CExev10a

Hunt Hypothesis

This detection identifies specific executable behaviors captured by the CExev10a YARA signature, potentially signaling early-stage reconnaissance or benign automation activities within the environment. Proactively hunting for this pattern in Azure Sentinel allows the SOC team to validate low-severity signals against known good baselines and uncover subtle anomalies that might precede more significant security incidents.

YARA Rule

rule CExev10a
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 81 EC 0C 02 [2] 56 BE 04 01 [2] 8D 85 F8 FE FF FF 56 50 6A ?? FF 15 54 10 40 ?? 8A 8D F8 FE FF FF 33 D2 84 C9 8D 85 F8 FE FF FF 74 16 }

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: Legitimate deployment of Microsoft Defender for Endpoint updates via the Windows Update Agent or WSUS server. The CExev10a rule often flags the execution of update binaries (usoclient.exe, wuauclt.exe) or associated PowerShell scripts that modify system configuration, which mimics the behavior of a new executable installation.

  • Suggested Filter: Exclude processes where ImageName contains \Windows\System32\usoclient.exe or \Windows\System32\wuauclt.exe, and restrict to parent processes named svchost.exe with service name Wuauserv.

• Scenario: Scheduled maintenance jobs executed by enterprise backup solutions (e.g., Veeam Backup & Replication or Commvault) that spawn temporary agents to scan disk volumes. These agents often instantiate new executable instances in the ProgramData directory, triggering the rule due to their dynamic creation and file system interaction patterns.

  • Suggested Filter: Create an exclusion for processes running under the user context of a dedicated service account (e.g., DOMAIN\VeeamService) where the ImageDirectory path starts with C:\ProgramData\ or D:\BackupAgents\.

• Scenario: Automated software distribution tasks performed by Microsoft Endpoint Configuration Manager (SCCM/MECM) during business hours. When SCCM pushes a new application package, it launches the ccmsetup.exe installer which may spawn child processes that match the YARA signature for new executable execution events.

  • Suggested Filter: Exclude events where the parent process is ccmexec.exe or ccmsetup.exe, and the command line arguments contain specific SCCM identifiers like /mp: or InstallPackage.

• Scenario: Development teams running automated build pipelines using Jenkins or GitLab CI runners on shared infrastructure. These runners frequently compile code, generating temporary executables in