Detection: O365 Email Send and Hard Delete Suspicious Behavior

Updated Date: 2025-01-20 ID: c97b3d72-0a47-46f9-b742-b89f1cc2d551 Author: Steven Dick Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies when an O365 email account sends and then hard deletes email with 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 activity. Threat actors often use this technique to prevent defenders and victims from knowing the account has been compromised. --- Some account owner legitimate behaviors can trigger this alert, however these actions may not be aligned with organizational expectations / best practice behaviors.

Search

 1`o365_management_activity` Workload=Exchange (Operation IN ("Send*")) OR (Operation IN ("HardDelete") AND Folder.Path IN ("\\Sent Items","\\Recoverable Items\\Deletions"))
 2
 3| 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))
 4
 5| eval sendtime = CASE(Operation IN ("Send","SendAs","SendOnBehalf"),_time)
 6
 7| eval deltime = CASE(Operation IN ("SoftDelete","HardDelete"),_time)
 8
 9| stats values(sender) as sender, values(ClientIPAddress) as src, values(ClientInfoString) as http_user_agent, values(Operation) as signature, values(file_name) as file_name, sum(file_size) as file_size, values(Folder.Path) as file_path, min(sendtime) as firstTime, max(deltime) as lastTime, dc(Operation) as opcount, count by subject,user
10
11| eval timediff = tonumber(lastTime) - tonumber(firstTime)
12
13| where opcount > 1 AND firstTime < lastTime  AND timediff < 3600
14
15| `security_content_ctime(firstTime)` 
16
17| `security_content_ctime(lastTime)`
18
19| `o365_email_send_and_hard_delete_suspicious_behavior_filter`

Data Source

Name	Platform	Sourcetype	Source
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_suspicious_behavior_filter	`search *`

o365_email_send_and_hard_delete_suspicious_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.

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 within a short timeframe

Risk Object	Risk Object Type	Risk Score	Threat Objects
user	user	20	subject, src

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