Detection rules › Sigma
Fax Service DLL Search Order Hijack
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.
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Execution | T1574.001 Hijack Execution Flow: DLL |
| Stealth | T1574.001 Hijack Execution Flow: DLL |
Event coverage
| Provider | Event | Title |
|---|---|---|
| Sysmon | Event ID 7 | Image loaded |
Rule body yaml
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
Stages and Predicates
Stage 0: condition
selection and not filterStage 1: selection
selection:
Image|endswith: '\fxssvc.exe'
ImageLoaded|endswith: 'ualapi.dll'
Stage 2: not filter
filter:
ImageLoaded|startswith: 'C:\Windows\WinSxS\'
Exclusions
Top-level NOT(...) conjuncts: predicates this rule actively suppresses.
| Field | Kind | Excluded values |
|---|---|---|
ImageLoaded | starts_with | C:\Windows\WinSxS\ |
Indicators
Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.
| Field | Kind | Values |
|---|---|---|
Image | ends_with |
|
ImageLoaded | ends_with |
|