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Description

This search is to detect a suspicious outbound e-mail from internal email to external email domain. This can be a good hunting query to monitor insider or outbound email traffic for not common domain e-mail. The idea is to parse the domain of destination email check if there is a minimum outbound traffic < 20 with attachment.

  • Type: Hunting

  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud

  • Last Updated: 2024-03-25
  • Author: Teoderick Contreras, Stanislav Miskovic, Splunk
  • ID: dc4dc3a8-ff54-11eb-8bf7-acde48001122

Annotations

ATT&CK

ATT&CK

ID Technique Tactic
T1048.003 Exfiltration Over Unencrypted Non-C2 Protocol Exfiltration
T1048 Exfiltration Over Alternative Protocol Exfiltration
Kill Chain Phase
  • Actions On Objectives
NIST
  • DE.AE
CIS20
  • CIS 10
CVE 


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`gsuite_gmail` num_message_attachments > 0 
| rex field=source.from_header_address "[^@]+@(?<source_domain>[^@]+)" 
| rex field=destination{}.address "[^@]+@(?<dest_domain>[^@]+)" 
| where source_domain="internal_test_email.com" and not dest_domain="internal_test_email.com" 
| eval phase="plan" 
| eval severity="low" 
| stats values(subject) as subject, values(source.from_header_address) as src_domain_list, count as numEvents, dc(source.from_header_address) as numSrcAddresses, min(_time) as firstTime max(_time) as lastTime by dest_domain phase severity 
| where numSrcAddresses < 20 
|sort - numSrcAddresses 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `gsuite_outbound_email_with_attachment_to_external_domain_filter`

Macros

The SPL above uses the following Macros:

:information_source: gsuite_outbound_email_with_attachment_to_external_domain_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

  • _time
  • source.from_header_address
  • destination.address
  • num_message_attachments
  • dest_domain
  • phase
  • severity

How To Implement

To successfully implement this search, you need to be ingesting logs related to gsuite having the file attachment metadata like file type, file extension, source email, destination email, num of attachment and etc.

Known False Positives

network admin and normal user may send this file attachment as part of their day to day work. having a good protocol in attaching this file type to an e-mail may reduce the risk of having a spear phishing attack.

Associated Analytic Story

RBA

Risk Score Impact Confidence Message
9.0 30 30 Suspicious email from $src_domain_list$ to $dest_domain$

:information_source: The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

Reference

Test Dataset

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 | version: 2