RosAsm2050aBetov

Hunt Hypothesis

This hunt hypothesis targets adversary behavior characterized by the execution of specific assembly patterns identified by the RosAsm2050aBetov YARA signature, which may indicate early-stage malware or obfuscated code deployment within the environment. A SOC team should proactively hunt for this indicator in Azure Sentinel to uncover low-severity anomalies that could represent novel threats or initial footholds before they escalate into more severe incidents.

YARA Rule

rule RosAsm2050aBetov
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 60 8B 5D 08 B9 08 00 00 00 BF [4] 83 C7 07 FD 8A C3 24 0F 04 30 3C 39 76 02 04 07 AA C1 EB 04 E2 EE FC 68 00 10 00 00 68 [4] 68 [4] 6A 00 FF 15 [4] 61 8B E5 5D C2 04 00 }

condition:
		$a0
}

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 4 specific false positive scenarios for the RosAsm2050aBetov detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Scheduled Antivirus Database Updates

  • Context: Enterprise endpoints running Microsoft Defender for Endpoint or Symantec Endpoint Protection often trigger this YARA signature during nightly definition updates. The update process involves assembling new heuristic signatures that mimic the assembly structure of the detected threat.
  • Filter/Exclusion: Create a whitelist exclusion based on the specific hash of the updater service (e.g., MsMpEng.exe or SymantecEngine.exe) and restrict detection to non-business hours (01:00–05:00 UTC) for these specific processes.

• Scenario: Automated Deployment via Configuration Management

  • Context: When Microsoft System Center Configuration Manager (SCCM) or Ansible Tower pushes a new application package to the fleet, the deployment agent extracts and assembles binaries in the temporary staging folder (C:\Windows\CCMCache or /tmp/ansible). This extraction activity frequently triggers the RosAsm signature due to the rapid creation of assembly artifacts.
  • Filter/Exclusion: Exclude file paths containing CCMCache, Ansible, or ChocolateyTemp. Additionally, filter alerts where the parent process is ccmsetup.exe or ansible-runner with a “Scheduled Task” trigger type.

• Scenario: CI/CD Pipeline Artifact Generation

  • Context: Internal build servers running Jenkins or GitLab Runners generate temporary assembly files during the compilation and packaging phase of software builds. The YARA rule may misinterpret these transient build artifacts as malicious executable assemblies, especially when the pipeline runs on shared infrastructure.
  • **Filter/