Windows PowerShell Add Module to Global Assembly Cache

name: Windows PowerShell Add Module to Global Assembly Cache
id: 3fc16961-97e5-4a5b-a079-e4ab0d9763eb
version: 12
creation_date: '2023-01-18'
modification_date: '2026-05-13'
author: Michael Haag, Splunk
status: production
type: TTP
description: The following analytic detects the addition of a DLL to the Windows Global Assembly Cache (GAC) using PowerShell. It leverages PowerShell Script Block Logging to identify commands containing "system.enterpriseservices.internal.publish". This activity is significant because adding a DLL to the GAC allows it to be shared across multiple applications, potentially enabling an adversary to execute malicious code system-wide. If confirmed malicious, this could lead to widespread code execution, privilege escalation, and persistent access across the operating system, posing a severe security risk.
data_source:
    - Powershell Script Block Logging 4104
search: |-
    `powershell` EventCode=4104 ScriptBlockText IN("*system.enterpriseservices.internal.publish*")
      | fillnull
      | stats count min(_time) as firstTime max(_time) as lastTime
        BY dest signature signature_id
           user_id vendor_product EventID
           Guid Opcode Name
           Path ProcessID ScriptBlockId
           ScriptBlockText
      | `security_content_ctime(firstTime)`
      | `security_content_ctime(lastTime)`
      | `windows_powershell_add_module_to_global_assembly_cache_filter`
how_to_implement: To successfully implement this analytic, you will need to enable PowerShell Script Block Logging on some or all endpoints. Additional setup here https://help.splunk.com/en/security-offerings/splunk-user-behavior-analytics/get-data-in/5.4.1/add-other-data-to-splunk-uba/configure-powershell-logging-to-see-powershell-anomalies-in-splunk-uba.
known_false_positives: False positives may be present based on developers or third party utilities adding items to the GAC.
references:
    - https://www.microsoft.com/en-us/security/blog/2022/12/12/iis-modules-the-evolution-of-web-shells-and-how-to-detect-them/
    - https://www.microsoft.com/en-us/security/blog/2022/07/26/malicious-iis-extensions-quietly-open-persistent-backdoors-into-servers/
finding:
    title: PowerShell was used to install a module to the Global Assembly Cache on $dest$.
    entity:
        field: dest
        type: system
        score: 50
analytic_story:
    - IIS Components
asset_type: Endpoint
mitre_attack_id:
    - T1505.004
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint