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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.

  • Type: TTP
  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud

  • Last Updated: 2024-05-24
  • Author: Michael Haag, Splunk
  • ID: a21e3484-c94d-11eb-b55b-acde48001122

Annotations

ATT&CK

ATT&CK

ID Technique Tactic
T1562 Impair Defenses Defense Evasion
T1059.001 PowerShell Execution
T1059 Command and Scripting Interpreter Execution
Kill Chain Phase
  • Exploitation
  • Installation
NIST
  • DE.CM
CIS20
  • CIS 10
CVE
1
2
3
4
5
`powershell` EventCode=4104 ScriptBlockText = *system.management.automation.amsi* 
| stats count min(_time) as firstTime max(_time) as lastTime by EventCode ScriptBlockText Computer user_id 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `unloading_amsi_via_reflection_filter`

Macros

The SPL above uses the following Macros:

:information_source: unloading_amsi_via_reflection_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

  • _time
  • ScriptBlockText
  • Opcode
  • Computer
  • UserID
  • EventCode

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://docs.splunk.com/Documentation/UBA/5.0.4.1/GetDataIn/AddPowerShell#Configure_module_logging_for_PowerShell.

Known False Positives

Potential for some third party applications to disable AMSI upon invocation. Filter as needed.

Associated Analytic Story

RBA

Risk Score Impact Confidence Message
49.0 70 70 Possible AMSI Unloading via Reflection using PowerShell on $Computer$

:information_source: The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

Reference

Test Dataset

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

source | version: 2