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Description

The following analytic utilizes PowerShell Script Block Logging (EventCode=4104) to identify suspicious PowerShell execution. Script Block Logging captures the command sent to PowerShell, the full command to be executed. Upon enabling, logs will output to Windows event logs. Dependent upon volume, enable on critical endpoints or all.
This analytic identifies the behavior of AMSI being tampered with. Implemented natively in many frameworks, the command will look similar to SEtValuE($Null,(New-OBJEct COLlECtionS.GenerIC.HAshSEt{[StrINg]))}$ReF=[ReF].AsSeMbLY.GeTTyPe("System.Management.Automation.Amsi"+"Utils") taken from Powershell-Empire.
During triage, review parallel processes using an EDR product or 4688 events. It will be important to understand the timeline of events around this activity. Review the entire logged PowerShell script block.

  • Type: TTP
  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
  • Datamodel:
  • Last Updated: 2021-06-09
  • Author: Michael Haag, Splunk
  • ID: a21e3484-c94d-11eb-b55b-acde48001122

ATT&CK

ID Technique Tactic
T1562 Impair Defenses Defense Evasion
`powershell` EventCode=4104 Message=*system.management.automation.amsi* 
| stats count min(_time) as firstTime max(_time) as lastTime by OpCode ComputerName User EventCode Message 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `unloading_amsi_via_reflection_filter`

Associated Analytic Story

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.

Required field

  • _time
  • Message
  • OpCode
  • ComputerName
  • User
  • EventCode

Kill Chain Phase

  • Exploitation

Known False Positives

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

RBA

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

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

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