MaxMpxCt Registry Value Changed

Hunt Hypothesis

Detects changes to the “MaxMpxCt” registry value. MaxMpxCt specifies the maximum outstanding network requests for the server per client, which is used when negotiating a Server Message Block (SMB) con

Detection Rule

Sigma (Original)

title: MaxMpxCt Registry Value Changed
id: 0e6a9e62-627e-496c-aef5-bfa39da29b5e
status: test
description: |
    Detects changes to the "MaxMpxCt" registry value.
    MaxMpxCt specifies the maximum outstanding network requests for the server per client, which is used when negotiating a Server Message Block (SMB) connection with a client. Note if the value is set beyond 125 older Windows 9x clients will fail to negotiate.
    Ransomware threat actors and operators (specifically BlackCat) were seen increasing this value in order to handle a higher volume of traffic.
references:
    - https://www.huntress.com/blog/blackcat-ransomware-affiliate-ttps
    - https://securityscorecard.com/research/deep-dive-into-alphv-blackcat-ransomware
    - https://www.intrinsec.com/alphv-ransomware-gang-analysis/?cn-reloaded=1
    - https://www.sentinelone.com/labs/blackcat-ransomware-highly-configurable-rust-driven-raas-on-the-prowl-for-victims/
author: Nasreddine Bencherchali (Nextron Systems)
date: 2024-03-19
tags:
    - attack.stealth
    - attack.t1070.005
logsource:
    category: registry_set
    product: windows
detection:
    selection:
        TargetObject|endswith: '\Services\LanmanServer\Parameters\MaxMpxCt'
    condition: selection
falsepositives:
    - Unknown
level: low

KQL (Azure Sentinel)

imRegistry
| where RegistryKey endswith "\\Services\\LanmanServer\\Parameters\\MaxMpxCt"

KQL (Microsoft 365 Defender)

DeviceRegistryEvents
| where RegistryKey endswith "\\Services\\LanmanServer\\Parameters\\MaxMpxCt"

Required Data Sources

False Positive Guidance

• Unknown

MITRE ATT&CK Context

• Technique: T1070.005 — T1070.005

References

• https://www.huntress.com/blog/blackcat-ransomware-affiliate-ttps
• https://securityscorecard.com/research/deep-dive-into-alphv-blackcat-ransomware
• https://www.intrinsec.com/alphv-ransomware-gang-analysis/?cn-reloaded=1
• https://www.sentinelone.com/labs/blackcat-ransomware-highly-configurable-rust-driven-raas-on-the-prowl-for-victims/
• SigmaHQ Rule