Cisco NVM - Rundll32 Abuse of MSHTML.DLL for Payload Download

name: Cisco NVM - Rundll32 Abuse of MSHTML.DLL for Payload Download
id: 18f0d27d-569e-4bc4-96e1-09b214fa73c0
version: 6
creation_date: '2025-07-01'
modification_date: '2026-07-14'
author: Nasreddine Bencherchali, Splunk
status: production
type: Anomaly
description: |
    This analytic detects suspicious use of `rundll32.exe` in combination with `mshtml.dll` and the export `RunHTMLApplication`.
    This behavior is often observed in malware to execute JavaScript or VBScript in memory, enabling payload staging or
    bypassing script execution policies and bypassing the usage of the "mshta.exe" binary.
    The detection leverages Cisco Network Visibility Module telemetry which offers network flow activity
    along with process information such as command-line arguments
    If confirmed malicious, this activity may indicate initial access or payload download.
data_source:
    - Cisco Network Visibility Module Flow Data
search: |
    `cisco_network_visibility_module_flowdata`
    process_name = "rundll32.exe"
    process_arguments = "*mshtml*"
    process_arguments IN ("*135*", "*RunHTMLApplication*")
    | stats count min(_time) as firstTime max(_time) as lastTime
            values(parent_process_arguments) as parent_process_arguments
            values(process_arguments) as process_arguments
            values(parent_process_hash) as parent_process_hash
            values(process_hash) as process_hash
            values(module_name_list) as module_name_list
            values(module_hash_list) as module_hash_list
            values(dest_port) as dest_port
            values(aliul) as additional_logged_in_users_list
            values(dest_hostname) as dest_hostname
            by src dest parent_process_path parent_process_integrity_level process_path process_name process_integrity_level process_id transport
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | table
      parent_process_integrity_level parent_process_path parent_process_arguments parent_process_hash
      process_integrity_level process_path process_name process_arguments process_hash process_id
      additional_logged_in_users_list module_name_list module_hash_list
      src dest_hostname dest dest_port transport firstTime lastTime
    | `cisco_nvm___rundll32_abuse_of_mshtml_dll_for_payload_download_filter`
how_to_implement: |
    This search requires Network Visibility Module logs, which includes the flow data sourcetype.
    This search uses an input macro named `cisco_network_visibility_module_flowdata`.
    We strongly recommend that you specify your environment-specific configurations
    (index, source, sourcetype, etc.) for Cisco Network Visibility Module logs.
    Replace the macro definition with configurations for your Splunk environment.
    The search also uses a post-filter macro designed to filter out known false positives.
    The logs are to be ingested using the Splunk Add-on for Cisco Endpoint Security Analytics (CESA) (https://splunkbase.splunk.com/app/4221).
known_false_positives: |
    `rundll32.exe` using `mshtml.dll` is rare in legitimate environments. However, edge cases might exist. Tuning may be needed in environments with custom automation scripts.
references:
    - https://lolbas-project.github.io/lolbas/Binaries/Rundll32/
    - https://redcanary.com/blog/threat-detection/threat-research-questions/
    - https://twitter.com/n1nj4sec/status/1421190238081277959
    - https://hyp3rlinx.altervista.org/advisories/MICROSOFT_WINDOWS_DEFENDER_TROJAN.WIN32.POWESSERE.G_MITIGATION_BYPASS_PART2.txt
    - http://hyp3rlinx.altervista.org/advisories/MICROSOFT_WINDOWS_DEFENDER_DETECTION_BYPASS.txt
intermediate_findings:
    entities:
        - field: src
          type: system
          score: 20
          message: $process_path$ was executed on $src$ leveraging the mshtml.dll and the RunHTMLApplication export to download a potentially suspicious file from $dest_hostname$.
threat_objects:
    - field: process_name
      type: process_name
analytic_story:
    - Cisco Network Visibility Module Analytics
asset_type: Endpoint
mitre_attack_id:
    - T1218.005
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint