Windows Webshell Strings — Sigma Rule

Rule metadata and linked tags

Author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems). Source status: test; severity: high. Source dates: 2017-02-19 / 2022-11-18.

{
  "category": "webserver"
}

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 8d919a9c208ef9060dfc10e81c9ae48c91b42d37af64b3a8ca156ed42e07d9e2

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: Windows Webshell Strings
id: 7ff9db12-1b94-4a79-ba68-a2402c5d6729
status: test
description: Detects common commands used in Windows webshells
references:
    - https://bad-jubies.github.io/RCE-NOW-WHAT/
    - https://m365internals.com/2022/10/07/hunting-in-on-premises-exchange-server-logs/
author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems)
date: 2017-02-19
modified: 2022-11-18
tags:
    - attack.persistence
    - attack.t1505.003
logsource:
    category: webserver
detection:
    selection_method:
        cs-method: 'GET'
    selection_keywords:
        # The "%20" is URL encoded version of the space
        # The "%2B" is URL encoded version of the "+"
        - '=whoami'
        - '=net%20user'
        - '=net+user'
        - '=net%2Buser'
        - '=cmd%20/c%'
        - '=cmd+/c+'
        - '=cmd%2B/c%'
        - '=cmd%20/r%'
        - '=cmd+/r+'
        - '=cmd%2B/r%'
        - '=cmd%20/k%'
        - '=cmd+/k+'
        - '=cmd%2B/k%'
        - '=powershell%'
        - '=powershell+'
        - '=tasklist%'
        - '=tasklist+'
        - '=wmic%'
        - '=wmic+'
        - '=ssh%'
        - '=ssh+'
        - '=python%'
        - '=python+'
        - '=python3%'
        - '=python3+'
        - '=ipconfig'
        - '=wget%'
        - '=wget+'
        - '=curl%'
        - '=curl+'
        - '=certutil'
        - '=copy%20%5C%5C'
        - '=dsquery%'
        - '=dsquery+'
        - '=nltest%'
        - '=nltest+'
    condition: all of selection_*
falsepositives:
    - Web sites like wikis with articles on os commands and pages that include the os commands in the URLs
    - User searches in search boxes of the respective website
level: high

Original YAML and metadata in JSON

False positives

• Web sites like wikis with articles on os commands and pages that include the os commands in the URLs
• User searches in search boxes of the respective website

Source references

• https://bad-jubies.github.io/RCE-NOW-WHAT/
• https://m365internals.com/2022/10/07/hunting-in-on-premises-exchange-server-logs/