Windows Fax Service ualapi.dll Side-Loading via fxssvc.exe

Flags fxssvc.exe loading ualapi.dll from unexpected paths, indicating potential DLL side-loading for privilege escalation.

FreeReviewedSigma · High · v2

Product: windows
Category: image_load
Author: NVISO (SigmaHQ), DRL 1.1
Published: 2020-05-04
Updated: 2026-07-31

Analysis Detection

ATT&CK techniques

Execution → Defense Evasion

Recon
Resource Dev
Initial Access
Execution
- T1574.001DLL
Persistence
- T1574.001DLL
Priv Esc
- T1574.001DLL
Defense Evasion
- T1574.001DLL
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule flags Windows DLL search-order side-loading behavior where fxssvc.exe loads ualapi.dll from an unexpected location, indicating the Fax-related service failed to find its expected dependency. Attackers can exploit this pattern to force the service to load a malicious DLL and gain execution in the service context. The detection relies on image load telemetry capturing the calling process (fxssvc.exe) and the loaded module name (ualapi.dll), while excluding benign loads from the WinSxS directory.

Reporting behind it

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Windows Fax Service ualapi.dll Side-Loading via fxssvc.exe

Pivot detection · T1574.001 · 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-fax-service-dll-search-order-hijack-via-ualapi-dll-side-loading-828af599

title: Windows Fax Service ualapi.dll Side-Loading via fxssvc.exe
id: c39467a7-bb06-4ced-a072-72d776d57b71
status: test
description: This rule flags Windows DLL search-order side-loading behavior where fxssvc.exe loads ualapi.dll from an unexpected location, indicating the Fax-related service failed to find its expected dependency. Attackers can exploit this pattern to force the service to load a malicious DLL and gain execution in the service context. The detection relies on image load telemetry capturing the calling process (fxssvc.exe) and the loaded module name (ualapi.dll), while excluding benign loads from the WinSxS directory.
references:
  - https://windows-internals.com/faxing-your-way-to-system/
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/image_load/image_load_side_load_ualapi.yml
author: NVISO, Huntrule Team
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
license: DRL-1.1
related:
  - id: 828af599-4c53-4ed2-ba4a-a9f835c434ea
    type: derived

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