Finds the custom xor decode loop for <PolishBankRAT_fdsvc>

Hunt Hypothesis

This hunt targets adversaries executing the specific XOR decoding routine associated with PolishBankRAT’s fdsvc component to identify early-stage malware activity within Azure Sentinel. Proactively hunting for this signature is critical because detecting this custom loop allows the SOC team to isolate potential banking sector compromises before they escalate into full data exfiltration or lateral movement events.

YARA Rule

rule PolishBankRAT_fdsvc_xor_loop 
{
	meta:

		author = "Booz Allen Hamilton Dark Labs"
		description = "Finds the custom xor decode loop for <PolishBankRAT_fdsvc>"

	strings:

		$loop = {0F B6 42 FF 48 8D 52 FF 30 42 01 FF CF 75 F1}

	condition:

		(uint16(0) == 0x5A4D and uint32(uint32(0x3C)) == 0x00004550) and $loop
}

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

Here are 4 specific false positive scenarios for the Finds the custom xor decode loop for <PolishBankRAT_fdsvc> detection rule, tailored for an enterprise environment:

• Antivirus Signature Updates via Windows Defender

  • Scenario: The daily scheduled update of Microsoft Defender Antivirus (via MpCmdRun.exe or MsMpEng.exe) often involves unpacking and scanning compressed signature definition files. During this process, the engine may perform custom XOR-based decompression on large .cab or .zip archives containing heuristic signatures, inadvertently triggering the specific byte-pattern match associated with the PolishBankRAT loop.
  • Suggested Filter: Exclude processes where ParentImage is MpCmdRun.exe or MsMpEng.exe, and the file path contains \ProgramData\Microsoft\Windows Defender\.

• Enterprise Backup Agent Data Deduplication

  • Scenario: Enterprise backup solutions like Veeam Backup & Replication or Commvault often run scheduled jobs that perform block-level deduplication. When processing large database dumps (e.g., SQL Server .bak files) or virtual machine snapshots, the backup agent’s compression engine utilizes XOR operations to identify redundant data blocks before transmission, mimicking the decoding loop signature of the malware.
  • Suggested Filter: Create an exclusion for known backup service executables such as VeeamAgent.exe, CommServe.exe, or OracleBackupService.exe running under a dedicated service account (e.g., DOMAIN\svc_backup).

• Software Deployment via SCCM/Intune

  • Scenario: During the deployment of enterprise applications using Microsoft Endpoint Configuration Manager (SCCM) or Intune, the client agent (ccmexec.exe) extracts application packages. These packages often contain custom installers that use proprietary XOR encoding for configuration manifests or embedded scripts to ensure data