Windows Rundll32 with Non-Standard File Extension

name: Windows Rundll32 with Non-Standard File Extension
id: f52b55ce-41ad-4802-9909-fbd7cc8410a5
version: 1
date: '2026-03-27'
author: Teoderick Contreras, Splunk
status: production
type: Anomaly
description: |-
    This analytic identifies the instance of rundll32.exe process loading a non-standard Windows modules file extension.
    This behavior is not common and can be associated with malicious activities, such as the Gh0st RAT backdoor. This technique is to evade possible detection by security tools that monitors a suspicious dll loading activity.
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_rundll32`
    (
        Processes.parent_process_path IN (
            "*:\\PerfLogs\\*",
            "*:\\ProgramData\\*",
            "*:\\Recycle.bin\\*",
            "*:\\Users\\Administrator\\Music\\*",
            "*:\\Users\\Public\\*",
            "*:\\Windows\\Cursors\\*",
            "*:\\Windows\\debug\\*",
            "*:\\Windows\\fonts\\*",
            "*:\\Windows\\INF\\*",
            "*:\\Windows\\Media\\*",
            "*:\\Windows\\Prefetch\\*",
            "*:\\Windows\\repair\\*",
            "*:\\Windows\\Tasks\\*",
            "*\\Temp\\*"
        )
        OR
        Processes.parent_process_name IN (
            "*cmd.exe*",
            "*cscript.exe*",
            "*mshta.exe*",
            "*powershell.exe*",
            "*pwsh.exe*",
            "*regsvr32.exe*",
            "*wscript.exe*"
        )
    )
    NOT Processes.process IN (
            "*.cpl*",
            "*.dll*",
            "*.drv*",
            "*.inf*",
            "*.mui*",
            "*.ocx*"
        )

    BY Processes.action Processes.dest Processes.original_file_name
       Processes.parent_process_exec Processes.parent_process_guid
       Processes.parent_process_id Processes.parent_process_path
       Processes.process_exec Processes.process_guid Processes.process_hash
       Processes.process_id Processes.process_integrity_level
       Processes.process_path Processes.user Processes.user_id
       Processes.vendor_product Processes.parent_process_name
       Processes.parent_process Processes.process_name Processes.process

    | `drop_dm_object_name(Processes)`

    | rex field=process "^(?<cmd_base_process>[^\s]+)\s+\"?(?<cmd_base_first_param>[^,^\"^\s]+).*?,(?<cmd_args>.*)$"

    | rex field=cmd_base_first_param "^(?<root>[^\\\\]+)\\\\(?<subdirs>[^\\\\]+)"

    | eval folder_count = mvcount(split(cmd_base_first_param, "\\"))

    | where (
                folder_count = 3
                AND
                NOT lower(subdirs) IN (
                                        "windows",
                                        "program files",
                                        "program files (x86)"
                                    )
            )
            OR
            like(cmd_base_first_param, "%:\\ProgramData\\%")
            OR
            like(cmd_base_first_param, "%:\\Users\\Public\\%")
            OR
            like(cmd_base_first_param, "%\\AppData\\Local\\Temp\\%")
            OR
            like(cmd_base_first_param, "%\\AppData\\Roaming\\%")

    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_rundll32_with_non_standard_file_extension_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: |-
    Certain legitimate Windows components, third-party applications, or administrative scripts may use rundll32.exe with non-standard or extensionless inputs during normal operation.
    Filtering and contextual analysis are required, focus on command-line arguments, parent process, file location, and prevalence across the environment before determining malicious intent.
references:
    - https://malpedia.caad.fkie.fraunhofer.de/details/win.ghost_rat
    - https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-rrasm/22ad9f0e-4349-43e0-92b3-37f7a9c7ca41
    - https://s7d2.scene7.com/is/content/cylance/prod/cylance-web/en-us/resources/knowledge-center/resource-library/reports/Op_Dust_Storm_Report.pdf
    - https://www.sentinelone.com/blog/the-curious-case-of-gh0st-malware/
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: An instance of [$parent_process_path$] launched [$process_name$] loading a non-standard DLL extension [$process$] in host [$dest$]
    risk_objects:
        - field: dest
          type: system
          score: 20
    threat_objects:
        - field: parent_process_name
          type: parent_process_name
        - field: process
          type: process
tags:
    analytic_story:
        - Living Off The Land
        - Suspicious Rundll32 Activity
        - Gh0st RAT
    asset_type: Endpoint
    mitre_attack_id:
        - T1218.011
    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/attack_techniques/T1218.011/rundll32_random_dll_ext/rundll32_random_ext.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog