PESpin V13betaX cyberbobnbsp nbsp SignByfly 20080311

Hunt Hypothesis

This hypothesis targets the presence of the PESpin V13betaX variant, a legacy Windows executable often associated with early-stage malware loaders or proof-of-concept exploits, indicating potential unauthorized code execution or initial access attempts. Proactively hunting for this specific signature in Azure Sentinel allows the SOC to identify dormant or overlooked legacy threats that may have been introduced via unmanaged endpoints or legacy application deployments, ensuring timely remediation before they evolve into more complex attack chains.

YARA Rule

rule PESpin_V13betaX_cyberbobnbsp_nbsp_SignByfly_20080311: PEiD
{
    strings:
        $a = { EB 01 ?? 60 E8 00 00 00 00 8B 1C 24 83 C3 12 81 2B E8 B1 06 00 FE 4B FD 82 2C 24 71 DF 46 00 0B E4 74 9E 75 01 C7 81 73 04 D7 7A F7 2F 81 73 19 77 00 43 B7 F6 C3 6B B7 00 00 F9 FF E3 C9 C2 08 00 A3 68 72 01 FF 5D 33 C9 41 E2 17 EB 07 ?? ?? ?? ?? ?? ?? ?? E8 01 00 00 00 ?? 5A 83 EA 0B FF E2 EB 04 ?? EB 04 ?? EB FB ?? ?? ?? ?? ?? ?? ?? 8B 42 3C 03 C2 ?? ?? ?? ?? ?? ?? EB 02 ?? ?? F9 72 08 73 0E F9 83 04 24 17 C3 E8 04 00 00 00 0F F5 73 11 EB 06 9A 72 ED 1F EB 07 F5 72 0E F5 72 F8 68 EB EC 83 04 24 07 F5 FF 34 24 C3 }
    condition:
        $a 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

• Legacy .NET Framework Development and Testing

  • Scenario: Developers building or testing applications using the .NET Framework 1.0 or 2.0 (released around 2002-2005) may use older versions of mscorwks.dll or related runtime components that match the specific byte patterns in the PESpin YARA rule, particularly if the rule targets specific version strings or checksums from that era.
  • Filter/Exclusion: Exclude files located in standard .NET Framework installation directories (e.g., C:\Windows\Microsoft.NET\Framework\v2.0.50727\ or v1.0.3705\) or files with the extension .dll that have a known good hash for the specific .NET version in use.

• Older Version Control Systems (VCS) and Build Tools

  • Scenario: Enterprises still using legacy build agents or version control clients (such as older versions of Visual SourceSafe, Perforce, or early TFS agents) may have cached binaries or helper executables compiled with older toolchains that inadvertently match the cyberbob or SignByfly string patterns if these are embedded in debug symbols or version resources.
  • Filter/Exclusion: Exclude processes or files originating from known legacy build tool directories (e.g., C:\Program Files\Microsoft Visual Studio 9.0\ or C:\Program Files (x86)\Perforce\) and exclude files with timestamps older than 10 years if the rule is designed to catch recent malware.

• Legacy Java or C++ Runtime Libraries

  • Scenario: Applications relying on older JREs (Java Runtime Environment) or legacy C++ runtime libraries (e.g., msvcr71.dll or `