Windows File Creation: System DLL Named .dll in Uncommon Locations

Alerts on creation of .dll files named like system DLLs in unexpected Windows directories.

FreeReviewedSigma · Medium · v2

Product: windows
Category: file_event
Author: Nasreddine Bencherchali (Nextron Systems) (SigmaHQ), DRL 1.1
Published: 2024-06-24
Updated: 2026-07-31

ATT&CK techniques

Defense Evasion

Recon
Resource Dev
Initial Access
Execution
Persistence
Priv Esc
Defense Evasion
- T1036.005Match Legitimate Resource Name or Location
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule flags creation of new .dll files whose filename ends with specific System DLL names (e.g., secur32.dll, tdh.dll) in locations outside typical Windows system directories. Attackers may use unexpected paths to stage or sideload DLLs to increase stealth and influence application loading behavior. It relies on Windows file creation telemetry capturing the target filename and matches both the DLL name suffix and the absence of common trusted path substrings.

Reporting behind it

Related detections9 linkedT1036.005 — drag to rearrange

Suspicious ALPHA SPIDER Rclone Exfiltration Tool Masquerading as System Binary (via process_creation)

Suspicious Axios NPM macOS Persistence Masquerading as Apple Service

Malicious Spoolsv Process Masquerading From Non-System Path (via process_creation)

Suspicious svchost.exe Execution From AppData Roaming Directory

Malicious StyleSmuggler (CVE-2026-75650) Post-Exploitation Implant Process (via process_creation)

Malicious WezRat Persistence via Chrome Updater Run Key

Malicious ValleyRAT RuntimeBroker Masquerade in RunTime Directory

Suspicious Execution of spools.exe from Public Directory

Malicious Fake Fortinet Patch Infostealer Execution (via process_creation)

Windows File Creation: System DLL Named .dll in Uncommon Locations

Pivot detection · T1036.005 · 9 related

Changelog

v2
Candidate ingested via manual entry.2026-07-31
v1
No changelog recorded for this version.2026-07-30

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

windows-file-creation-system-dll-names-in-uncommon-directories-13c02350

title: "Windows File Creation: System DLL Named .dll in Uncommon Locations"
id: d26241cf-b5af-4638-b6db-269d425a97f6
status: test
description: This rule flags creation of new .dll files whose filename ends with specific System DLL names (e.g., secur32.dll, tdh.dll) in locations outside typical Windows system directories. Attackers may use unexpected paths to stage or sideload DLLs to increase stealth and influence application loading behavior. It relies on Windows file creation telemetry capturing the target filename and matches both the DLL name suffix and the absence of common trusted path substrings.
references:
  - Internal Research
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/file/file_event/file_event_win_creation_system_dll_files.yml
author: Nasreddine Bencherchali (Nextron Systems), Huntrule Team
date: 2024-06-24
tags:
  - attack.stealth
  - attack.t1036.005
logsource:
  category: file_event
  product: windows
detection:
  selection:
    TargetFilename|endswith:
      - \secur32.dll
      - \tdh.dll
  filter_main_generic:
    TargetFilename|contains:
      - C:\$WINDOWS.~BT\
      - C:\$WinREAgent\
      - C:\Windows\SoftwareDistribution\
      - C:\Windows\System32\
      - C:\Windows\SysWOW64\
      - C:\Windows\WinSxS\
      - C:\Windows\uus\
  condition: selection and not 1 of filter_main_*
falsepositives:
  - Third party software might bundle specific versions of system DLLs.
level: medium
regression_tests_path: regression_data/rules/windows/file/file_event/file_event_win_creation_system_dll_files/info.yml
license: DRL-1.1
related:
  - id: 13c02350-4177-4e45-ac17-cf7ca628ff5e
    type: derived

Convert

This rule as a query you can run. Uses no unlock credit.

SplunkSPL