Unloading AMSI via Reflection

Name:Unloading AMSI via Reflection
id:a21e3484-c94d-11eb-b55b-acde48001122
version:8
date:2025-06-24
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/
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:'$info_max_time$'
tags:
  analytic_story:
    - 'Malicious PowerShell'
    - 'Hermetic Wiper'
    - 'Data Destruction'
  asset_type:Endpoint
  mitre_attack_id:
    - 'T1059.001'
    - 'T1562'
  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/T1059.001/powershell_script_block_logging/windows-powershell-xml.log
  source: XmlWinEventLog:Microsoft-Windows-PowerShell/Operational
  sourcetype: XmlWinEventLog
manual_test:None