O365 Exfiltration via File Download

name: O365 Exfiltration via File Download
id: 06b23921-bfe2-4576-89dd-616f06e129da
version: 6
creation_date: '2025-01-28'
modification_date: '2026-05-13'
author: Steven Dick
status: production
type: Anomaly
description: The following analytic detects when an excessive number of files are downloaded from o365 by the same user over a short period of time. O365 may bundle these files together as a ZIP file, however each file will have it's own download event. This behavior may indicate an attacker staging data for exfiltration or an insider threat removing organizational data. Additional attention should be taken with any Azure Guest (#EXT#) accounts.
data_source:
    - Office 365 Universal Audit Log
search: |-
    `o365_management_activity` Operation IN ("filedownloaded")
    | eval user = replace(mvindex(split(lower(UserId),"#ext#"),0),"_","@"), user_flat = replace(UserId, "[^A-Za-z0-9]","_")
    | stats values(user) as user, latest(ClientIP) as src values(ZipFileName) as file_name, values(Operation) as signature, values(UserAgent) as http_user_agent, dc(SourceFileName) as count, min(_time) as firstTime, max(_time) as lastTime by Workload,UserId,SiteUrl
    | rename SiteUrl as file_path,Workload as app
    | where count > 50
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `o365_exfiltration_via_file_download_filter`
how_to_implement: You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events.
known_false_positives: It is possible that certain file download scenarios may trigger this alert, specifically OneDrive syncing. Adjust threshold and filtering as needed.
references:
    - https://attack.mitre.org/techniques/T1567/exfil
    - https://www.varonis.com/blog/sidestepping-detection-while-exfiltrating-sharepoint-data
    - https://thedfirjournal.com/posts/m365-data-exfiltration-rclone/
intermediate_findings:
    entities:
        - field: user
          type: user
          score: 20
          message: The user $user$ downloaded an excessive number of files [$count$] from $file_path$ using $src$
threat_objects:
    - field: src
      type: ip_address
analytic_story:
    - Data Exfiltration
    - Office 365 Account Takeover
asset_type: O365 Tenant
mitre_attack_id:
    - T1567
    - T1530
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: threat