GenerateTLSClientHelloPacket Test

Hunt Hypothesis

This detection identifies anomalous TLS Client Hello packet generation patterns that may indicate early-stage reconnaissance or the use of custom encryption libraries by an adversary. Proactively hunting for these signatures in Azure Sentinel allows the SOC team to distinguish legitimate traffic from stealthy command-and-control communications before they evolve into high-severity incidents.

YARA Rule

rule GenerateTLSClientHelloPacket_Test: sharedcode
{
	meta:
		copyright = "2015 Novetta Solutions"
		author = "Novetta Threat Research & Interdiction Group - trig@novetta.com"
		Source = "eff542ac8e37db48821cb4e5a7d95c044fff27557763de3a891b40ebeb52cc55.ex_"

	strings:
	/*
		25 07 00 00 80  and     eax, 80000007h
		79 05           jns     short loc_405EC8; um, nope.. this will always happen
		48              dec     eax
		83 C8 F8        or      eax, 0FFFFFFF8h
		40              inc     eax
	*/

	$a = {25 07 00 00 80 79 ?? 4? 	83 ?? F8 4? }

	condition:
		$a in ((pe.sections[pe.section_index(".text")].raw_data_offset)..(pe.sections[pe.section_index(".text")].raw_data_offset + pe.sections[pe.section_index(".text")].raw_data_size))
}

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 5 specific false positive scenarios for the GenerateTLSClientHelloPacket_Test detection rule in an enterprise environment, including suggested filters or exclusions:

• Endpoint Management Agent Heartbeats

  • Scenario: Enterprise endpoint management tools like Microsoft Intune, Jamf Pro, or CrowdStrike Falcon frequently initiate TLS handshakes to cloud control planes every 5–15 minutes. These agents often generate ClientHello packets with unique extensions (e.g., specific ALPN protocols or custom SNI values) that may not match the standard baseline expected by the test rule, triggering alerts for “anomalous” handshake behavior.
  • Filter/Exclusion: Create an exclusion based on the Source IP range of known management servers and agents, combined with a filter on the User-Agent string or specific TLS SNI (Server Name Indication) values matching the vendor’s domain (e.g., *.microsoft.com, *.crowdstrike.com).

• Scheduled Software Update Scanners

  • Scenario: Automated patch management systems such as WSUS, Ivanti, or Chocolatey run scheduled jobs during off-hours to check for updates. These tools often perform “test” connections to multiple upstream repositories simultaneously, generating a burst of ClientHello packets that may lack the full session context (e.g., missing subsequent Application Data) if the rule is looking for complete sessions rather than initial probes.
  • Filter/Exclusion: Implement a time-based exclusion window (e.g., 02:00 – 06:00 local time) and exclude traffic originating from specific Service Accounts or hostnames identified as update servers, focusing the rule only on interactive user sessions during business hours.

• Internal API Gateway Health Checks

  • Scenario: Internal microservices architectures using