Cmd Launched with Hidden Start Flags to Suspicious Targets — Sigma Rule

Rule metadata and linked tags

Author: Vladan Sekulic, Swachchhanda Shrawan Poudel (Nextron Systems). Source status: experimental; severity: medium. Source dates: 2026-01-24 / not supplied.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: 322fca5ce3e60c1c04bd9988dcf5855f9f2c586ba81c76b9d03604ec595ec8d4

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: Cmd Launched with Hidden Start Flags to Suspicious Targets
id: 5a6b7c8d-9e0f-1a2b-3c4d-5e6f7a8b9c0d
status: experimental
description: |
    Detects cmd.exe executing commands with the "start" utility using "/b" (no window) or "/min" (minimized) flags.
    To reduce false positives from standard background tasks, detection is restricted to scenarios where the target is a known script extension or located in suspicious temporary/public directories.
    This technique was observed in Chaos, DarkSide, and Emotet malware campaigns.
references:
    - https://www.fortinet.com/blog/threat-research/evolution-of-chaos-ransomware-faster-smarter-and-more-dangerous
    - https://www.fortinet.com/blog/threat-research/newly-discovered-function-in-darkside-ransomware-variant-targets-disk-partitions
    - https://www.fortinet.com/blog/threat-research/ms-office-files-involved-in-emotet-trojan-campaign-pt-one
    - https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/start
tags:
    - attack.stealth
    - attack.t1564.003
author: Vladan Sekulic, Swachchhanda Shrawan Poudel (Nextron Systems)
date: 2026-01-24
logsource:
    category: process_creation
    product: windows
detection:
    selection_cmd_img:
        - Image|endswith: '\cmd.exe'
        - OriginalFileName: 'Cmd.Exe'
    selection_cmd_hidden_start_1:
        CommandLine|contains|windash:
            - 'start '
            - 'start/b'
            - 'start/min'
    selection_cmd_hidden_start_2:
        CommandLine|contains|windash:
            - '/b '
            - '/b"'
            - '/min '
            - '/min"'
    selection_cli_uncommon_location:
        CommandLine|contains:
            - ':\Perflogs\'
            - ':\Temp\'
            - ':\Users\Default\'
            - ':\Windows\Temp\'
            - '\AppData\Roaming\'
            - '\Contacts\'
            - '\Documents\'
            - '\Downloads\'
            - '\Favorites\'
            - '\Favourites\'
            - '\inetpub\'
            - '\Music\'
            - '\Photos\'
            - '\Temporary Internet\'
            - '\Users\Public\'
            - '\Videos\'
    selection_cli_susp_extension:
        CommandLine|contains:
            - '.bat'
            - '.cmd'
            - '.cpl'
            - '.hta'
            - '.js'
            - '.ps1'
            - '.scr'
            - '.vbe'
            - '.vbs'
    selection_cli_susp_pattern:
        CommandLine|contains:
            - ' -nop '
            - ' -sta '
            - '.downloadfile(' # PowerShell download command
            - '.downloadstring(' # PowerShell download command
            - '-noni '
            - '-w hidden '
    condition: all of selection_cmd_* and 1 of selection_cli_*
falsepositives:
    - Legitimate administrative scripts running from temporary folders.
    - Niche software updaters utilizing hidden batch files in ProgramData.
level: medium # Can be increased after an initial baseline and tuning
regression_tests_path: regression_data/rules/windows/process_creation/proc_creation_win_cmd_launched_with_hidden_start_flag/info.yml

Original YAML and metadata in JSON

False positives

• Legitimate administrative scripts running from temporary folders.
• Niche software updaters utilizing hidden batch files in ProgramData.

Source references

• https://www.fortinet.com/blog/threat-research/evolution-of-chaos-ransomware-faster-smarter-and-more-dangerous
• https://www.fortinet.com/blog/threat-research/newly-discovered-function-in-darkside-ransomware-variant-targets-disk-partitions
• https://www.fortinet.com/blog/threat-research/ms-office-files-involved-in-emotet-trojan-campaign-pt-one
• https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/start