Unveiling OpenVPN: Fingerprinting with ACK Patterns and Timing Analysis

:::info Authors:

(1) Diwen Xue, University of Michigan;

(2) Reethika Ramesh, University of Michigan;

(3) Arham Jain, University of Michigan;

(4) Arham Jain, Merit Network, Inc.;

(5) J. Alex Halderman, University of Michigan;

(6) Jedidiah R. Crandall, Arizona State University/Breakpointing Bad;

(7) Roya Ensaf, University of Michigan.

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Abstract and 1 Introduction

2 Background & Related Work

3 Challenges in Real-world VPN Detection

4 Adversary Model and Deployment

5 Ethics, Privacy, and Responsible Disclosure

6 Identifying Fingerprintable Features and 6.1 Opcode-based Fingerprinting

6.2 ACK-based Fingerprinting

6.3 Active Server Fingerprinting

6.4 Constructing Filters and Probers

7 Fine-tuning for Deployment and 7.1 ACK Fingerprint Thresholds

7.2 Choice of Observation Window N

7.3 Effects of Packet Loss

7.4 Server Churn for Asynchronous Probing

7.5 Probe UDP and Obfuscated OpenVPN Servers

8 Real-world Deployment Setup

9 Evaluation & Findings and 9.1 Results for control VPN flows

9.2 Results for all flows

10 Discussion and Mitigations

11 Conclusion

12 Acknowledgement and References

Appendix

OpenVPN engages in a TLS-style handshake with its peer over the control channel. Since TLS is designed to operate over a reliable layer, OpenVPN implements an explicit acknowledgement and re-transmission mechanism for its control channel messages [30]. Specifically, incoming P- Control packets are acknowledged by PACK packets, which do not carry any TLS payloads and are uniform in size (Note these ACK packets are carried over by TCP as payload and are not the same as TCP ACK flags). Moreover, these ACK packets are seen mostly only in the early stage of a flow, during the handshake phase, and are not used in the actual data transfer channel, which can run over an unreliable layer.

\ To our knowledge, we are the first to devise fingerprinting attempts based on the distinct protocol-layer ACKs against OpenVPN. Previously, the unique timing pattern in meek’s TCP-level ACK traffic has rendered the obfuscation tool vulnerable to detection [67]. For OpenVPN, the presence of explicit ACK packets, uniform in size and only seen in some parts of a session, provides another fingerprintable feature. Specifically, we first identify a likely ACK packet of a session by locating an initial packet exchange sequence of C- >S (Client-Reset), S->C (Server-Reset), C->S (ACK), C->S (Control), as illustrated in Figure 1. For vanilla OpenVPN and XOR-based obfuscation, the first ACK packet usually appears as the third (data) packet transmitted in a session. For tunnels or obfuscators that have their own handshake or key exchange process (e.g., Stunnel, SSH tunnel, or Obfsproxy), this counting is offset by the number of tunnel handshake packets. Next, we group packets into 10-packet bins, and we derive the ACK fingerprint for each flow by counting the number of packets in each bin that have the same size as the identified ACK packet. For OpenVPN flows, we expect to observe a high number of ACK packets in early bins and an absence of them in later bins. (Later in the session, Control and ACK packets can be exchanged again to transfer random key materials, but it is not expected to be observed within our observation window N.) This approach proves effective to fingerprint vanilla OpenVPN as well as obfuscated services running over encrypted tunnels that lack random padding. We quantify exact fingerprinting thresholds in Section 7.1.

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:::info This paper is available on arxiv under CC BY 4.0 DEED license.

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