Fax Service DLL Search Order Hijack

Status: test
Severity: high
Log source: category image_load, product windows
Author: NVISO
Source: github.com/SigmaHQ/sigma

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.

Known false positives

Unlikely

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1574.001` Hijack Execution Flow: DLL
Stealth	`T1574.001` Hijack Execution Flow: DLL

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 7: Image loaded

Rule body

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 filter

Stage 1: `selection`

selection:
    Image|endswith: '\fxssvc.exe'
    ImageLoaded|endswith: 'ualapi.dll'

Stage 2: `not filter`

filter:
    ImageLoaded|startswith: 'C:\Windows\WinSxS\'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`ImageLoaded`	starts_with	`C:\Windows\WinSxS\`	`excludes:ImageLoaded` `field:"ImageLoaded" value:"C:\Windows\WinSxS\"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`\fxssvc.exe`	`field:"Image" kind:ends_with value:"\fxssvc.exe"`
`ImageLoaded`	ends_with	`ualapi.dll`	`field:"ImageLoaded" kind:ends_with value:"ualapi.dll"`