Look for DCP RijnDael EncryptECB

Hunt Hypothesis

The hypothesis is that the detection rule identifies potential encryption activity associated with the DCP RijnDael algorithm, which may indicate data exfiltration or encryption for malicious purposes. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify early signs of data manipulation or covert communication channels used by adversaries.

YARA Rule

rule DCP_RIJNDAEL_EncryptECB {
	meta:
		author = "_pusher_"
		description = "Look for DCP RijnDael EncryptECB"
		date = "2016-07"
	strings:
		$c0 = { 53 56 57 55 83 C4 B4 89 0C 24 8D 74 24 08 8D 7C 24 28 80 78 30 00 75 16 B9 ?? ?? ?? ?? B2 01 A1 ?? ?? ?? ?? E8 ?? ?? ?? ?? E8 ?? ?? ?? ?? 8B 0A 89 0F 8B CA 83 C1 04 8B 09 8D 5F 04 89 0B 8B CA 83 C1 08 8B 09 8D 5F 08 89 0B 83 C2 0C 8B 12 8D 4F 0C 89 11 8B 50 58 83 EA 02 85 D2 0F 82 3B 01 00 00 42 89 54 24 04 33 D2 8B 0F 8B DA C1 E3 02 33 4C D8 5C 89 0E 8D 4F 04 8B 09 33 4C D8 60 8D 6E 04 89 4D 00 8D 4F 08 8B 09 33 4C D8 64 8D 6E 08 89 4D 00 8D 4F 0C 8B 09 33 4C D8 68 8D 5E 0C 89 0B 33 C9 8A 0E 8D 0C 8D }
	condition:
		$c0
}

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 system administrator is using 7-Zip to encrypt files using ECB mode for archival purposes.
  Filter/Exclusion: Check for the presence of 7-Zip in the process name or command line arguments, or filter by file extensions commonly used for archives (e.g., .7z, .zip).

• Scenario: A scheduled backup job is running using Veeam Backup & Replication, which occasionally uses ECB mode encryption for certain data types.
  Filter/Exclusion: Filter by process name veeam.exe or check for known backup job IDs in the event log or task scheduler.

• Scenario: An IT admin is manually encrypting sensitive data using GnuPG (GPG) with ECB mode for legacy compatibility.
  Filter/Exclusion: Check for the presence of gpg in the process name or command line, or filter by specific file types associated with GPG encryption (e.g., .gpg, .asc).

• Scenario: A Windows Update task or Group Policy processing is triggering encryption-related activity due to system configuration changes.
  Filter/Exclusion: Filter by process names such as wuauclt.exe, gpupdate.exe, or check for system event IDs related to policy changes or updates.

• Scenario: A database encryption task using SQL Server TDE (Transparent Data Encryption) is executing, which may involve ECB-like encryption during data processing.
  Filter/Exclusion: Filter by process names such as sqlservr.exe or check for SQL Server event logs indicating encryption operations.