Fax Service DLL Search Order Hijack

Description

The Fax service attempts to load ualapi.dll, which is non-existent. An attacker can then (side)load their own malicious DLL using this service.

Query · sigma

selection:
  Image|endswith: \fxssvc.exe
  ImageLoaded|endswith: ualapi.dll
filter:
  ImageLoaded|startswith: C:\Windows\WinSxS\
condition: selection and not filter

Known false positives

  • Unlikely
Raw source Fax Service DLL Search Order Hijack · Sigma
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Published by SigmaHQ/sigma ↗, licensed under Detection Rule License 1.1 ↗. Reproduced here unmodified. 
title: Fax Service DLL Search Order Hijack
id: 828af599-4c53-4ed2-ba4a-a9f835c434ea
status: test
description: The Fax service attempts to load ualapi.dll, which is non-existent. An attacker can then (side)load their own malicious DLL using this service.
references:
    - https://windows-internals.com/faxing-your-way-to-system/
author: NVISO
date: 2020-05-04
modified: 2022-06-02
tags:
    - attack.privilege-escalation
    - attack.persistence
    - attack.execution
    - attack.stealth
    - attack.t1574.001
logsource:
    category: image_load
    product: windows
detection:
    selection:
        Image|endswith: '\fxssvc.exe'
        ImageLoaded|endswith: 'ualapi.dll'
    filter:
        ImageLoaded|startswith: 'C:\Windows\WinSxS\'
    condition: selection and not filter
falsepositives:
    - Unlikely
level: high

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