OpenSSL DSA

Hunt Hypothesis

The OpenSSL DSA rule detects potential adversary use of OpenSSL-based DSA key generation to establish secure communication channels, which may indicate covert data exfiltration or command and control activities. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate early-stage adversarial network activity that could evade traditional detection methods.

YARA Rule

rule OpenSSL_DSA
{
	meta:
		author="_pusher_"
		date="2016-08"
	strings:	
		$a0 = "bignum_data" wide ascii nocase
		$a1 = "DSA_METHOD" wide ascii nocase
		$a2 = "PDSA" wide ascii nocase
		$a3 = "dsa_mod_exp" wide ascii nocase
		$a4 = "bn_mod_exp" wide ascii nocase
		$a5 = "dsa_do_verify" wide ascii nocase
		$a6 = "dsa_sign_setup" wide ascii nocase
		$a7 = "dsa_do_sign" wide ascii nocase
		$a8 = "dsa_paramgen" wide ascii nocase
		$a9 = "BN_MONT_CTX" wide ascii nocase
	condition:
		7 of ($a*)
}

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 10 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: System update or patching using OpenSSL tools
  Description: A legitimate system update or patching process may involve OpenSSL tools that generate DSA keys as part of certificate management.
  Filter/Exclusion: Exclude processes related to yum, apt, or pacman (package managers), or filter by process.name containing update or patch.

• Scenario: Scheduled certificate rotation using OpenSSL
  Description: Automated certificate rotation tasks may use OpenSSL to generate new DSA keys, which could trigger the rule.
  Filter/Exclusion: Exclude processes with process.name containing certbot, openssl, or cron, and filter by process.args containing cert or rotate.

• Scenario: Admin task to generate DSA keys for internal use
  Description: System administrators may manually generate DSA keys for internal purposes, such as internal authentication or key management.
  Filter/Exclusion: Exclude processes with user.name set to root or admin, or filter by process.args containing genpkey or genrsa.

• Scenario: Integration with third-party security tools using OpenSSL
  Description: Some third-party security tools or APIs may use OpenSSL to handle cryptographic operations, including DSA key generation.
  Filter/Exclusion: Exclude processes with process.name containing splunk, logstash, or siem, or filter by process.args containing api or integration.

• Scenario: Development environment using OpenSSL for testing
  Description: Developers may use OpenSSL in a test environment to generate DSA keys for testing cryptographic functions or protocols.
  Filter/Exclusion: Exclude processes with process.name containing dev, test, or docker, or filter by process.args containing test or `