Auto-generated rule - file-3665415_sys

Hunt Hypothesis

Unusual file system activity involving the file “3665415_sys” may indicate potential adversary tampering or persistence mechanisms. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential compromise early.

YARA Rule

rule Regin_Sample_1 
{

    meta:
        description = "Auto-generated rule - file-3665415_sys"
        author = "@MalwrSignatures"
        date = "26.11.14"
        hash = "773d7fab06807b5b1bc2d74fa80343e83593caf2"

    strings:
        $s0 = "Getting PortName/Identifier failed - %x" fullword ascii
        $s1 = "SerialAddDevice - error creating new devobj [%#08lx]" fullword ascii
        $s2 = "External Naming Failed - Status %x" fullword ascii
        $s3 = "------- Same multiport - different interrupts" fullword ascii
        $s4 = "%x occurred prior to the wait - starting the" fullword ascii
        $s5 = "'user registry info - userPortIndex: %d" fullword ascii
        $s6 = "Could not report legacy device - %x" fullword ascii
        $s7 = "entering SerialGetPortInfo" fullword ascii
        $s8 = "'user registry info - userPort: %x" fullword ascii
        $s9 = "IoOpenDeviceRegistryKey failed - %x " fullword ascii
        $s10 = "Kernel debugger is using port at address %X" fullword ascii
        $s12 = "Release - freeing multi context" fullword ascii
        $s13 = "Serial driver will not load port" fullword ascii
        $s14 = "'user registry info - userAddressSpace: %d" fullword ascii
        $s15 = "SerialAddDevice: Enumeration request, returning NO_MORE_ENTRIES" fullword ascii
        $s20 = "'user registry info - userIndexed: %d" fullword ascii

    condition:
        all of them and filesize < 110KB and filesize > 80KB
}

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 16 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: System file integrity check using Microsoft Defender
  Description: A scheduled task runs Microsoft Defender’s file integrity check, which may trigger the rule due to normal file scanning activity.
  Filter/Exclusion: process.parent_process_name : "Windows Defender Antivirus Service"

• Scenario: Windows Update installation
  Description: During a Windows Update installation, system files are temporarily modified or replaced, which may be flagged by the rule.
  Filter/Exclusion: process.name : "wuauclt.exe" or process.parent_process_name : "svchost.exe"

• Scenario: Scheduled backup job using Veeam Backup & Replication
  Description: A backup job initiated by Veeam may involve copying or modifying system files, leading to false positives.
  Filter/Exclusion: process.name : "vmbackup.exe" or process.parent_process_name : "Veeam Backup Service"

• Scenario: PowerShell script running as part of system maintenance
  Description: An admin-run PowerShell script (e.g., for log cleanup or registry maintenance) may trigger the rule due to file access.
  Filter/Exclusion: process.name : "powershell.exe" and process.user : "Administrator" with a custom field indicating it’s a known maintenance script.

• Scenario: Antivirus signature update
  Description: An antivirus tool (e.g., Bitdefender, Kaspersky) may update its signature database, which can involve reading or writing system files.
  Filter/Exclusion: process.name : "bdagent.exe" or process.name : "kavsvc.exe" depending on the antivirus tool.