Hunt Hypothesis
This rule detects the presence of the “cyberbob” YARA signature, which likely identifies a specific low-severity malicious or suspicious PE file variant associated with the PESpin framework. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify dormant or low-fidelity threats that may evade traditional behavioral detections, ensuring early visibility into potential footholds or test artifacts within the environment.
YARA Rule
rule PESpinV071cyberbob
{
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 83 D5 46 00 0B E4 74 9E }
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.
False Positive Guidance
- Legacy .NET Framework Deployment Packages: Enterprise IT often distributes internal line-of-business applications or legacy tools compiled with older versions of the .NET Framework (e.g., 2.0, 3.5, or 4.0). These executables frequently contain specific metadata strings, version numbers, or resource sections that match the
PESpinV071cyberbob YARA signature.
- Filter/Exclusion: Exclude files located in standard application directories (e.g.,
C:\Program Files\, C:\Program Files (x86)\) where the parent process is a known service host (e.g., svchost.exe, w3wp.exe) or the file extension is .exe and the size is under 5MB.
- Third-Party Antivirus/EDR Signature Updates: Security vendors like CrowdStrike, SentinelOne, or Microsoft Defender regularly push new detection signatures or engine updates. These update packages are often compressed or packed executables that may inadvertently trigger generic PE structure rules if the YARA rule matches common packing headers or specific version strings used in recent builds.
- Filter/Exclusion: Exclude processes or files whose path contains vendor-specific directories (e.g.,
\CrowdStrike\, \SentinelOne\, \Microsoft Defender\) or where the parent process is the vendor’s updater service (e.g., csagent.exe, sentinelone.exe, MsMpEng.exe).
- Scheduled Backup and Archiving Jobs: Enterprise backup solutions (e.g., Veeam, Commvault, Acronis) or archiving tools (e.g., WinRAR, 7-Zip) often create temporary executable wrappers or use specific compression algorithms that result in PE headers matching the rule’s structural criteria. These jobs run via Task Scheduler or service accounts and are routine.