OpenSSL BN_mod_exp_simple

Hunt Hypothesis

The OpenSSL BN_mod_exp_simple rule detects potential misuse of cryptographic operations that could indicate an adversary leveraging weak exponentiation methods to compromise cryptographic keys. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential vulnerabilities in cryptographic implementations that could be exploited for key recovery or data exfiltration.

YARA Rule

rule OpenSSL_BN_mod_exp_simple
{	meta:
		author = "Maxx"
		description = "OpenSSL BN_mod_exp_simple"
	strings:
		$c0 = { B8 98 02 00 00 E8 ?? ?? ?? ?? 8B 84 24 A4 02 00 00 55 56 33 ED 50 89 6C 24 1C 89 6C 24 18 E8 ?? ?? ?? ?? 8B F0 83 C4 04 3B F5 89 74 24 0C 75 1B 8B 8C 24 A4 02 00 00 6A 01 51 E8 ?? ?? ?? ?? 83 C4 08 5E 5D 81 C4 98 02 00 00 C3 53 57 8B BC 24 BC 02 00 00 57 E8 ?? ?? ?? ?? 57 E8 ?? ?? ?? ?? 8B D8 83 C4 08 3B DD 0F 84 71 02 00 00 8D 54 24 28 52 E8 ?? ?? ?? ?? 8B AC 24 BC 02 00 00 8B 84 24 B4 02 00 00 57 55 8D 4C 24 34 50 51 C7 44 24 30 01 00 00 00 E8 ?? ?? ?? ?? 83 C4 14 85 C0 0F }
	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: Scheduled TLS Certificate Renewal via Let’s Encrypt CLI
  Description: The OpenSSL BN_mod_exp_simple function is used during automated certificate renewal by certbot or acme.sh.
  Filter/Exclusion: Check for process names like certbot, acme.sh, or command-line arguments containing renew or certificates.

• Scenario: System-wide OpenSSL Upgrade via Package Manager
  Description: During a system-wide OpenSSL upgrade using apt, yum, or zypper, the BN_mod_exp_simple function may be invoked as part of the upgrade process.
  Filter/Exclusion: Filter by process names like apt, yum, or zypper, or check for package names like openssl or openssl1.1.

• Scenario: Internal Key Management System Performing Batch Exponentiation
  Description: A custom internal key management tool (e.g., keymgmt-cli) may use OpenSSL for batch cryptographic operations, including BN_mod_exp_simple.
  Filter/Exclusion: Filter by process names matching the internal tool (e.g., keymgmt-cli) or check for known internal IP ranges or user accounts.

• Scenario: Automated Security Audit Tool Running OpenSSL Tests
  Description: Tools like OpenSSL itself or openssl-speed may run test cases that invoke BN_mod_exp_simple as part of benchmarking or validation.
  Filter/Exclusion: Filter by process names like openssl, openssl-speed, or check for command-line arguments containing test or bench.

• Scenario: Backup and Restore of SSL/TLS Configuration Files
  Description: During backup or restore operations, OpenSSL may be used to decrypt or re-encrypt configuration files, triggering BN_mod_exp_simple.
  Filter/Exclusion: Filter by process names like rsync, `