Looks for crypt32 CryptBinaryToStringA function

Hunt Hypothesis

The crypt32 CryptBinaryToStringA function may indicate an adversary attempting to decode or manipulate binary data, potentially as part of credential theft or data exfiltration. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential data manipulation or sensitive information processing activities.

YARA Rule

rule Crypt32_CryptBinaryToString_API {
	meta:
		author = "_pusher_"
		description = "Looks for crypt32 CryptBinaryToStringA function"
		date = "2016-08"
	strings:
		$crypt32 = "crypt32.dll" wide ascii nocase
		$CryptBinaryToStringA = "CryptBinaryToStringA" wide ascii
	condition:
		$crypt32 and ($CryptBinaryToStringA)
}

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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: Windows Update or System Update Process
  Description: The CryptBinaryToStringA function may be called during system updates or Windows Update processes as part of certificate validation.
  Filter/Exclusion: Check the process name (ImageLoaded field) for svchost.exe or wuauclt.exe and exclude those processes.

• Scenario: Scheduled Task Running Certificate Validation
  Description: A legitimate scheduled task may use certificate validation routines that invoke CryptBinaryToStringA.
  Filter/Exclusion: Filter by TaskName or CommandLine to exclude known system or enterprise scheduled tasks (e.g., Task Scheduler tasks with known names).

• Scenario: Microsoft Intune or Endpoint Protection Policy Enforcement
  Description: Intune or other endpoint protection tools may use cryptographic functions during policy enforcement or certificate checks.
  Filter/Exclusion: Exclude processes associated with Microsoft Intune (Microsoft.Intune.MAM) or endpoint protection agents (e.g., MpCmdRun.exe).

• Scenario: Certificate Management via PowerShell or MMC
  Description: Administrators may use PowerShell or the Microsoft Management Console (MMC) to manage certificates, which can trigger the CryptBinaryToStringA function.
  Filter/Exclusion: Filter by ProcessName to exclude powershell.exe or mmc.exe when running known certificate management tasks.

• Scenario: Legacy Application Using Cryptographic APIs
  Description: Some legacy applications or internal tools may use the crypt32 library for cryptographic operations, including certificate handling.
  Filter/Exclusion: Exclude processes associated with known internal tools or legacy applications (e.g., MyLegacyApp.exe) or filter by User to exclude non-admin users.