Suspicious Group And Account Reconnaissance Activity Using Net.EXE — Sigma Rule

Rule metadata and linked tags

Author: Florian Roth (Nextron Systems), omkar72, @svch0st, Nasreddine Bencherchali (Nextron Systems). Source status: test; severity: medium. Source dates: 2019-01-16 / 2023-03-02.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 9d72f8de7778f0505a080d9ca09b08743b86d8df573d505f580977b4ef8bb3fb

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: Suspicious Group And Account Reconnaissance Activity Using Net.EXE
id: d95de845-b83c-4a9a-8a6a-4fc802ebf6c0
status: test
description: |
    Detects suspicious reconnaissance command line activity on Windows systems using Net.EXE
    Check if the user that executed the commands is suspicious (e.g. service accounts, LOCAL_SYSTEM)
references:
    - https://redcanary.com/blog/how-one-hospital-thwarted-a-ryuk-ransomware-outbreak/
    - https://thedfirreport.com/2020/10/18/ryuk-in-5-hours/
    - https://research.nccgroup.com/2022/08/19/back-in-black-unlocking-a-lockbit-3-0-ransomware-attack/
author: Florian Roth (Nextron Systems), omkar72, @svch0st, Nasreddine Bencherchali (Nextron Systems)
date: 2019-01-16
modified: 2023-03-02
tags:
    - attack.discovery
    - attack.t1087.001
    - attack.t1087.002
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith:
              - '\net.exe'
              - '\net1.exe'
        - OriginalFileName:
              - 'net.exe'
              - 'net1.exe'
    # Covers group and localgroup flags
    selection_group_root:
        CommandLine|contains:
            - ' group '
            - ' localgroup '
    selection_group_flags:
        CommandLine|contains:
            # Add more groups for other languages
            - 'domain admins'
            - ' administrator' # Typo without an 'S' so we catch both
            - ' administrateur' # Typo without an 'S' so we catch both
            - 'enterprise admins'
            - 'Exchange Trusted Subsystem'
            - 'Remote Desktop Users'
            - 'Utilisateurs du Bureau à distance' # French for "Remote Desktop Users"
            - 'Usuarios de escritorio remoto' # Spanish for "Remote Desktop Users"
            - ' /do' # short for domain
    filter_group_add:
        # This filter is added to avoid the potential case where the point is not recon but addition
        CommandLine|contains: ' /add'
    # Covers 'accounts' flag
    selection_accounts_root:
        CommandLine|contains: ' accounts '
    selection_accounts_flags:
        CommandLine|contains: ' /do' # short for domain
    condition: selection_img and ((all of selection_group_* and not filter_group_add) or all of selection_accounts_*)
falsepositives:
    - Inventory tool runs
    - Administrative activity
level: medium

Original YAML and metadata in JSON

False positives

• Inventory tool runs
• Administrative activity

Source references

• https://redcanary.com/blog/how-one-hospital-thwarted-a-ryuk-ransomware-outbreak/
• https://thedfirreport.com/2020/10/18/ryuk-in-5-hours/
• https://research.nccgroup.com/2022/08/19/back-in-black-unlocking-a-lockbit-3-0-ransomware-attack/