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Description

This search is to detect a suspicious attachment file extension in Gsuite email that may related to spear phishing attack. This file type is commonly used by malware to lure user to click on it to execute malicious code to compromised targetted machine. But this search can also catch some normal files related to this file type that maybe send by employee or network admin.

  • Type: Anomaly

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

  • Last Updated: 2021-08-16
  • Author: Teoderick Contreras, Splunk
  • ID: 6d663014-fe92-11eb-ab07-acde48001122

Annotations

ATT&CK
ID Technique Tactic
T1566.001 Spearphishing Attachment Initial Access
T1566 Phishing Initial Access
Kill Chain Phase
  • Exploitation
NIST
CIS20
CVE 
1
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7

`gsuite_gmail` "attachment{}.file_extension_type" IN ("pl", "py", "rb", "sh", "bat", "exe", "dll", "cpl", "com", "js", "vbs", "ps1", "reg","swf", "cmd", "go") 
| eval phase="plan" 
| eval severity="medium" 
| stats count min(_time) as firstTime max(_time) as lastTime values(attachment{}.file_extension_type) as email_attachments, values(attachment{}.sha256) as attachment_sha256, values(payload_size) as payload_size by destination{}.service num_message_attachments  subject destination{}.address source.address phase severity 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `gsuite_email_suspicious_attachment_filter`

Macros

The SPL above uses the following Macros:

Note that gsuite_email_suspicious_attachment_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required field

  • _time
  • attachment{}.file_extension_type
  • attachment{}.sha256
  • destination{}.service
  • num_message_attachments
  • payload_size
  • subject
  • destination{}.address
  • source.address

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
49.0 70 70 suspicious email from $source.address$ to $destination{}.address$

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

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