Detects an executable encrypted with a 4 byte XOR (also used for Derusbi Trojan)

Hunt Hypothesis

The hypothesis is that the detection identifies adversaries using a 4-byte XOR encryption method to obfuscate malicious payloads, a technique commonly associated with the Derusbi Trojan. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential advanced persistent threats that may evade traditional detection mechanisms.

YARA Rule

rule XOR_4byte_Key
{

    meta:
        description = "Detects an executable encrypted with a 4 byte XOR (also used for Derusbi Trojan)"
        author = "Florian Roth"
        reference = "http://blog.airbuscybersecurity.com/post/2015/11/Newcomers-in-the-Derusbi-family"
        date = "2015-12-15"
        score = 60

   strings:
      /* Op Code */
      $s1 = { 85 C9 74 0A 31 06 01 1E 83 C6 04 49 EB F2 }
      /*
      test    ecx, ecx
      jz      short loc_590170
      xor     [esi], eax
      add     [esi], ebx
      add     esi, 4
      dec     ecx
      jmp     short loc_590162
      */

   condition:
      uint16(0) == 0x5a4d and filesize < 900KB and all 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 1 string patterns in its detection logic.

References

• http://blog.airbuscybersecurity.com/post/2015/11/Newcomers-in-the-Derusbi-family
• Source Rule

False Positive Guidance

• Scenario: System Restore Point Creation
  Description: When a system restore point is created, the system may generate temporary files that are encrypted using a 4-byte XOR for internal consistency checks.
  Filter/Exclusion: Exclude files with the SystemRestore or VolumeShadowCopy attributes in the file system metadata.

• Scenario: Scheduled Task Execution of a Legitimate Tool
  Description: A scheduled task runs a legitimate tool (e.g., 7-Zip, WinRAR, or PowerShell scripts) that uses 4-byte XOR encryption for data compression or obfuscation.
  Filter/Exclusion: Exclude processes associated with known legitimate tools using the ProcessName field (e.g., 7z.exe, PowerShell.exe) or check for known benign command-line arguments.

• Scenario: Administrative Tool with Encrypted Configuration Files
  Description: An administrative tool (e.g., Sysinternals Suite, Process Explorer, or LogParser) may store encrypted configuration files using 4-byte XOR for security.
  Filter/Exclusion: Exclude files located in known administrative directories (e.g., C:\Windows\System32, C:\Program Files\) or filter by file extensions like .cfg, .ini, or .xml.

• Scenario: Encrypted Backup Files from Third-Party Backup Software
  Description: A third-party backup tool (e.g., Veeam, Acronis, or Dell Backup and Recovery) may use 4-byte XOR encryption to secure backup files.
  Filter/Exclusion: Exclude files with known backup extensions (e.g., .vbk, .vib, .acn) or check for presence of backup software-specific metadata.

• Scenario: Encrypted Payload in a Legitimate Software Update
  Description: A legitimate software update