Windows Change Default File Association For No File Ext

SPLUNK detection rule by Teoderick Contreras, Splunk · Apache-2.0

The following analytic detects attempts to change the default file association for files without 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. If confirmed malicious, this could allow attackers to execute arbitrary code by tricking users into opening files, potentially leading to system compromise or data exfiltration.

name: Windows Change Default File Association For No File Ext
id: dbdf52ad-d6a1-4b68-975f-0a10939d8e38
version: 9
creation_date: '2022-12-05'
modification_date: '2026-05-13'
author: Teoderick Contreras, Splunk
status: removed
type: TTP
description: The following analytic detects attempts to change the default file association for files without 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. If confirmed malicious, this could allow attackers to execute arbitrary code by tricking users into opening files, potentially leading to system compromise or data exfiltration.
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 *" AND Processes.process="* HKCR\\*" AND Processes.process="*\\shell\\open\\command*" AND  Processes.process= *Notepad.exe* 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)` | rex field=process "Notepad\.exe (?<file_name_association>.*$)" | rex field=file_name_association "\.(?<extension>[^\.]*$)" | where isnull(extension) and isnotnull(file_name_association) | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `windows_change_default_file_association_for_no_file_ext_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: unknown
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 ("$dest$") starthoursago=168  | 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: $info_min_time$
      latest_offset: $info_max_time$
rba:
    message: process with commandline $process$ set or change the file association of a file with no file extension on $dest$
    risk_objects:
        - field: dest
          type: system
          score: 80
    threat_objects: []
tags:
    analytic_story:
        - Prestige Ransomware
        - Compromised Windows Host
    asset_type: Endpoint
    mitre_attack_id:
        - T1546.001
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    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
deprecation_info:
    reason: Detection has been deprecated since it has been replaced with a better named detection that reflect a much better consistent logic
    removed_in_version: 5.18.0
    replacement_content:
        - Windows Change File Association Command To Notepad

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