OpenVPN is Open to VPN Fingerprinting (2403.03998v1)

Abstract: VPN adoption has seen steady growth over the past decade due to increased public awareness of privacy and surveillance threats. In response, certain governments are attempting to restrict VPN access by identifying connections using "dual use" DPI technology. To investigate the potential for VPN blocking, we develop mechanisms for accurately fingerprinting connections using OpenVPN, the most popular protocol for commercial VPN services. We identify three fingerprints based on protocol features such as byte pattern, packet size, and server response. Playing the role of an attacker who controls the network, we design a two-phase framework that performs passive fingerprinting and active probing in sequence. We evaluate our framework in partnership with a million-user ISP and find that we identify over 85% of OpenVPN flows with only negligible false positives, suggesting that OpenVPN-based services can be effectively blocked with little collateral damage. Although some commercial VPNs implement countermeasures to avoid detection, our framework successfully identified connections to 34 out of 41 "obfuscated" VPN configurations. We discuss the implications of the VPN fingerprintability for different threat models and propose short-term defenses. In the longer term, we urge commercial VPN providers to be more transparent about their obfuscation approaches and to adopt more principled detection countermeasures, such as those developed in censorship circumvention research.

• The paper introduces a robust two-phase system combining passive filtering and active probing that identifies over 85% of vanilla and 67% of obfuscated OpenVPN flows.
• It details how specific handshake opcodes, ACK sequences, and server responses to malformed packets enable effective fingerprinting using over 15TB of real-world ISP traffic.
• The research warns that current obfuscation methods are inadequate, urging VPN providers to adopt advanced random padding and standardized techniques to improve anonymity.

An Analysis of OpenVPN Fingerprinting Vulnerabilities

The research paper titled "OpenVPN is Open to VPN Fingerprinting" provides a comprehensive investigation into the susceptibility of OpenVPN, a prevalent protocol for commercial VPN services, to fingerprinting attacks. The paper is motivated by the increasing efforts of ISPs and governments to track or block VPN traffic as a means of maintaining control over data flows within their networks, utilizing deep packet inspection (DPI) technologies.

Key Contributions and Findings

This paper proposes a robust, two-phase system for identifying OpenVPN traffic through a combination of passive filtering and active probing. The analysis considers several real-world scenarios, leveraging insights into protocol features that are potentially susceptible to fingerprinting. The researchers focus on the following key aspects:

1. Fingerprinting Mechanisms: The authors identify three features for fingerprinting OpenVPN traffic: opcode patterns in the handshake messages, distinct ACK packet sequences during initial communication, and server behavior when faced with malformed packets.
2. Infrastructure and Methodology: The paper leverages a collaboration with Merit, a regional ISP, to deploy their framework on a server monitoring 20% of the ISP's traffic. This deployment ramps up to 15 TB of traffic processed per day, scrutinizing over 2 billion flows for potential OpenVPN connections.
3. Results in Control and Real-world Traffic: The system successfully identifies over 85% of vanilla OpenVPN flows and more than two-thirds of tested obfuscated configurations. The latter highlights the inadequacies of many current obfuscation methods in achieving unobservability, defying some providers' claims of being undetectable.
4. Sources of Vulnerability: Many "obfuscation" techniques such as XOR patches, tunnel-based methods without proper padding, and co-location with non-obfuscated servers render these VPN services vulnerable. For instance, vanishing patterns in byte-level packet attributes and packet size distributions are effectively exploited for detection.
5. Active Probing Success: The active probing phase proves particularly effective at confirming OpenVPN servers, even those that use HMAC protections to thwart unsolicited packets.

Implications and Future Directions

Practical Implications: The paper demonstrates that identifying OpenVPN traffic can be achieved with minimal false positives, making it feasible for censoring authorities to block or throttle VPNs efficiently. It raises concerns about the reliability of current obfuscation techniques advertised by commercial VPN providers.

Theoretical Implications: The paper underlines the broader challenge in designing camouflaged network protocols. The fingerprinting vulnerabilities identified here, despite ongoing obfuscation attempts, reveal the necessity for more advanced techniques in maintaining the anonymity and usability of VPNs in censorious environments.

Recommendations for Mitigation: The authors suggest short-term safeguards for VPN providers, including server isolation practices and the use of random padding. In the long term, they advocate for the adoption of standardized obfuscation technologies, akin to strategies used in other circumvention tools such as Tor.

Conclusion

The research provides poignant insights into the open vulnerabilities within VPN ecosystems, particularly focusing on OpenVPN's susceptibility to DPI techniques. This paper serves as a catalyst for further research and development in obfuscation methodologies and highlights the necessity for VPNs to evolve continuously in the face of increasingly sophisticated network adversaries. The call to action for VPN providers is clear: to develop more resilient, transparent, and standardized protections against monitoring and censorship attempts.