wsc.dll

Part of the DLL icon HijackLibs project.

Type

DLL Search Order Hijacking (1 EXE) DLLs specified by an application without a path are searched for in fixed locations in a specific order. By putting a malicious wsc.dll in a location that is searched in before the actual DLL, the legitimate application will execute arbitrary code upon normal execution. See also MITRE ATT&CK® technique T1574.001: Hijack Execution Flow: DLL Search Order Hijacking.

Vendor

Avast

Resources

Last updated

Unknown

Expected Locations

The file wsc.dll is normally found in the following path:

%PROGRAMFILES%\AVAST Software\Avast

Vulnerable Executables

The following executable attempts to load wsc.dll:

wsc_proxy.exe

File hash available

Detection

Below a sample Sigma rule that will find processes that loaded wsc.dll located in a folder that is not one of the expected locations (see above).

title: Possible DLL Hijacking of wsc.dll
id: 2764371b-9122-48a3-7130-5b9ff8861115
status: experimental
description: Detects possible DLL hijacking of wsc.dll by looking for suspicious image loads, loading this DLL from unexpected locations.
references:
    - https://hijacklibs.net/entries/3rd_party/avast/wsc.html
author: "Matt Green"
date: 2022-08-15
tags:
    - attack.defense_evasion
    - attack.T1574.001
logsource:
    product: windows
    category: image_load
detection:
    selection:
        ImageLoaded: '*\wsc.dll'
    filter:
        ImageLoaded:
            - 'c:\program files\AVAST Software\Avast\*'
            - 'c:\program files (x86)\AVAST Software\Avast\*'

    condition: selection and not filter
falsepositives:
    - False positives are likely. This rule is more suitable for hunting than for generating detections.

title: Possible preparation for wsc.dll DLL Hijacking
id: 2764372b-9122-48a3-7130-5b9ff8861115
status: experimental
description: Detects possible DLL hijacking of wsc.dll by looking for suspicious file writes of this DLL, to unexpected locations.
references:
    - https://hijacklibs.net/entries/3rd_party/avast/wsc.html
author: "Matt Green"
date: 2022-08-15
tags:
    - attack.defense_evasion
    - attack.T1574.001
logsource:
    product: windows
    category: file_event
detection:
    selection:
        TargetFileName: '*\wsc.dll'
    filter:
        TargetFileName:
            - 'c:\program files\AVAST Software\Avast\*'
            - 'c:\program files (x86)\AVAST Software\Avast\*'

    condition: selection and not filter
falsepositives:
    - False positives are likely. This rule is more suitable for hunting than for generating detections.

Note that this rule is also included in the Sigma feed that comprises all DLL Hijacking entries part of this project.

FAQs

Why should I care about this?
DLL Hijacking enables the execution of malicious code through a signed and/or trusted executable. Defensive measures such as AV and EDR solutions may not pick up on this activity out of the box, and allow-list applications such as AppLocker may not block the execution of the untrusted code. There are numerous examples of threat actors that have been observed to leaverage DLL Hijacking to achieve their objectives. As such, this project wants to encourage you to monitor for unusual activity involving wsc.dll.

How do I abuse this vulnerability?
As a red teamer, you will have to compile your own version of wsc.dll. There are various guides on how this can be achieved.

How could the vendor have prevented this vulnerability?
Most DLL Hijacking vulnerabilities are introduced by the 'lazy' loading of DLL files, which relies on Windows' default DLL search order. Explicitly specifying where a required DLL is located is easy and often already helps a lot. This doesn't have to hurt portability if Windows API calls are used to obtain paths, e.g. GetSystemDirectory to get the path of the System32 folder. Even better is to check the signature of required DLLs prior to loading them; most platforms, frameworks and/or runtimes offer means to verify DLL signatures with minimal performance impact.

This DLL Hijack doesn't seem to work (anymore), why is it still included?
Luckily, vendors regularly patch vulnerable applications in order to prevent DLL Hijacking from taking place. Nevertheless, older versions will remain vulnerable; for that reason, the entry won't be deleted from this project. To help others, you may want to open a pull request updating the 'precondition' tag on this entry to make the community aware of the reduced scope.

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