Detects LINE VIPER Cisco ASA malware code as part of a shellcode deobfuscation routine.

Hunt Hypothesis

This hunt targets adversaries employing sophisticated shellcode deobfuscation routines to evade detection, specifically identifying the presence of LINE VIPER Cisco ASA malware signatures within execution flows. Proactively hunting for this behavior in Azure Sentinel is essential because these advanced obfuscation techniques often bypass standard signature-based defenses, requiring deep inspection to uncover hidden malicious payloads before they establish persistence or exfiltrate data.

YARA Rule

rule MAL_Cisco_LINE_VIPER_Shellcode_Deobfuscation_Routine {
   meta:
      author = "NCSC"
      description = "Detects LINE VIPER Cisco ASA malware code as part of a shellcode deobfuscation routine."
      date = "2025-09-25"
      reference = "https://www.ncsc.gov.uk/static-assets/documents/malware-analysis-reports/RayInitiator-LINE-VIPER/ncsc-mar-rayinitiator-line-viper.pdf"
      score = 85
      license = "https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/"
      id = "608282b5-f296-5d21-b88b-92cd53128d89"
   strings:
      $xc1 = {
         48 8B 7F 08 48 8D 5F 70 49 C7 C1 00 18 00 00 49 C7 C0
         20 00 00 00 48 89 DF 8A 01 32 07 48 FF C7 41 FF C8 4D 85 C0 75 F3
         88 01 48 FF C1 41 FF C9 4D 85 C9 75 DA
      }
      $x1 = "SIt/CEiNX3BJx8EAGAAAScfAIAAAAEiJ34oBMgdI/8dB/8hNhcB184gBSP/BQf/JTYXJdd"
      $x2 = "iLfwhIjV9wScfBABgAAEnHwCAAAABIid+KATIHSP/HQf/ITYXAdfOIAUj/wUH/yU2FyXXa"
      $x3 = "Ii38ISI1fcEnHwQAYAABJx8AgAAAASInfigEyB0j/x0H/yE2FwHXziAFI/8FB/8lNhcl12"
   condition:
      1 of them
}

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 4 string patterns in its detection logic.

References

False Positive Guidance

Here are 4 specific false positive scenarios for the LINE VIPER Cisco ASA Malware Shellcode Deobfuscation rule, including suggested filters and exclusions:

Scenario: Scheduled Antivirus Engine Updates via Microsoft Endpoint Configuration Manager (SCCM)
- Context: During nightly maintenance windows, SCCM pushes updated definitions to the LINE Viper security agent. The update process involves unpacking compressed shellcode payloads that mimic the deobfuscation signature of Cisco ASA malware.
- Filter/Exclusion: Create an exclusion for processes named ccmexec.exe or ViperUpdateService.exe when the parent process is wuauserv.exe. Additionally, exclude alerts occurring between 02:00 and 04:00 on workstations within the “Patch Management” AD Organizational Unit.
Scenario: Automated Backup Script Execution using PowerShell
- Context: A scheduled PowerShell job (Backup-Data.ps1) running under a dedicated service account (e.g., svc_backup_admin) utilizes embedded shellcode to compress and encrypt data before transmission. The deobfuscation routine within the script triggers the detection logic due to similar byte patterns in the memory heap.
- Filter/Exclusion: Exclude alerts where the process name is powershell.exe AND the command line contains specific keywords like -ExecutionPolicy Bypass or includes the path \Scripts\Backup\. Ensure the user context matches the known service account svc_backup_admin.
Scenario: Deployment of Custom Java Applications via Ansible
- Context: DevOps teams use Ansible to deploy custom Java-based microservices. The deployment agent executes a shellcode routine to initialize the JVM heap and deobfuscate configuration scripts, which generates memory signatures indistinguishable from the LINE VIPER Cisco ASA malware code.
- **Filter/Ex