The research paper[1] has more details. AIUI, knowing the client certificate's private key allows the attacker to calculate the TLS master secret if the server

The research paper[1] has more details. AIUI, knowing the client certificate's private key allows the attacker to calculate the TLS master secret if the server supports a non-ephemeral (EC)DH cipher and the attacker has access to a valid certificate (with certain properties) for the site (not the private key, just the certificate, which can be obtained by visiting the site, from Certificate Transparency logs or other sources). [1]: https://www.usenix.org/system/files/conference/woot15/woot15-paper-hlauschek.pdf https://www.usenix.org/system/files/conference/woot15/woot15...