Fax Service DLL Search Order Hijack

Hunt Hypothesis

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.

Detection Rule

Sigma (Original)

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

KQL (Microsoft 365 Defender)

DeviceImageLoadEvents
| where (InitiatingProcessFolderPath endswith "\\fxssvc.exe" and FolderPath endswith "ualapi.dll") and (not(FolderPath startswith "C:\\Windows\\WinSxS\\"))

Required Data Sources

False Positive Guidance

• Unlikely

MITRE ATT&CK Context

• Tactic: Privilege Escalation
• Tactic: Persistence
• Tactic: Execution
• Technique: T1574.001 — T1574.001

References

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