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

Updated Date: 2025-07-03 ID: 18f0d27d-569e-4bc4-96e1-09b214fa73c0 Author: Nasreddine Bencherchali, Splunk Type: Anomaly Product: Splunk Enterprise Security

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.

Search

 1`cisco_network_visibility_module_flowdata`
 2process_name = "rundll32.exe"
 3process_arguments = "*mshtml*"
 4process_arguments IN ("*135*", "*RunHTMLApplication*")
 5
 6| stats count min(_time) as firstTime max(_time) as lastTime
 7        values(parent_process_arguments) as parent_process_arguments
 8        values(process_arguments) as process_arguments
 9        values(parent_process_hash) as parent_process_hash
10        values(process_hash) as process_hash
11        values(module_name_list) as module_name_list
12        values(module_hash_list) as module_hash_list
13        values(dest_port) as dest_port
14        values(aliul) as additional_logged_in_users_list
15        values(dest_hostname) as dest_hostname
16        by src dest parent_process_path parent_process_integrity_level process_path process_name process_integrity_level process_id transport
17
18| `security_content_ctime(firstTime)`
19
20| `security_content_ctime(lastTime)`
21
22| table
23  parent_process_integrity_level parent_process_path parent_process_arguments parent_process_hash
24  process_integrity_level process_path process_name process_arguments process_hash process_id
25  additional_logged_in_users_list module_name_list module_hash_list
26  src dest_hostname dest dest_port transport firstTime lastTime
27
28| `cisco_nvm___rundll32_abuse_of_mshtml_dll_for_payload_download_filter`

Data Source

Name	Platform	Sourcetype	Source
Cisco Network Visibility Module Flow Data	Network	`'cisco:nvm:flowdata'`	`'not_applicable'`

Macros Used

Name	Value
cisco_network_visibility_module_flowdata	`sourcetype="cisco:nvm:flowdata"`
cisco_nvm___rundll32_abuse_of_mshtml_dll_for_payload_download_filter	`search *`

cisco_nvm___rundll32_abuse_of_mshtml_dll_for_payload_download_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1218.005	Mshta	Defense Evasion

Exploitation

DE.AE

CIS 10

Default Configuration

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

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

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.

Associated Analytic Story

Cisco Network Visibility Module Analytics

Risk Based Analytics (RBA)

Risk Message:

$process_path$ was executed on $src$ leveraging the mshtml.dll and the RunHTMLApplication export to download a potentially suspicious file from $dest_hostname$.

Risk Object	Risk Object Type	Risk Score	Threat Objects
src	system	40	process_name

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`not_applicable`	`cisco:nvm:flowdata`
Integration	✅ Passing	Dataset	`not_applicable`	`cisco:nvm:flowdata`

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