Unloading AMSI via Reflection

name: Unloading AMSI via Reflection
id: a21e3484-c94d-11eb-b55b-acde48001122
version: 13
creation_date: '2021-06-09'
modification_date: '2026-05-13'
author: Michael Haag, Splunk
status: production
type: TTP
description: The following analytic detects the tampering of AMSI (Antimalware Scan Interface) via PowerShell reflection. It leverages PowerShell Script Block Logging (EventCode=4104) to capture and analyze suspicious PowerShell commands, specifically those involving `system.management.automation.amsi`. This activity is significant as it indicates an attempt to bypass AMSI, a critical security feature that helps detect and block malicious scripts. If confirmed malicious, this could allow an attacker to execute harmful code undetected, leading to potential system compromise and data exfiltration.
data_source:
    - Powershell Script Block Logging 4104
search: |-
    `powershell` EventCode=4104 ScriptBlockText = *system.management.automation.amsi*
      | 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)`
      | `unloading_amsi_via_reflection_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: Potential for some third party applications to disable AMSI upon invocation. Filter as needed.
references:
    - 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.
    - https://blog.palantir.com/tampering-with-windows-event-tracing-background-offense-and-defense-4be7ac62ac63
    - https://static1.squarespace.com/static/552092d5e4b0661088167e5c/t/59c1814829f18782e24f1fe2/1505853768977/Windows+PowerShell+Logging+Cheat+Sheet+ver+Sept+2017+v2.1.pdf
    - https://www.crowdstrike.com/blog/investigating-powershell-command-and-script-logging/
finding:
    title: Possible AMSI Unloading via Reflection using PowerShell on $dest$
    entity:
        field: dest
        type: system
        score: 50
analytic_story:
    - Malicious PowerShell
    - Hermetic Wiper
    - Data Destruction
asset_type: Endpoint
mitre_attack_id:
    - T1059.001
    - T1685
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint