PowerShell Download Via Net.WebClient - PowerShell Classic — Sigma Rule

Rule metadata and linked tags

Author: Florian Roth (Nextron Systems). Source status: test; severity: low. Source dates: 2017-03-05 / 2026-04-28.

{
  "product": "windows",
  "category": "ps_classic_start"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 66bef8fdc9ecf4b3ca7b1375f06a30f987bc4728d19151f555f12dcbbbadb7ba

Detection logic

Original source YAML. Source-tag agreement is not proof of complete semantic coverage. This rule has not been compiled for a SIEM backend or validated against live telemetry here.

title: PowerShell Download Via Net.WebClient - PowerShell Classic
id: 3236fcd0-b7e3-4433-b4f8-86ad61a9af2d
related:
    - id: 65531a81-a694-4e31-ae04-f8ba5bc33759
      type: derived
status: test
description: |
    Detects PowerShell download activity, via the .DownloadFile() or .DownloadString() methods of the Net.WebClient class.
    This technique is often abused by attackers to download additional payloads.
references:
    - https://www.trendmicro.com/en_us/research/22/j/lv-ransomware-exploits-proxyshell-in-attack.html
author: Florian Roth (Nextron Systems)
date: 2017-03-05
modified: 2026-04-28
tags:
    - attack.execution
    - attack.command-and-control
    - attack.t1059.001
    - attack.t1105
logsource:
    product: windows
    category: ps_classic_start
detection:
    selection_webclient:
        Data|contains: 'Net.WebClient'
    selection_download:
        Data|contains:
            - '.DownloadFile('
            - '.DownloadString('
    condition: all of selection_*
falsepositives:
    - This activity may be used by legitimate software, such as patch management tools or software updaters. Investigate any such activity and apply the necessary filter.
level: low

Original YAML and metadata in JSON

False positives

• This activity may be used by legitimate software, such as patch management tools or software updaters. Investigate any such activity and apply the necessary filter.

Source references

• https://www.trendmicro.com/en_us/research/22/j/lv-ransomware-exploits-proxyshell-in-attack.html