maindll mutex

Hunt Hypothesis

The detection of a mutex named “maindll” may indicate an adversary attempting to coordinate or synchronize malicious activities across multiple instances of a process. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential persistence or coordination mechanisms used by adversaries to maintain control or evade detection.

YARA Rule

rule maindll_mutex
{
   
    meta:
        author = "Matt Brooks, @cmatthewbrooks"
        desc = "Matches on the maindll mutex"
        ref = "https://citizenlab.org/2016/04/between-hong-kong-and-burma/"
        
    strings:
        $mutex = "h31415927tttt"
        
    condition:
        //MZ header  //PE signature
        uint16(0) == 0x5A4D and uint32(uint32(0x3C)) == 0x00004550 and $mutex
}

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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A legitimate system update or patching tool (e.g., Microsoft Update, Windows Server Update Services) creates a mutex named maindll_mutex during installation.
  Filter/Exclusion: Check for the presence of known update tools or processes (e.g., wusa.exe, msiexec.exe) in the process tree or command line arguments.

• Scenario: A scheduled task (e.g., via Task Scheduler) runs a legitimate application (e.g., Microsoft SQL Server Agent) that uses a mutex named maindll_mutex to prevent multiple instances.
  Filter/Exclusion: Filter by process name or command line containing “SQL Server” or “Task Scheduler” and check for scheduled task context.

• Scenario: A system administration task (e.g., using PowerShell or Group Policy) temporarily creates a mutex to synchronize operations across multiple servers.
  Filter/Exclusion: Exclude processes running under the SYSTEM account or with elevated privileges, or filter by known administrative scripts.

• Scenario: A third-party application (e.g., Adobe Acrobat, Microsoft Office) uses a mutex named maindll_mutex to manage internal synchronization.
  Filter/Exclusion: Check for known application names in the process name or command line, or filter by application-specific process IDs.

• Scenario: A legitimate service (e.g., IIS, Exchange, or Active Directory) creates a mutex during startup to ensure single instance operation.
  Filter/Exclusion: Filter by service name or process name (e.g., w3wp.exe, msiexec.exe, lsass.exe) and check for service startup context.