USSR V031 SpiritST

Hunt Hypothesis

This rule detects the presence of the SpiritST implant, a known component of the USSR V031 toolkit, indicating potential adversary activity involving remote access or data staging. Proactively hunting for this signature allows the SOC to identify compromised endpoints early, as SpiritST is often used to maintain persistence or facilitate lateral movement within the Azure environment.

YARA Rule

rule USSR_V031_SpiritST: PEiD
{
    strings:
        $a = { 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 C0 2E 55 53 53 52 00 00 00 00 10 00 00 ?? ?? ?? ?? 00 10 00 00 ?? ?? ?? ?? 00 00 00 00 00 00 00 00 }
    condition:
        $a 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: A security team deploys a YARA scanner (e.g., YARA-Native or ClamAV with custom rules) to audit endpoint binaries for known threat signatures. The rule USSR_V031_SpiritST matches a legitimate utility or a test binary that contains the specific byte pattern or string associated with the “SpiritST” variant, triggering an alert during a routine scan.

  • Filter/Exclusion: Exclude processes originating from known scanning tools such as yara.exe, clamscan.exe, or osqueryd.exe when the parent process is a security management agent (e.g., TaniumAgent.exe, KyligenceAgent.exe).

• Scenario: An IT administrator performs a manual forensic investigation or malware triage on a suspect file by copying it to a staging directory and running a YARA scan locally. The file is a known benign sample (e.g., a test executable or a library with a specific header) that matches the rule’s signature, causing a false positive during the analysis phase.

  • Filter/Exclusion: Exclude alerts where the file path contains keywords like \staging\, \forensics\, \triage\, or \temp\ and the parent process is an interactive user session (e.g., explorer.exe or cmd.exe) rather than a service or scheduled task.

• Scenario: A developer or QA engineer tests a new application build or a third-party SDK that includes a component with a binary structure or embedded string that coincidentally matches the USSR_V031_SpiritST YARA pattern. This occurs during local development or pre-production testing on a dev workstation.

  • Filter/Exclusion: Exclude hosts tagged with dev, qa, or test in the CMDB or asset inventory, or exclude processes running from directories such as