O365 Email Send and Hard Delete Suspicious Behavior

name: O365 Email Send and Hard Delete Suspicious Behavior
id: c97b3d72-0a47-46f9-b742-b89f1cc2d551
version: 6
creation_date: '2025-01-23'
modification_date: '2026-05-13'
author: Steven Dick
status: production
type: Anomaly
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.
data_source:
    - Office 365 Universal Audit Log
search: |-
    `o365_management_activity` Workload=Exchange (Operation IN ("Send*")) OR (Operation IN ("HardDelete") AND Folder.Path IN ("\\Sent Items","\\Recoverable Items\\Deletions"))
    | 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))
    | eval sendtime = CASE(Operation IN ("Send","SendAs","SendOnBehalf"),_time)
    | eval deltime = CASE(Operation IN ("SoftDelete","HardDelete"),_time)
    | 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
    | eval timediff = tonumber(lastTime) - tonumber(firstTime)
    | where opcount > 1 AND firstTime < lastTime  AND timediff < 3600
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `o365_email_send_and_hard_delete_suspicious_behavior_filter`
how_to_implement: 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.
references:
    - https://attack.mitre.org/techniques/T1114/
    - https://www.hhs.gov/sites/default/files/help-desk-social-engineering-sector-alert-tlpclear.pdf
    - https://intelligence.abnormalsecurity.com/attack-library/threat-actor-convincingly-impersonates-employee-requesting-direct-deposit-update-in-likely-ai-generated-attack
intermediate_findings:
    entities:
        - field: user
          type: user
          score: 20
          message: The user $user$ sent and hard deleted an email within a short timeframe
threat_objects:
    - field: src
      type: ip_address
    - field: subject
      type: email_subject
analytic_story:
    - Office 365 Account Takeover
    - Office 365 Collection Techniques
    - Suspicious Emails
    - Data Destruction
asset_type: O365 Tenant
mitre_attack_id:
    - T1114.001
    - T1070.008
    - T1485
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: threat