PESpinV03cyberbob

Hunt Hypothesis

This YARA rule targets specific executable artifacts associated with the “cyberbob” signature, potentially indicating the presence of a low-severity malware strain or a benign tool that has been repurposed for offensive operations. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify dormant or stealthy endpoints that may be executing known malicious code, thereby reducing the risk of lateral movement or data exfiltration before it escalates to a higher-severity incident.

YARA Rule

rule PESpinV03cyberbob
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 01 68 60 E8 00 00 00 00 8B 1C 24 83 C3 12 81 2B E8 B1 06 00 FE 4B FD 82 2C 24 B7 CD 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 EA EB 01 EB EB 0D FF E8 01 00 00 00 EA 5A 83 EA 0B FF E2 8B 95 CB 2C 40 00 8B 42 3C 03 C2 89 85 D5 2C 40 00 41 C1 E1 07 8B 0C 01 03 CA 8B 59 10 03 DA 8B 1B 89 9D E9 2C 40 00 53 8F 85 B6 2B 40 00 BB ?? 00 00 00 B9 75 0A 00 00 8D BD 7E 2D 40 00 4F 30 1C 39 FE CB E2 F9 68 3C 01 00 00 59 8D BD B6 36 40 00 C0 0C 39 02 E2 FA E8 02 00 00 00 FF 15 5A 8D 85 1F 53 56 00 BB 54 13 0B 00 D1 E3 2B C3 FF E0 E8 01 00 00 00 68 E8 1A 00 00 00 8D 34 28 B9 08 00 00 00 B8 [4] 2B C9 83 C9 15 0F A3 C8 0F 83 81 00 00 00 8D B4 0D DC 2C 40 00 }

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 developer or DevOps engineer is compiling a custom C/C++ application using the Microsoft Visual Studio Build Tools (specifically cl.exe or link.exe) on a build server or developer workstation. The resulting executable may contain specific section headers or import table structures that match the YARA signature if the rule targets common compiler artifacts or specific string patterns.
  • Filter/Exclusion: Exclude processes where the parent process is cl.exe, msbuild.exe, or devenv.exe and the file path resides in standard development directories (e.g., C:\Users\<User>\Source\, C:\Builds\, or C:\dev\).
• Scenario: An IT administrator is running a legitimate third-party utility or installer (such as 7z.exe for compression, NSIS installers, or Inno Setup compilers) that creates temporary executables or self-extracting archives. These tools often generate PE files with specific section names or alignment patterns that can trigger generic PE-based YARA rules.
  • Filter/Exclusion: Exclude files located in temporary directories (C:\Windows\Temp\, C:\Users\<User>\AppData\Local\Temp\) where the parent process is a known archiver or installer tool (7z.exe, nsis.exe, innosetup.exe) and the file extension is .exe or .msi.
• Scenario: A security team or application owner is running a legitimate digital signature verification tool (like signtool.exe from Windows SDK) or a code integrity checker (like ic32.exe or ic64.exe from Sysinternals) that loads and inspects PE files in memory or on disk. The inspection process may create shadow copies or temporary instances of the target binary that match the rule’s structural criteria.