| ID | Technique | Tactic |
|---|---|---|
| T1070.008 | Clear Mailbox Data | Stealth |
| T1485 | Data Destruction | Impact |
Detection: O365 Email Send Attachments Excessive Volume
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.
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, _time max=0
12 [
13
14| search `o365_management_activity` Workload=Exchange Operation IN ("Send","SendAs","SendOnBehalf")
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 = 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\((\d+)(b\)$)", "\2"))/1024/1024,2)), InternetMessageId = lower('Item.InternetMessageId')
17
18| bin _time span=1hr
19
20| eval file_name = mvfilter(NOT match(file_name, "\.jpg
21|\.png
22|\.jpeg
23|\.gif "))
24
25| search file_name=*
26
27| stats values(sender) as sender, values(ClientIPAddress) as src, values(ClientInfoString) as http_user_agent, values(Operation) as signature, values(file_name) as file_name, sum(file_size) as file_size, values(Folder.Path) as file_path, min(-time) as firstTime, max(-time) as lastTime, dc(file_name) as count by _time,user,InternetMessageId
28
29| where count > 25
30
31| eval file_name = mvjoin(file_name,"
32|
33|")
34 ]
35
36| eval file_name = split(file_name,"
37|
38|")
39
40| stats values(sender) as sender, values(recipient) as recipient, values(http_user_agent) as http_user_agent, values(signature) as signature, values(file_name) as file_name, max(file_size) as file_size, min(firstTime) as firstTime, max(lastTime) as lastTime max(count) as count by subject,user,Organization,InternetMessageId
41
42| eval recipient = mvmap(recipient, if(match(mvindex(split(lower(recipient),"@"),1),mvindex(split(lower(user),"@"),1)), null(),recipient))
43
44| search recipient = *
45
46| `security_content_ctime(firstTime)`
47
48| `security_content_ctime(lastTime)`
49
50| `o365_email_send_attachments_excessive_volume_filter`
Data Source
| Name | Platform | Sourcetype | Source |
|---|---|---|---|
| Office 365 Universal Audit Log | Other | 'o365:management:activity' |
'o365' |
Macros Used
| Name | Value |
|---|---|
| security_content_ctime | convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$) |
| o365_email_send_attachments_excessive_volume_filter | search * |
o365_email_send_attachments_excessive_volume_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
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 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.
Associated Analytic Story
Intermediate Findings
| Message | Entity Field | Entity Type | Risk Score |
|---|---|---|---|
| The user $user$ sent an excessive number of email attachments [$count$] to external recipient(s) within a short timeframe | user | user | 20 |
Threat Objects
| Field | Type |
|---|---|
| recipient | email_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