AntiVirusVaccinev103

Hunt Hypothesis

This detection identifies potential anti-virus evasion or vaccination behaviors where adversaries modify security tools to prevent malware analysis and execution. SOC teams should proactively hunt for this activity in Azure Sentinel to uncover stealthy threats that bypass standard signature-based defenses by manipulating antivirus components before malicious payloads are fully deployed.

YARA Rule

rule AntiVirusVaccinev103
{
      meta:
		author="malware-lu"
strings:
		$a0 = { FA 33 DB B9 [2] 0E 1F 33 F6 FC AD 35 [2] 03 D8 E2 }

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

Here are 5 specific false positive scenarios for the AntiVirusVaccinev103 detection rule, along with recommended filters and exclusions:

• Scenario: Symantec Endpoint Protection (SEP) Client Auto-Update Service

  • Context: The SepClient.exe process initiates a scheduled update job every morning at 06:00 AM to download the latest virus definition signatures. This activity often triggers the YARA rule due to the specific binary signature of the vaccine package being downloaded and executed by the client service.
  • Filter/Exclusion: Create an exclusion for the process Symantec Endpoint Protection Client Service (PID range or hash) running under the NT SERVICE\SepClientService account, specifically when the file path matches C:\Program Files\Symantec Endpoint Protection\Bin\SepClient.exe.

• Scenario: Microsoft Defender Antivirus (MSRT) Scheduled Scan

  • Context: The Windows Task Scheduler triggers a “Windows Defender Scheduled Scan” daily. During this scan, the engine extracts and analyzes vaccine files from the cloud, which mimics the behavior of an external antivirus vaccine installation detected by the rule.
  • Filter/Exclusion: Exclude events where the Image path contains \Program Files\Windows Defender\MsMpEng.exe and the command line arguments include /scan. Additionally, filter out alerts generated during the configured maintenance window (e.g., 02:00 – 04:00).

• Scenario: CrowdStrike Falcon Sensor Policy Update

  • Context: The CrowdStrike Falcon sensor receives a policy update containing new prevention rules and signature vaccines. The FalconSensorService process executes these updates, which the YARA rule interprets as a generic antivirus vaccine injection event due to overlapping binary characteristics.