Fax Service DLL Search Order Hijack — Sigma Rule

Rule metadata and linked tags

Author: NVISO. Source status: test; severity: high. Source dates: 2020-05-04 / 2022-06-02.

{
  "category": "image_load",
  "product": "windows"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 4262cd7d500811aa42c720fcd6e9bf11863bb1eb163f3e8d9938ad5f316c0e59

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

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

Original YAML and metadata in JSON

False positives

• Unlikely

Source references

• https://windows-internals.com/faxing-your-way-to-system/