Detection: Common Ransomware Extensions

Updated Date: 2025-03-03 ID: a9e5c5db-db11-43ca-86a8-c852d1b2c0ec Author: David Dorsey, Michael Haag, Splunk, Steven Dick Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects modifications to files with extensions commonly associated with ransomware. It leverages the Endpoint.Filesystem data model to identify changes in file extensions that match known ransomware patterns. This activity is significant because it suggests an attacker is attempting to encrypt or alter files, potentially leading to severe data loss and operational disruption. If confirmed malicious, this activity could result in the encryption of critical data, rendering it inaccessible and causing significant damage to the organization's data integrity and availability.

 1
 2| tstats `security_content_summariesonly` min(_time) as firstTime max(_time) as lastTime count latest(Filesystem.user) as user values(Filesystem.file_path) as file_path from datamodel=Endpoint.Filesystem by Filesystem.file_name Filesystem.dest _time span=1h 
 3| `drop_dm_object_name(Filesystem)` 
 4| rex field=file_name "(?<file_extension>\.[^\.]+)$" 
 5| rex field=file_path "(?<true_file_path>([^\\\]*\\\)*).*" 
 6| stats min(firstTime) as firstTime max(lastTime) as lastTime latest(user) as user dc(true_file_path) as path_count dc(file_name) as file_count latest(true_file_path) as file_path by dest file_name 
 7| `security_content_ctime(lastTime)` 
 8| `security_content_ctime(firstTime)` 
 9| `ransomware_extensions` 
10| where path_count > 1 OR file_count > 20 
11| `common_ransomware_extensions_filter`

Data Source

Name Platform Sourcetype Source
Sysmon EventID 11 Windows icon Windows 'xmlwineventlog' 'XmlWinEventLog:Microsoft-Windows-Sysmon/Operational'

Macros Used

Name Value
ransomware_extensions `lookup update=true ransomware_extensions_lookup Extensions AS file_name OUTPUT Extensions Name
common_ransomware_extensions_filter search *
common_ransomware_extensions_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
T1485 Data Destruction Impact
Actions on Objectives
DE.CM
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 Notable Yes
Rule Title %name%
Rule Description %description%
Notable Event Fields user, dest
Creates Risk Event True
This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

You must be ingesting data that records the filesystem activity from your hosts to populate the Endpoint Filesystem data model node. To see the additional metadata, add the following fields, if not already present, please review the detailed documentation on how to create a new field within Incident Review

Known False Positives

It is possible for a legitimate file with these extensions to be created. If this is a true ransomware attack, there will be a large number of files created with these extensions.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

The device $dest$ wrote $file_count$ files to $path_count$ path(s) with the $Extensions$ extension. This extension and behavior may indicate a $Name$ ransomware attack.

Risk Object Risk Object Type Risk Score Threat Objects
dest system 90 No Threat Objects
user user 90 No Threat Objects

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation ✅ Passing N/A N/A N/A
Unit ✅ Passing Dataset XmlWinEventLog:Microsoft-Windows-Sysmon/Operational XmlWinEventLog
Integration ✅ Passing Dataset XmlWinEventLog:Microsoft-Windows-Sysmon/Operational XmlWinEventLog

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: 12