NetNTLM Downgrade Attack - Registry — Sigma Rule

Rule metadata and linked tags

Author: Florian Roth (Nextron Systems), wagga, Nasreddine Bencherchali (Splunk STRT). Source status: test; severity: high. Source dates: 2018-03-20 / 2024-12-03.

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

Pinned original Sigma rule · Detection Rule License 1.1

Source SHA-256: d90cf4d9480182a6f575b90a3bd311851497b5b4a245d547a4ed0d953aad6f5d

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: NetNTLM Downgrade Attack - Registry
id: d67572a0-e2ec-45d6-b8db-c100d14b8ef2
status: test
description: Detects NetNTLM downgrade attack
references:
    - https://web.archive.org/web/20171113231705/https://www.optiv.com/blog/post-exploitation-using-netntlm-downgrade-attacks
    - https://www.ultimatewindowssecurity.com/wiki/page.aspx?spid=NSrpcservers
author: Florian Roth (Nextron Systems), wagga, Nasreddine Bencherchali (Splunk STRT)
date: 2018-03-20
modified: 2024-12-03
tags:
    - attack.persistence
    - attack.defense-impairment
    - attack.t1685
    - attack.t1112
logsource:
    product: windows
    category: registry_event
detection:
    selection_regkey:
        TargetObject|contains|all:
            - 'SYSTEM\'
            - 'ControlSet'
            - '\Control\Lsa'
    selection_value_lmcompatibilitylevel:
        TargetObject|endswith: '\lmcompatibilitylevel'
        Details:
            - 'DWORD (0x00000000)'
            - 'DWORD (0x00000001)'
            - 'DWORD (0x00000002)'
    selection_value_ntlmminclientsec:
        TargetObject|endswith: '\NtlmMinClientSec'
        Details:
            - 'DWORD (0x00000000)' # No Security
            - 'DWORD (0x00000010)' # Only Integrity
            - 'DWORD (0x00000020)' # Only confidentiality
            - 'DWORD (0x00000030)' # Both Integrity and confidentiality
    selection_value_restrictsendingntlmtraffic:
        # Note: The obvious values with issues are 0x00000000 (allow all) and 0x00000001 (audit).
        # 0x00000002 can be secure but only if "ClientAllowedNTLMServers" is properly configured
        # Hence all values should be monitored and investigated
        TargetObject|endswith: '\RestrictSendingNTLMTraffic'
    condition: selection_regkey and 1 of selection_value_*
falsepositives:
    - Services or tools that set the values to more restrictive values
level: high

Original YAML and metadata in JSON

False positives

• Services or tools that set the values to more restrictive values

Source references

• https://web.archive.org/web/20171113231705/https://www.optiv.com/blog/post-exploitation-using-netntlm-downgrade-attacks
• https://www.ultimatewindowssecurity.com/wiki/page.aspx?spid=NSrpcservers