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OpenConnect Protocol Documentation

Overview​

This section contains comprehensive documentation of the OpenConnect VPN Protocol, which is a reverse-engineered implementation of Cisco's Secure Socket Layer (SSL) VPN protocol, also known as the Cisco SSL Tunnel Protocol or AnyConnect Protocol.

About the Protocol​

The OpenConnect protocol is used by Cisco AnyConnect/Secure Client to establish secure VPN connections. Through extensive reverse engineering and analysis of Cisco Secure Client 5.x binaries, we've documented the protocol specifications, cryptographic implementations, and authentication mechanisms.

What You'll Find Here​

📡 Protocol Specifications​

  • Cryptographic Standards: TLS 1.2/1.3, DTLS 1.2/1.3, cipher suites
  • Authentication Methods: Username/password, certificates, OTP, SAML
  • Certificate Management: PKI infrastructure, certificate pinning
  • Network Virtual Management (NVM): Telemetry and monitoring

🔍 Reverse Engineering​

  • Binary Analysis: Decompilation tools and methodologies (Ghidra, Reko, angr)
  • Analysis Workflow: Step-by-step reverse engineering process
  • Findings: Security vulnerabilities and implementation insights
  • Best Practices: Reverse engineering for interoperability

📚 Protocol Reference​

  • RFC Draft: Proposed standardization of OpenConnect protocol
  • Version Differences: Cisco Secure Client 5.1.2 vs 5.1.12 vs 5.x+
  • Version Summaries: Comprehensive analysis of protocol changes
  • Compatibility Guide: Cross-version interoperability

Purpose​

This documentation serves multiple purposes:

  1. Interoperability: Enable open-source implementations (wolfguard, openconnect)
  2. Security Research: Document security mechanisms and potential vulnerabilities
  3. Protocol Standardization: Provide basis for RFC submission
  4. Education: Help developers understand modern VPN protocols

Legal & Ethical Notice​

This documentation is produced through clean-room reverse engineering for interoperability purposes only, which is legally protected in most jurisdictions. We:

  • ✅ DO NOT distribute or modify Cisco binaries
  • ✅ DO NOT circumvent security mechanisms
  • ✅ DO document publicly observable protocol behavior
  • ✅ DO follow responsible disclosure for vulnerabilities

Contributing​

Found inaccuracies or have additional protocol insights? See our Contributing Guide for how to submit improvements.

Status: Active Development Protocol Version: Cisco Secure Client 5.1.12+ Last Updated: 2025-10-29