
  | where DetectionMethods != "" 
  | extend detection= tostring(parse_json(DetectionMethods).Phish) 
  | where detection has "File detonation reputation" or detection has "File detonation"
  | summarize total=count() by bin(Timestamp, 1d) 
  | order by Timestamp asc
version: 1.0.0

Required Data Sources

MITRE ATT&CK Context

• Tactic: InitialAccess
• Technique: T1566 — T1566

References

• [Source Query](https://github.com/Azure/Azure-Sentinel/blob/main/Hunting Queries/Microsoft 365 Defender/Email and Collaboration Queries/Attachment/Safe attachment detection.yaml)

False Positive Guidance

• Scenario: Scheduled Backup Job with Encrypted Files
  Description: A scheduled backup job compresses and encrypts files using tools like Veeam Backup & Replication or Commvault, which may trigger Safe Attachments due to the presence of encrypted content.
  Filter/Exclusion: Exclude files with known encryption headers (e.g., .7z, .zip, .tar.gz) or use a filter based on the source IP of the backup server.

• Scenario: Admin Task to Send Secure Email
  Description: An admin sends a secure email using Microsoft Exchange Online with Safe Attachments enabled, which may flag the email as a detection due to the use of encrypted or sanitized attachments.
  Filter/Exclusion: Exclude emails sent from admin accounts (e.g., [email protected]) or use a sender IP filter to exclude internal mail servers.

• Scenario: Automated Report Generation with Encrypted Attachments
  Description: A Power BI or Power Automate job generates a report with an encrypted attachment (e.g., using 7-Zip or WinRAR) and sends it via email, triggering Safe Attachments.
  Filter/Exclusion: Exclude attachments with specific file extensions (e.g., .7z, .zip) or use a file content signature to identify known encryption tools.

• Scenario: User-Initiated File Sharing via SharePoint
  Description: A user shares a file via Microsoft SharePoint that contains encrypted content, which may be flagged by Safe Attachments during content scanning.
  Filter/Exclusion: Exclude files uploaded from internal SharePoint sites or use a file type filter to exclude known encrypted file formats.

• Scenario: System Log File with Base64 Encoded Data
  Description: A system log file (e.g.,