Detection: O365 Email Send and Hard Delete Exfiltration Behavior

Updated Date: 2025-01-20 ID: dd7798cf-c4f5-4114-ad0f-beacd9a33708 Author: Steven Dick Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies when an O365 email account sends and then hard deletes an email to an external recipient within a short period (within 1 hour). This behavior may indicate a compromised account where the threat actor is attempting to remove forensic artifacts or evidence of exfiltration activity. This behavior is often seen when threat actors want to reduce the probability of detection by the compromised account owner.

Search

 1`o365_messagetrace` Status=Delivered
 2
 3| eval mailtime = _time
 4
 5| bin _time span=1hr
 6
 7| eval user = lower(SenderAddress), recipient = lower(RecipientAddress)
 8
 9| eval InternetMessageId = lower(MessageId)
10
11| join InternetMessageId, user, max=0
12  [
13  
14| search `o365_management_activity` Workload=Exchange (Operation IN ("Send*")) OR (Operation IN ("HardDelete") AND Folder.Path IN ("\\Sent Items","\\Recoverable Items\\Deletions"))
15  
16| eval user = lower(UserId), sender = lower(CASE(isnotnull(SendAsUserSmtp),SendAsUserSmtp,isnotnull(SendOnBehalfOfUserSmtp),SendOnBehalfOfUserSmtp,true(),MailboxOwnerUPN)), subject = trim(CASE(Operation IN ("Send","SendAs","SendOnBehalf"),'Item.Subject',Operation IN ("SoftDelete","HardDelete"),'AffectedItems{}.Subject')), -time = _time,file_name = CASE(Operation IN ("Send","SendAs","SendOnBehalf"),split('Item.Attachments',"; "),Operation IN ("SoftDelete","HardDelete"),split('AffectedItems{}.Attachments',"; ")), file_size = CASE(Operation IN ("Send","SendAs","SendOnBehalf"),round(tonumber('Item.SizeInBytes')/1024/1024,2),true(),round(tonumber(replace(file_name, "(.+)\s\((\d+)(b\)$)", "\2"))/1024/1024,2)), InternetMessageId = lower('Item.InternetMessageId')
17  
18| eval sendtime = CASE(Operation IN ("Send","SendAs","SendOnBehalf"),_time)
19  
20| eval deltime = CASE(Operation IN ("SoftDelete","HardDelete"),_time)
21  
22| bin _time span=1hr
23  
24| stats values(sender) as sender, values(ClientInfoString) as http_user_agent, values(InternetMessageId) as InternetMessageId, values(file_name) as file_name, sum(file_size) as file_size, values(sendtime) as firstTime, values(deltime) as lastTime values(Operation) as signature, dc(Operation) as opcount, count by _time,subject,user
25  
26| where opcount > 1 AND firstTime < lastTime
27  ]
28
29| stats values(sender) as sender, values(http_user_agent) as http_user_agent, values(signature) as signature, values(file_name) as file_name, sum(file_size) as file_size, min(firstTime) as firstTime, max(lastTime) as lastTime  count by subject,user,recipient,Organization
30
31| eval externalRecipient = if(match(lower(recipient),mvindex(split(lower(Organization),"."),0)),0,1)
32
33| where externalRecipient = 1
34
35| `security_content_ctime(firstTime)` 
36
37| `security_content_ctime(lastTime)`
38
39| `o365_email_send_and_hard_delete_exfiltration_behavior_filter`

Data Source

Name	Platform	Sourcetype	Source
Office 365 Reporting Message Trace	N/A	`'o365:reporting:messagetrace'`	`'o365'`
Office 365 Universal Audit Log	N/A	`'o365:management:activity'`	`'o365'`

Macros Used

Name	Value
o365_management_activity	`sourcetype=o365:management:activity`
o365_email_send_and_hard_delete_exfiltration_behavior_filter	`search *`

o365_email_send_and_hard_delete_exfiltration_behavior_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
T1114.001	Local Email Collection	Collection
T1070.008	Clear Mailbox Data	Defense Evasion
T1485	Data Destruction	Impact

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events AND Message Trace events.

Known False Positives

Users that habitually/proactively cleaning the recoverable items folder may trigger this alert.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

The user $user$ sent and hard deleted an email to an external recipient [$recipient$] within a short timeframe

Risk Object	Risk Object Type	Risk Score	Threat Objects
recipient	user	40	subject
user	user	40	subject

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: 1