Detects RayInitiator GRUB bootkit stage 3 install phase code that searches for the 'client-cert-fail' string.

Hunt Hypothesis

This detection identifies the Stage 3 installation behavior of a RayInitiator GRUB bootkit specifically searching for the “client-cert-fail” string to establish persistence and potentially bypass certificate validation checks. SOC teams should proactively hunt for this signature in Azure Sentinel because early identification of this bootkit phase allows for immediate containment before the adversary fully embeds itself into the system’s boot process, thereby preventing deep-rooted compromise that is difficult to remediate post-infection.

YARA Rule

rule MAL_Cisco_RayInitiator_Stage_3 {
   meta:
      author = "NCSC"
      description = "Detects RayInitiator GRUB bootkit stage 3 install phase code that searches for the 'client-cert-fail' string."
      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 = "ea42c20b-7362-5d2d-9ce2-ad010f3f880d"
   strings:
      $xc1 = {
         48 81 EE 00 00 00 08 48 B8 63 6C 69 65 6E 74 2D 63 49
         B8 65 72 74 2D 66 61 69 6C 48 FF C6 48 39 D6 0F 87 D2
         00 00 00 48 8B 3E 48 39 C7
      }
   condition:
      $xc1
}

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.

References

False Positive Guidance

Here are 4 specific false positive scenarios for the RayInitiator GRUB bootkit stage 3 install phase detection rule, including targeted filters and exclusions:

Scenario: Automated Certificate Rotation via Ansible Playbook
- Context: An automated nightly Ansible playbook runs on Linux servers to rotate client certificates. During the “client-cert-fail” validation step of the rotation script, the system triggers a GRUB update process that scans for this specific string to verify boot-time certificate availability.
- Filter/Exclusion: Exclude events where the process_name matches ansible-playbook or ansible-runner and the command line arguments contain --certificate-rotation. Alternatively, whitelist the specific user account (e.g., svc_ansible) executing these tasks during the maintenance window (02:00–04:00 UTC).
Scenario: Endpoint Protection Agent Self-Repair
- Context: The enterprise endpoint protection suite (e.g., CrowdStrike Falcon or Carbon Black) performs a scheduled self-repair task. When the agent detects a potential mismatch in the boot-time certificate chain, it invokes the RayInitiator module to re-verify the ‘client-cert-fail’ string within the GRUB stage 3 configuration before rebooting the service.
- Filter/Exclusion: Filter out alerts where the parent_process_name is the endpoint agent executable (e.g., falcon.sys, cb.exe) and the event source is identified as a “Scheduled Maintenance Task” rather than an interactive user session.
Scenario: Golden Image Deployment via SCCM or Intune
- Context: During a mass deployment of a new OS golden image using Microsoft Endpoint Configuration Manager (SCCM) or Intune, the provisioning script executes a GRUB configuration check. The script explicitly searches for ‘client-cert-fail