Regsvr32 DLL Execution With Suspicious File Extension

Status: test
Severity: high
Log source: category process_creation, product windows
Author: Florian Roth (Nextron Systems), frack113
Source: github.com/SigmaHQ/sigma

Detects the execution of REGSVR32.exe with DLL files masquerading as other files

Known false positives

Unlikely

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1218.010` System Binary Proxy Execution: Regsvr32

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation

Rule body

title: Regsvr32 DLL Execution With Suspicious File Extension
id: 089fc3d2-71e8-4763-a8a5-c97fbb0a403e
related:
    - id: 8e2b24c9-4add-46a0-b4bb-0057b4e6187d
      type: obsolete
status: test
description: Detects the execution of REGSVR32.exe with DLL files masquerading as other files
references:
    - https://thedfirreport.com/2021/11/29/continuing-the-bazar-ransomware-story/
    - https://blog.talosintelligence.com/2021/10/threat-hunting-in-large-datasets-by.html
    - https://guides.lib.umich.edu/c.php?g=282942&p=1885348
    - https://harfanglab.io/insidethelab/uac-0057-pressure-ukraine-poland/
author: Florian Roth (Nextron Systems), frack113
date: 2021-11-29
modified: 2025-08-27
tags:
    - attack.stealth
    - attack.t1218.010
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith: '\regsvr32.exe'
        - OriginalFileName: 'REGSVR32.EXE'
    selection_cli:
        CommandLine|endswith:
            # Add more image extensions
            # https://twitter.com/Max_Mal_/status/1542461200797163522/photo/3
            - '.bin'
            - '.bmp'
            - '.cr2'
            - '.dat'
            - '.eps'
            - '.gif'
            - '.ico'
            - '.jpeg'
            - '.jpg'
            - '.log'
            - '.nef'
            - '.orf'
            - '.png'
            - '.raw'
            - '.rtf'
            - '.sr2'
            - '.temp'
            - '.tif'
            - '.tiff'
            - '.tmp'
            - '.txt'
    condition: all of selection_*
falsepositives:
    - Unlikely
level: high

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_img`

selection_img:
    - Image|endswith: '\regsvr32.exe'
    - OriginalFileName: 'REGSVR32.EXE'

Stage 2: `selection_cli`

selection_cli:
    CommandLine|endswith:
        - '.bin'
        - '.bmp'
        - '.cr2'
        - '.dat'
        - '.eps'
        - '.gif'
        - '.ico'
        - '.jpeg'
        - '.jpg'
        - '.log'
        - '.nef'
        - '.orf'
        - '.png'
        - '.raw'
        - '.rtf'
        - '.sr2'
        - '.temp'
        - '.tif'
        - '.tiff'
        - '.tmp'
        - '.txt'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	ends_with	`.bin` corpus 2 (sigma 1, elastic 1) `.bmp` `.cr2` `.dat` corpus 7 (sigma 6, elastic 1) `.eps` `.gif` corpus 3 (sigma 3) `.ico` `.jpeg` corpus 3 (sigma 3) `.jpg` corpus 2 (sigma 2) `.log` corpus 3 (sigma 2, elastic 1) `.nef` `.orf` `.png` corpus 3 (sigma 3) `.raw` `.rtf` `.sr2` `.temp` corpus 2 (sigma 2) `.tif` `.tiff` `.tmp` corpus 3 (sigma 2, elastic 1) `.txt` corpus 5 (sigma 4, elastic 1)	`field:"CommandLine" kind:ends_with`
`Image`	ends_with	`\regsvr32.exe` corpus 64 (sigma 64)	`field:"Image" kind:ends_with value:"\regsvr32.exe"`
`OriginalFileName`	eq	`REGSVR32.EXE` corpus 37 (sigma 17, elastic 15, splunk 5)	`field:"OriginalFileName" kind:eq value:"REGSVR32.EXE"`

Regsvr32 DLL Execution With Suspicious File Extension

Known false positives

MITRE ATT&CK coverage

Telemetry coverage

Rule body

Stages and Predicates

Stage 0: condition

Stage 1: selection_img

Stage 2: selection_cli

Indicators

Stage 0: `condition`

Stage 1: `selection_img`

Stage 2: `selection_cli`