kryptor5

Hunt Hypothesis

This rule identifies the presence of the Kryptor5 malware family, which is commonly used for initial access and credential theft in ransomware operations. Proactively hunting for this signature allows the SOC to detect dormant or early-stage infections before they progress to lateral movement or encryption phases within the Azure environment.

YARA Rule

rule kryptor5
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 03 [3] E9 EB 6C 58 40 FF E0 }

condition:
		$a0 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 developer or DevOps engineer uses a legitimate open-source encryption utility (such as gpg, openssl enc, or a custom Go/Rust binary named kryptor5 for local data obfuscation) on a workstation or build agent. The binary’s name or hash matches the YARA signature intended for the malicious variant, causing a false positive on the host where the tool is installed.

  • Filter/Exclusion: Exclude processes where the parent process is a known development tool (e.g., code.exe, idea64.exe, docker.exe) or restrict the detection to servers and domain controllers, excluding workstations and build agents. Additionally, allowlist the specific SHA256 hash of the legitimate binary if it is consistently used.

• Scenario: An IT administrator runs a scheduled maintenance job using a custom PowerShell script or batch file that invokes a helper binary named kryptor5.exe to encrypt/decrypt configuration files or secrets during a patching cycle. The binary is located in a non-standard directory (e.g., C:\Temp\ or C:\Scripts\) and is executed by cmd.exe or powershell.exe, triggering the rule if it checks for unusual parent-child relationships or file paths.

  • Filter/Exclusion: Allowlist the specific file path (e.g., C:\Scripts\kryptor5.exe) or the parent process powershell.exe when the command line contains specific keywords like -Encrypt or -Decrypt. Ensure the binary is signed by the internal CA and add its certificate thumbprint to the allowlist.

• Scenario: A CI/CD pipeline (e.g., Jenkins, Azure DevOps, or GitHub Actions) runs a build agent that downloads and executes a temporary tool named kryptor5 as part of a dependency resolution or artifact packaging step. The process is