Windows PowerShell Add Module to Global Assembly Cache

name: Windows PowerShell Add Module to Global Assembly Cache
id: 3fc16961-97e5-4a5b-a079-e4ab0d9763eb
version: 10
date: '2026-03-10'
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/
drilldown_searches:
    - name: View the detection results for - "$Computer$"
      search: '%original_detection_search% | search  Computer = "$Computer$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$Computer$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$Computer$") starthoursago=168  | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
rba:
    message: PowerShell was used to install a module to the Global Assembly Cache on $dest$.
    risk_objects:
        - field: dest
          type: system
          score: 50
    threat_objects: []
tags:
    analytic_story:
        - IIS Components
    asset_type: Endpoint
    mitre_attack_id:
        - T1505.004
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1505.004/pwsh_publish_powershell.log
          source: XmlWinEventLog:Microsoft-Windows-PowerShell/Operational
          sourcetype: XmlWinEventLog