DTLS 1.2 Client ChangeCipherSpec Handling Flaw

Executive Summary

This vulnerability involves a (D)TLS 1.2 client accepting a ChangeCipherSpec message before sending its ClientKeyExchange. Without a master secret derived, the client installs read keys from a known key and checks the server's Finished message against it. An attacker can exploit out-of-order ChangeCipherSpec to complete the handshake and send data the client accepts as authentic. The client's own traffic remains encrypted, so the attacker cannot read it, and the real server never completes the handshake.

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Detection Guidance

Detecting this vulnerability requires checking if your wolfSSL version is vulnerable and monitoring for unusual TLS handshake patterns. Use 'wolfssl --version' to check the installed version. If it is below 5.9.4, the system is likely vulnerable. Inspect network traffic for TLS handshakes where ChangeCipherSpec (CCS) messages appear before ClientKeyExchange (CKE). Tools like Wireshark can analyze TLS handshake sequences for anomalies.

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Impact Analysis

An attacker in a man-in-the-middle position or any fake server for PSK connections could complete the handshake and send malicious data to the client, which the client would accept as authentic. This could lead to unauthorized actions or data manipulation, though the attacker cannot decrypt the client's traffic.

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Compliance Impact

This vulnerability could impact compliance with GDPR and HIPAA by enabling man-in-the-middle attacks that bypass authentication, potentially exposing sensitive data during TLS handshakes. Weak session handling and premature encryption may violate encryption and integrity requirements in these regulations.

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Mitigation Strategies

Update wolfSSL to version 5.9.4 or later to address the vulnerability by enforcing stricter validation of ChangeCipherSpec and ClientKeyExchange sequencing.

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