Detection: Windows Change File Association Command To Notepad

Description

The following analytic detects attempts to change the command value of a file association of an extension to open with Notepad.exe. It leverages data from Endpoint Detection and Response (EDR) agents, focusing on specific command-line patterns and registry modifications. This activity is significant as it can indicate an attempt to manipulate file handling behavior, a technique observed in APT and ransomware attacks like Prestige. After changing the extension of all encrypted files to a new one, Prestige ransomware modifies the file association for that extension to open with Notepad.exe in order to display a ransom note.

Search

 1
 2| tstats `security_content_summariesonly` 
 3  count min(_time) as firstTime 
 4  max(_time) as lastTime
 5
 6from datamodel=Endpoint.Processes where 
 7
 8(
 9  (`process_reg` AND Processes.process="* add *")
10  OR
11  (`process_powershell` AND Processes.process IN ("*New-ItemProperty*", "*Set-ItemProperty*", "* sp *"))
12)
13
14Processes.process IN ("*HKCR\\*", "*HKEY_CLASSES_ROOT\\*")
15Processes.process = "*\\shell\\open\\command*"
16Processes.process = "*Notepad.exe*"

The exclusion below aims to filter the default notepad association as well as links to the notepad package from the Microsoft Store.

 1
 2NOT Processes.process IN ("*\\Applications\\notepad.exe\\*", "*\\WindowsApps\\Microsoft.WindowsNotepad*")
 3
 4by Processes.action Processes.dest Processes.original_file_name Processes.parent_process 
 5   Processes.parent_process_exec Processes.parent_process_guid Processes.parent_process_id 
 6   Processes.parent_process_name Processes.parent_process_path Processes.process 
 7   Processes.process_exec Processes.process_guid Processes.process_hash Processes.process_id 
 8   Processes.process_integrity_level Processes.process_name Processes.process_path Processes.user 
 9   Processes.user_id Processes.vendor_product
10
11
12| `drop_dm_object_name(Processes)`
13
14| `security_content_ctime(firstTime)` 
15
16| `security_content_ctime(lastTime)` 
17
18| `windows_change_file_association_command_to_notepad_filter`

Data Source

Name	Platform	Sourcetype	Source
CrowdStrike ProcessRollup2	N/A	'crowdstrike:events:sensor'	'crowdstrike'
Sysmon EventID 1	Windows	'XmlWinEventLog'	'XmlWinEventLog:Microsoft-Windows-Sysmon/Operational'
Windows Event Log Security 4688	Windows	'XmlWinEventLog'	'XmlWinEventLog:Security'

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

The detection is based on data that originates from Endpoint Detection and Response (EDR) agents. These agents are designed to provide security-related telemetry from the endpoints where the agent is installed. To implement this search, you must ingest logs that contain the process GUID, process name, and parent process. Additionally, you must ingest complete command-line executions. These logs must be processed using the appropriate Splunk Technology Add-ons that are specific to the EDR product. The logs must also be mapped to the Processes node of the Endpoint data model. Use the Splunk Common Information Model (CIM) to normalize the field names and speed up the data modeling process.

Known False Positives

Unlikely

Associated Analytic Story

• Compromised Windows Host

• Prestige Ransomware

Risk Based Analytics (RBA)

Risk Message:

Process with commandline $process$ set the execution command of a file association to notepad.exe on $dest$

References

• https://www.microsoft.com/en-us/security/blog/2022/10/14/new-prestige-ransomware-impacts-organizations-in-ukraine-and-poland/

Detection Testing

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: 1