Windows Change File Association Command To Notepad

name: Windows Change File Association Command To Notepad
id: 339155d6-34cb-4788-9d00-e67f190af93a
version: 6
creation_date: '2025-10-13'
modification_date: '2026-05-13'
author: Teoderick Contreras, Nasreddine Bencherchali, Splunk
status: production
type: TTP
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.
data_source:
    - Sysmon EventID 1
    - Windows Event Log Security 4688
    - CrowdStrike ProcessRollup2
search: |
    | tstats `security_content_summariesonly`
      count min(_time) as firstTime
      max(_time) as lastTime
    
    from datamodel=Endpoint.Processes where
    
    (
      (`process_reg` AND Processes.process="* add *")
      OR
      (`process_powershell` AND Processes.process IN ("*New-ItemProperty*", "*Set-ItemProperty*", "* sp *"))
    )
    
    Processes.process IN ("*HKCR\\*", "*HKEY_CLASSES_ROOT\\*")
    Processes.process = "*\\shell\\open\\command*"
    Processes.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.
    ```
    
    NOT Processes.process IN ("*\\Applications\\notepad.exe\\*", "*\\WindowsApps\\Microsoft.WindowsNotepad*")
    
    by Processes.action Processes.dest Processes.original_file_name Processes.parent_process
       Processes.parent_process_exec Processes.parent_process_guid Processes.parent_process_id
       Processes.parent_process_name Processes.parent_process_path Processes.process
       Processes.process_exec Processes.process_guid Processes.process_hash Processes.process_id
       Processes.process_integrity_level Processes.process_name Processes.process_path Processes.user
       Processes.user_id Processes.vendor_product
    
    | `drop_dm_object_name(Processes)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_change_file_association_command_to_notepad_filter`
how_to_implement: |
    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: No false positives have been identified at this time.
references:
    - https://www.microsoft.com/en-us/security/blog/2022/10/14/new-prestige-ransomware-impacts-organizations-in-ukraine-and-poland/
drilldown_searches:
    - name: View the detection results for - "$dest$"
      search: '%original_detection_search% | search  dest = "$dest$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$dest$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$dest$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
finding:
    title: Process with commandline $process$ set the execution command of a file association to notepad.exe on $dest$
    entity:
        field: dest
        type: system
        score: 50
analytic_story:
    - Prestige Ransomware
    - Compromised Windows Host
asset_type: Endpoint
mitre_attack_id:
    - T1546.001
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/malware/prestige_ransomware/sysmon.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog
      test_type: unit