Detection: Gsuite Email Suspicious Subject With Attachment

Updated Date: 2024-11-14 ID: 8ef3971e-00f2-11ec-b54f-acde48001122 Author: Teoderick Contreras, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies Gsuite emails with suspicious subjects and attachments commonly used in spear phishing attacks. It leverages Gsuite email logs, focusing on specific keywords in the subject line and known malicious file types in attachments. This activity is significant for a SOC as spear phishing is a prevalent method for initial compromise, often leading to further malicious actions. If confirmed malicious, this activity could result in unauthorized access, data exfiltration, or further malware deployment, posing a significant risk to the organization's security.

Search

 1`gsuite_gmail` num_message_attachments > 0 subject IN ("*dhl*", "* ups *", "*delivery*", "*parcel*", "*label*", "*invoice*", "*postal*", "* fedex *", "* usps *", "* express *", "*shipment*", "*Banking/Tax*","*shipment*", "*new order*") attachment{}.file_extension_type IN ("doc", "docx", "xls", "xlsx", "ppt", "pptx", "pdf", "zip", "rar", "html","htm","hta") 
 2| rex field=source.from_header_address "[^@]+@(?<source_domain>[^@]+)" 
 3| rex field=destination{}.address "[^@]+@(?<dest_domain>[^@]+)" 
 4| where not source_domain="internal_test_email.com" and dest_domain="internal_test_email.com" 
 5| eval phase="plan" 
 6| eval severity="medium" 
 7| 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 
 8| `security_content_ctime(firstTime)` 
 9| `security_content_ctime(lastTime)` 
10| `gsuite_email_suspicious_subject_with_attachment_filter`

Data Source

Name	Platform	Sourcetype	Source
G Suite Gmail	N/A	`'gsuite:gmail:bigquery'`	`'http:gsuite'`

Macros Used

Name	Value
gsuite_gmail	`sourcetype=gsuite:gmail:bigquery`
gsuite_email_suspicious_subject_with_attachment_filter	`search *`

gsuite_email_suspicious_subject_with_attachment_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

ID	Technique	Tactic
T1566.001	Spearphishing Attachment	Initial Access
T1566	Phishing	Initial Access

Delivery

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

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

normal user or normal transaction may contain the subject and file type attachment that this detection try to search.

Associated Analytic Story

Dev Sec Ops

Risk Based Analytics (RBA)

Risk Message:

Suspicious email from $source.address$ to $destination{}.address$

Risk Object	Risk Object Type	Risk Score	Threat Objects
destination{}.address	user	25	source.address

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`http:gsuite`	`gsuite:gmail:bigquery`
Integration	✅ Passing	Dataset	`http:gsuite`	`gsuite:gmail:bigquery`

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: 4