XPEORv099b

Hunt Hypothesis

This YARA rule targets specific executable patterns associated with the XPEOR malware family, which is frequently used for initial access and credential theft. Proactively hunting for this signature helps identify compromised hosts before the adversary establishes persistence or moves laterally within the Azure environment.

YARA Rule

rule XPEORv099b
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 00 00 00 00 5D 8B CD 81 ED 7A 29 40 00 89 AD 0F 6D 40 00 }
	$a1 = { E8 [4] 5D 8B CD 81 ED 7A 29 40 ?? 89 AD 0F 6D 40 }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Since the specific YARA logic for XPEORv099b is not provided in the prompt, I will assume this rule targets a common enterprise pattern often associated with this naming convention: detection of signed executables or scripts performing file system operations (e.g., copying, moving, or archiving) in non-standard directories, or detecting specific known benign tools that match a generic “suspicious behavior” signature.

Given the generic nature of the rule ID, the most likely false positive scenarios involve legitimate administrative or deployment tools that interact with the file system in ways that mimic malicious activity (e.g., staging, caching, or temporary file creation).

Here are 3-5 specific false positive scenarios with suggested filters:

• Scenario: Microsoft Endpoint Configuration Manager (MECM/SCCM) Client Deployment

  • Trigger: The SCCM client service (CCMEXEC.EXE or CCMEXE32.EXE) copies application packages or software distribution files to the C:\Windows\CCM or C:\_SMSTaskSequence directories. If the YARA rule flags executables performing file writes/copies in system or hidden directories, this legitimate deployment activity can trigger the alert.
  • Filter/Exclusion: Exclude processes where the parent process is CCMEXEC.EXE or WUAEXE.EXE (Windows Update Agent) and the file path contains \CCM\ or \_SMSTaskSequence\.

• Scenario: Chocolatey Package Manager Installation

  • Trigger: The choco.exe or choc.exe utility downloads and extracts packages to C:\ProgramData\chocolatey\ or C:\Users\<user>\AppData\Local\Temp\. If the rule detects a signed executable creating temporary files or extracting archives in user-specific temp directories, this standard