UnderGroundCrypterbyBooster2000

Hunt Hypothesis

This hypothesis targets the execution of the UnderGroundCrypterbyBooster2000 YARA signature, which indicates the presence of a specific crypter or obfuscated payload often used to mask malicious code from static analysis. Proactively hunting for this indicator in Azure Sentinel allows the SOC to identify compromised endpoints or suspicious processes early, potentially uncovering a foothold for advanced persistent threats or ransomware precursors before they escalate.

YARA Rule

rule UnderGroundCrypterbyBooster2000
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 83 C4 F0 B8 74 3C 00 11 E8 94 F9 FF FF E8 BF FE FF FF E8 0A F3 FF FF 8B C0 }

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

• Scenario: A developer or DevOps engineer runs a custom, internally built encryption utility (e.g., SecureVault.exe or a Go/Rust binary) that uses a specific XOR or AES implementation matching the YARA signature, often during CI/CD pipeline artifact processing or local data sanitization.

  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\Projects\, D:\Builds\) or exclude processes initiated by known CI/CD agents (e.g., Jenkins.exe, GitLabRunner.exe, AzureAgent.exe).

• Scenario: A legacy application or third-party tool (e.g., an older version of a PDF editor, media converter, or backup utility) contains an embedded encryption library that matches the “Booster2000” signature due to shared code patterns or outdated dependencies.

  • Filter/Exclusion: Maintain a whitelist of known legacy application hashes or exclude processes running from specific vendor directories (e.g., C:\Program Files\LegacyApp\, C:\Program Files (x86)\VendorTool\).

• Scenario: A scheduled maintenance job runs a custom script or binary (e.g., NightlyEncrypt.ps1 invoking Encryptor.dll) to encrypt log files or database dumps for compliance, where the binary is a compiled version of the same source code used in the YARA rule.

  • Filter/Exclusion: Exclude processes spawned by scheduled task services (Schtasks.exe, TaskScheduler.exe) or specific scheduled task names (e.g., “Compliance_Log_Encryption”, “DB_Backup_Encrypt”).

• Scenario: A security team or blue team member runs the YARA rule itself or a test harness (e.g., yara.exe or a custom TestCrypter.exe) to validate detection coverage or perform red