O365 Email Send Attachments Excessive Volume
SPLUNK detection rule by Steven Dick · Apache-2.0
The following analytic identifies when an O365 email account sends an excessive number of email attachments to external recipients within a short period (within 1 hour). This behavior may indicate a compromised account where the threat actor is attempting to exfiltrate data from the mailbox. Threat actors may attempt to transfer data through email as a simple means of exfiltration from the compromised mailbox. Some account owner legitimate behaviors can trigger this alert, however these actions may not be aligned with organizational expectations / best practice behaviors.
name: O365 Email Send Attachments Excessive Volume
id: 70a050a2-8537-488a-a628-b60a9558d96a
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 an excessive number of email attachments to external recipients within a short period (within 1 hour). This behavior may indicate a compromised account where the threat actor is attempting to exfiltrate data from the mailbox. Threat actors may attempt to transfer data through email as a simple means of exfiltration from the compromised mailbox. 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_messagetrace` Status=Delivered
| eval mailtime = _time
| bin _time span=1hr
| eval user = lower(SenderAddress), recipient = lower(RecipientAddress)
| eval InternetMessageId = lower(MessageId)
| join InternetMessageId, user, _time max=0
[
| search `o365_management_activity` Workload=Exchange Operation IN ("Send","SendAs","SendOnBehalf")
| 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 = trim(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\((\d+)(b\)$)", "\2"))/1024/1024,2)), InternetMessageId = lower('Item.InternetMessageId')
| bin _time span=1hr
| eval file_name = mvfilter(NOT match(file_name, "\.jpg |\.png |\.jpeg |\.gif "))
| search file_name=*
| stats values(sender) as sender, values(ClientIPAddress) as src, values(ClientInfoString) as http_user_agent, values(Operation) as signature, values(file_name) as file_name, sum(file_size) as file_size, values(Folder.Path) as file_path, min(-time) as firstTime, max(-time) as lastTime, dc(file_name) as count by _time,user,InternetMessageId
| where count > 25
| eval file_name = mvjoin(file_name,"||")
]
| eval file_name = split(file_name,"||")
| stats values(sender) as sender, values(recipient) as recipient, values(http_user_agent) as http_user_agent, values(signature) as signature, values(file_name) as file_name, max(file_size) as file_size, min(firstTime) as firstTime, max(lastTime) as lastTime max(count) as count by subject,user,Organization,InternetMessageId
| eval recipient = mvmap(recipient, if(match(mvindex(split(lower(recipient),"@"),1),mvindex(split(lower(user),"@"),1)), null(),recipient))
| search recipient = *
| `security_content_ctime(firstTime)`
| `security_content_ctime(lastTime)`
| `o365_email_send_attachments_excessive_volume_filter`
how_to_implement: You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events AND Message Trace events.
known_false_positives: Users or processes that are send a large number of attachments may trigger this alert, adjust thresholds accordingly.
references:
- https://attack.mitre.org/techniques/T1114/
- https://www.hhs.gov/sites/default/files/help-desk-social-engineering-sector-alert-tlpclear.pdf
- https://intelligence.abnormalsecurity.com/attack-library/threat-actor-convincingly-impersonates-employee-requesting-direct-deposit-update-in-likely-ai-generated-attack
drilldown_searches:
- name: View the detection results for - "$user$"
search: '%original_detection_search% | search user = "$user$"'
earliest_offset: $info_min_time$
latest_offset: $info_max_time$
- name: View risk events for the last 7 days for - "$user$"
search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
earliest_offset: 7d
latest_offset: "0"
- name: Investigate Email for $user$
search: '`o365_management_activity` Workload=Exchange (Operation IN ("Send*")) AND Item.Attachments=* AND UserId = "$user$"'
earliest_offset: $info_min_time$
latest_offset: $info_max_time$
intermediate_findings:
entities:
- field: user
type: user
score: 20
message: The user $user$ sent an excessive number of email attachments [$count$] to external recipient(s) within a short timeframe
threat_objects:
- field: recipient
type: email_address
analytic_story:
- Office 365 Account Takeover
- Suspicious Emails
asset_type: O365 Tenant
mitre_attack_id:
- T1070.008
- T1485
product:
- Splunk Enterprise
- Splunk Enterprise Security
- Splunk Cloud
category: cloud
security_domain: threat
tests:
- name: True Positive Test
attack_data:
- data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1114/o365_suspect_email_actions/o365_exchange_suspect_events.log
source: o365
sourcetype: o365:management:activity
- data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1114/o365_suspect_email_actions/o365_messagetrace_suspect_events.log
source: o365_messagetrace
sourcetype: o365:reporting:messagetrace
test_type: unit
CIRCL