SLServer unknown string

Hunt Hypothesis

The hypothesis is that the detection identifies potential malicious activity involving an unknown string associated with SLServer, which may indicate an adversary attempting to execute or communicate through an unrecognized payload. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential compromise from unknown or evolving threats.

YARA Rule

rule SLServer_unknown_string
{
    meta:
        author = "Matt Brooks, @cmatthewbrooks"
        desc = "Searches for a unique string."
        ref = "https://citizenlab.org/2016/04/between-hong-kong-and-burma/"
    
    strings:
        $string = "test-b7fa835a39"
        
    condition:
        //MZ header //PE signature
        uint16(0) == 0x5A4D and uint32(uint32(0x3C)) == 0x00004550 and $string
}

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 system administrator is using PowerShell to script a routine system cleanup, and the script includes a string that matches the YARA rule (e.g., SLServer in a log file path).
  Filter/Exclusion: Check for process.name == "powershell.exe" and process.parent.name == "explorer.exe" or process.parent.name == "task scheduler".

• Scenario: A scheduled job runs nightly to process log files, and the log file path contains the string SLServer as part of a legitimate server name (e.g., C:\Logs\SLServer-App-Logs).
  Filter/Exclusion: Filter by file.path containing SLServer-App-Logs or use a whitelist of known log directories.

• Scenario: A database backup tool (e.g., SQL Server Backup Utility) generates a temporary file with the string SLServer in its filename during a backup operation.
  Filter/Exclusion: Exclude files created by process.name == "sqlbackup.exe" or process.name == "sqlservr.exe".

• Scenario: An IT support tool (e.g., Microsoft Endpoint Manager or Microsoft Intune) includes the string SLServer in its configuration files or logs during a policy deployment.
  Filter/Exclusion: Filter by process.name containing microsoft-intune or endpoint-manager, or check for process.parent.name == "explorer.exe".

• Scenario: A custom application (e.g., ServiceNow or Jira) uses the string SLServer in its internal logging or configuration files, which is part of its normal operation.
  Filter/Exclusion: Exclude files or processes associated with the application (e.g., `process.name == “servicenow.exe