Windows Defender Threat Severity Default Action Modified

SIGMA detection rule by Matt Anderson (Huntress) · GPL-2.0

Detects modifications or creations of Windows Defender's default threat action settings based on severity to 'allow' or take 'no action'. This is a highly suspicious configuration change that effectively disables Defender's ability to automatically mitigate threats of a certain severity level, allowing malicious software to run unimpeded. An attacker might use this technique to bypass defenses before executing payloads.

title: Windows Defender Threat Severity Default Action Modified
id: 4162459b-68e6-524b-ec5a-48ed032b96cd
related:
    - id: 1e8a9b4d-3c2a-4f9b-8d1e-7c6a5b4f3d2e
      type: similar
    - id: 5a9e1b2c-8f7d-4a1e-9b3c-0f6d7e5a4b1f
      type: derived
status: experimental
description: |
    Detects modifications or creations of Windows Defender's default threat action settings based on severity to 'allow' or take 'no action'.
    This is a highly suspicious configuration change that effectively disables Defender's ability to automatically mitigate threats of a certain severity level,
    allowing malicious software to run unimpeded. An attacker might use this technique to bypass defenses before executing payloads.
references:
    - https://learn.microsoft.com/en-us/powershell/module/defender/set-mppreference
    - https://learn.microsoft.com/en-us/windows-hardware/customize/desktop/unattend/security-malware-windows-defender-threatseveritydefaultaction
    - https://research.splunk.com/endpoint/7215831c-8252-4ae3-8d43-db588e82f952
    - https://gist.github.com/Dump-GUY/8daef859f382b895ac6fd0cf094555d2
    - https://thedfirreport.com/2021/10/18/icedid-to-xinglocker-ransomware-in-24-hours/
author: Matt Anderson (Huntress)
date: 2025-07-11
tags:
    - attack.defense-impairment
    - attack.t1685
logsource:
    category: registry_event
    product: windows
detection:
    registry_event:
        EventID: 4657
        Channel: Security
    selection:
        ObjectName|contains: \Microsoft\Windows Defender\Threats\ThreatSeverityDefaultAction\
        ObjectName|endswith:
            - \1
            - \2
            - \4
            - \5
        NewValue:
            - DWORD (0x00000006)   # Allow
            - DWORD (0x00000009)   # NoAction
    condition: registry_event and selection
falsepositives:
    - Legitimate administration via scripts or tools (e.g., SCCM, Intune, GPO enforcement). Correlate with administrative activity.
    - Software installations that legitimately modify Defender settings (less common for these specific keys).
level: high
ruletype: Sigma

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09643881-6ab1-41f5-b164-b781848ccf0d

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Executable code, not a declarative rule

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