VxTrivial25

Hunt Hypothesis

This rule detects the presence of the VxTrivial25 YARA signature, which identifies specific malicious code patterns often associated with trivial or low-complexity malware variants. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify dormant or low-severity threats that may have slipped past initial detection layers, ensuring comprehensive coverage of the attack surface.

YARA Rule

rule VxTrivial25
{
      meta:
		author="malware-lu"
strings:
		$a0 = { B4 4E FE C6 CD 21 B8 ?? 3D BA ?? 00 CD 21 93 B4 40 CD }

condition:
		$a0 at pe.entry_point
}

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 security team executes a targeted memory scan using Volatility3 or Rekall on a compromised endpoint to identify injected code or hidden processes. The YARA rule VxTrivial25 often matches generic x86/x64 shellcode patterns or common library function prologues (e.g., mov eax, [esp+4]) that are prevalent in standard memory dumps, triggering a false positive on the analysis tool’s own memory space or the dump file itself.

  • Filter/Exclusion: Exclude alerts where the process name is volatility3, rekall, or memscan, or where the file path contains /memory/ or .dmp. Additionally, suppress alerts if the parent process is a known forensic tool like falcon or crowdstrike.

• Scenario: An IT administrator runs Sysinternals Process Monitor (ProcMon) to troubleshoot application performance issues. The tool captures extensive system calls and memory accesses, and its internal logging mechanism or the specific DLLs it loads (such as dbghelp.dll or ntdll.dll during stack walking) may contain byte sequences that match the trivial pattern defined in VxTrivial25, especially if the rule targets common Windows API entry points.

  • Filter/Exclusion: Exclude alerts where the process name is ProcMon.exe or Process Monitor. If the alert is file-based, exclude paths containing \Sysinternals\ or \Tools\.

• Scenario: A DevOps engineer uses Docker or Podman to build a container image, and the build process involves compiling C/C++ code or linking static libraries. The resulting binary or intermediate object files may contain standard compiler-generated code (e.g., from GCC or Clang) that matches the generic assembly patterns in `VxTrivial