Detection: O365 Exfiltration via File Download

Updated Date: 2026-05-13 ID: 06b23921-bfe2-4576-89dd-616f06e129da Author: Steven Dick Type: Anomaly Product: Splunk Enterprise Security

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.

Search

 1`o365_management_activity` Operation IN ("filedownloaded")
 2
 3| eval user = replace(mvindex(split(lower(UserId),"#ext#"),0),"_","@"), user_flat = replace(UserId, "[^A-Za-z0-9]","_")
 4
 5| 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
 6
 7| rename SiteUrl as file_path,Workload as app
 8
 9| where count > 50
10
11| `security_content_ctime(firstTime)`
12
13| `security_content_ctime(lastTime)`
14
15| `o365_exfiltration_via_file_download_filter`

Data Source

Name	Platform	Sourcetype	Source
Office 365 Universal Audit Log	Other	`'o365:management:activity'`	`'o365'`

Macros Used

Name	Value
o365_management_activity	`sourcetype=o365:management:activity`
o365_exfiltration_via_file_download_filter	`search *`

o365_exfiltration_via_file_download_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1530	Data from Cloud Storage	Collection
T1567	Exfiltration Over Web Service	Exfiltration

Actions on Objectives

Exploitation

DE.AE

CIS 10

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Finding (Notable)	No
Creates Intermediate Finding (Risk Event)	Yes

Anomaly detections generate Intermediate Findings (Risk Events). They do not generate a Finding (Notable) directly.

Implementation

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.

Associated Analytic Story

Intermediate Findings

Message	Entity Field	Entity Type	Risk Score
The user $user$ downloaded an excessive number of files [$count$] from $file_path$ using $src$	user	user	20

Threat Objects

Field	Type
src	ip_address

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`o365`	`o365:management:activity`
Integration	✅ Passing	Dataset	`o365`	`o365:management:activity`

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 6