PowerShell ScriptBlock WebClient Download Calls

Alert on PowerShell ScriptBlock text that uses System.Net.WebClient to download files or strings from the Internet.

FreeReviewedSigma · Medium · v2

Product: windows
Category: ps_script
Author: Florian Roth (Nextron Systems) (SigmaHQ), DRL 1.1
Published: 2017-03-05
Updated: 2026-07-31

ATT&CK techniques

Execution

Recon
Resource Dev
Initial Access
Execution
- T1059.001PowerShell
Persistence
Priv Esc
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule flags PowerShell activity where a ScriptBlock contains references to System.Net.WebClient and download-related method calls such as DownloadFile, DownloadFileAsync, DownloadString, or DownloadStringAsync. Attackers commonly use these .NET WebClient methods to pull additional code or payload content into a PowerShell session. The detection relies on Script Block Logging telemetry that includes the ScriptBlockText content.

Reporting behind it

Related detections9 linkedT1059.001 — drag to rearrange

Suspicious Script Host Spawning Hidden PowerShell (via process_creation)

Suspicious PowerShell Reflective Assembly Load With GZip Decompression (via process_creation)

Suspicious PowerShell Invoke-Expression with Replace Obfuscation

Suspicious PowerShell Archive Extraction to AppData Roaming

Malicious PowerShell Spawned by Masqueraded NVIDIA GeForce Experience Binary

Suspicious PowerShell Hidden Unrestricted Execution via ClickFix

Suspicious PowerShell Spawned by cscript in Script Chain

Suspicious Script Interpreter Spawned by Explorer via ClickFix Run Dialog (via process_creation)

Suspicious PowerShell Download Cradle via ClickFix Fake CAPTCHA (via process_creation)

PowerShell ScriptBlock WebClient Download Calls

Pivot detection · T1059.001 · 9 related

Changelog

v2
Candidate ingested via manual entry.2026-07-31
v1
No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

powershell-scriptblock-downloads-via-system-net-webclient-403c2cc0

title: PowerShell ScriptBlock WebClient Download Calls
id: 9b9ac524-ce38-4009-8cff-ce2b56638877
related:
  - id: 65531a81-a694-4e31-ae04-f8ba5bc33759
    type: derived
  - id: 403c2cc0-7f6b-4925-9423-bfa573bed7eb
    type: derived
status: test
description: This rule flags PowerShell activity where a ScriptBlock contains references to System.Net.WebClient and download-related method calls such as DownloadFile, DownloadFileAsync, DownloadString, or DownloadStringAsync. Attackers commonly use these .NET WebClient methods to pull additional code or payload content into a PowerShell session. The detection relies on Script Block Logging telemetry that includes the ScriptBlockText content.
references:
  - https://learn.microsoft.com/en-us/dotnet/api/system.net.webclient.downloadstring?view=net-8.0
  - https://learn.microsoft.com/en-us/dotnet/api/system.net.webclient.downloadfile?view=net-8.0
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/powershell/powershell_script/posh_ps_susp_download.yml
author: Florian Roth (Nextron Systems), Huntrule Team
date: 2017-03-05
modified: 2022-12-02
tags:
  - attack.execution
  - attack.t1059.001
logsource:
  product: windows
  category: ps_script
  definition: "Requirements: Script Block Logging must be enabled"
detection:
  webclient:
    ScriptBlockText|contains: System.Net.WebClient
  download:
    ScriptBlockText|contains:
      - .DownloadFile(
      - .DownloadFileAsync(
      - .DownloadString(
      - .DownloadStringAsync(
  condition: webclient and download
falsepositives:
  - PowerShell scripts that download content from the Internet
level: medium
license: DRL-1.1

Convert

This rule as a query you can run. Uses no unlock credit.

SplunkSPL