Rules for DuBrute Bruteforcer

Hunt Hypothesis

This detection rule identifies potential brute force attacks by monitoring for repeated authentication failures that align with the DuBrute behavioral patterns. The SOC team should proactively hunt for these events in Azure Sentinel to distinguish legitimate user activity from early-stage credential stuffing attempts before they escalate into successful account compromises.

YARA Rule

rule dubrute : bruteforcer toolkit
{
    meta:
        author = "Christian Rebischke (@sh1bumi)"
        date = "2015-09-05"
        description = "Rules for DuBrute Bruteforcer"
        in_the_wild = true
        family = "Hackingtool/Bruteforcer"
    
    strings:
        $a = "WBrute"
        $b = "error.txt"
        $c = "good.txt"
        $d = "source.txt"
        $e = "bad.txt"
        $f = "Generator IP@Login;Password"

    condition:
        //check for MZ Signature at offset 0
        uint16(0) == 0x5A4D 

        and 

        //check for dubrute specific strings
        $a and $b and $c and $d and $e and $f 
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 6 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the DuBrute Bruteforcer detection rule in an enterprise environment, along with recommended filters or exclusions:

• Scheduled Password Rotation via Service Accounts

  • Scenario: Automated scripts (e.g., using Ansible, PowerShell Desired State Configuration, or Chef) run nightly to rotate credentials for shared service accounts across domain controllers. These jobs often attempt multiple login attempts with slightly varying parameters before successfully authenticating, mimicking a brute-force pattern.
  • Filter/Exclusion: Exclude source IP addresses belonging to the Configuration Management Infrastructure subnet and filter events where the LogonType is 3 (Network) or 10 (RemoteInteractive) originating from known service account names (e.g., svc_ansible_rotation, svc_chef_client).

• Enterprise MDM Device Enrollment and Policy Sync

  • Scenario: During business hours, mobile devices managed by Microsoft Intune, Jamf Pro, or VMware Workspace ONE perform background policy synchronization. If a device loses connectivity briefly during the sync process, it may retry authentication multiple times in rapid succession against the Exchange Online or Azure AD endpoints, triggering the brute-force threshold.
  • Filter/Exclusion: Create an exclusion rule for source IPs associated with the MDM Gateway and filter for specific UserAgent strings containing “Intune.MAM”, “JamfPro”, or “WorkspaceONE”. Additionally, exclude events where the failure reason is specifically “Authentication policy mismatch” rather than “Invalid credentials.”

• Legacy Application Batch Processing Jobs

  • Scenario: Legacy line-of-business applications (e.g., a custom SAP module or an on-premise Salesforce connector) utilize a shared database account to run end-of-day batch jobs. These jobs often execute thousands of transactions