Fax Service DLL Search Order Hijack

calendar May 15, 2025 · attack.persistence attack.defense-evasion attack.t1574.001  ·
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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.

Sigma rule (View on GitHub)

 1title: Fax Service DLL Search Order Hijack
 2id: 828af599-4c53-4ed2-ba4a-a9f835c434ea
 3status: test
 4description: 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.
 5references:
 6    - https://windows-internals.com/faxing-your-way-to-system/
 7author: NVISO
 8date: 2020-05-04
 9modified: 2022-06-02
10tags:
11    - attack.persistence
12    - attack.defense-evasion
13    - attack.t1574.001
14logsource:
15    category: image_load
16    product: windows
17detection:
18    selection:
19        Image|endswith: '\fxssvc.exe'
20        ImageLoaded|endswith: 'ualapi.dll'
21    filter:
22        ImageLoaded|startswith: 'C:\Windows\WinSxS\'
23    condition: selection and not filter
24falsepositives:
25    - Unlikely
26level: high

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