huntrule

Windows Rundll32 Execution Masquerading as Image Files via Image Extensions

Flags rundll32.exe executions whose command line references image file extensions used for DLL masquerading.

FreeUnreviewedSigmahighv1
title: Windows Rundll32 Execution Masquerading as Image Files via Image Extensions
id: 6866b25c-47b9-4838-9843-89511e001ac0
related:
  - id: 089fc3d2-71e8-4763-a8a5-c97fbb0a403e
    type: similar
  - id: 4aa6040b-3f28-44e3-a769-9208e5feb5ec
    type: derived
status: test
description: This rule identifies process creation events where rundll32.exe is executed while the command line includes common image file extensions (e.g., .png, .jpg, .gif). Attackers may use rundll32 to load DLL functionality while disguising the argument as an image to evade casual inspection and some content-based defenses. The detection relies on Windows process creation telemetry, matching the image path/end with rundll32.exe (and OriginalFileName) and the presence of specified image extensions in the command line.
references:
  - https://www.zscaler.com/blogs/security-research/onenote-growing-threat-malware-distribution
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_rundll32_susp_execution_with_image_extension.yml
author: Hieu Tran, Huntrule Team
date: 2023-03-13
tags:
  - attack.stealth
  - attack.t1218.011
logsource:
  category: process_creation
  product: windows
detection:
  selection_img:
    - Image|endswith: \rundll32.exe
    - OriginalFileName: RUNDLL32.exe
  selection_cli:
    CommandLine|contains:
      - .bmp
      - .cr2
      - .eps
      - .gif
      - .ico
      - .jpeg
      - .jpg
      - .nef
      - .orf
      - .png
      - .raw
      - .sr2
      - .tif
      - .tiff
  condition: all of selection_*
falsepositives:
  - Unknown
level: high
license: DRL-1.1

What it detects

This rule identifies process creation events where rundll32.exe is executed while the command line includes common image file extensions (e.g., .png, .jpg, .gif). Attackers may use rundll32 to load DLL functionality while disguising the argument as an image to evade casual inspection and some content-based defenses. The detection relies on Windows process creation telemetry, matching the image path/end with rundll32.exe (and OriginalFileName) and the presence of specified image extensions in the command line.

Known false positives

  • Unknown

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.