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Description

The following analytic detects the use of .NET classes in PowerShell to download a URL payload directly into memory, a common fileless malware staging technique. It leverages PowerShell Script Block Logging (EventCode=4104) to identify suspicious PowerShell commands involving system.net.webclient, system.net.webrequest, and IO.MemoryStream. This activity is significant as it indicates potential fileless malware execution, which is harder to detect and can bypass traditional file-based defenses. If confirmed malicious, this technique could allow attackers to execute code in memory, evade detection, and maintain persistence in the environment.

  • Type: TTP

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

  • Last Updated: 2024-05-12
  • Author: Steven Dick
  • ID: 103affa6-924a-4b53-aff4-1d5075342aab

Annotations

ATT&CK

ATT&CK

ID Technique Tactic
T1059.001 PowerShell Execution
T1105 Ingress Tool Transfer Command And Control
T1027.011 Fileless Storage Defense Evasion
Kill Chain Phase
  • Installation
  • Command and Control
  • Exploitation
NIST
  • DE.CM
CIS20
  • CIS 10
CVE 


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`powershell` EventCode=4104  ScriptBlockText IN ("*system.net.webclient*","*system.net.webrequest*") AND ScriptBlockText="*IO.MemoryStream*" 
| eval Path = case(isnotnull(Path),Path,true(),"unknown") 
| stats count min(_time) as firstTime max(_time) as lastTime list(ScriptBlockText) as command values(Path) as file_name values(UserID) as user by ActivityID, Computer, EventCode 
| rename Computer as dest, EventCode as signature_id 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `powershell_webrequest_using_memory_stream_filter`

Macros

The SPL above uses the following Macros:

:information_source: powershell_webrequest_using_memory_stream_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
  • EventCode
  • ActivityID
  • Computer
  • ScriptBlockText

How To Implement

The following analytic requires PowerShell operational logs to be imported. Modify the powershell macro as needed to match the sourcetype or add index. This analytic is specific to 4104, or PowerShell Script Block Logging.

Known False Positives

Unknown, possible custom scripting.

Associated Analytic Story

RBA

Risk Score Impact Confidence Message
80.0 100 80 Powershell webrequest to memory stream behavior. Possible fileless malware staging on $dest$ by $user$.

: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