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I know in TLS, the client would send a CertificateVerify message for the server to confirm the client's identity through means such as CA but what if the client never sent this information?

Is it possible for an attacker to use this opportunity to hijack the client's session through packet sniffing and create it's own "pre-master secret" to communicate with the server?

schroeder
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Spencer Chow
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  • Hijack the session? What session? This is all still in the handshake. And how would the client certificate be verified? What certificate would the attacker use? – schroeder Jul 07 '20 at 19:57
  • Probable duplicate: https://security.stackexchange.com/questions/138411/how-does-a-server-validate-the-certificate-verify-message-in-ssl-tls – schroeder Jul 07 '20 at 19:57
  • This question is asked based on a bunch of misunderstandings, which is going to make it difficult to answer. Sniffing alone is not enough to hijack a session. Injecting packets when one party is slow won't work. Client certificates are not used to establish the secret. – schroeder Jul 08 '20 at 07:33

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Client certificates are not involved in the creation of the pre-master secret in the first place. This means your threat scenario is not possible. It is also pretty common that no client certificates are used at all in the TLS handshake but only a server certificate, i.e. in many cases no CertificateVerify is even used within the TLS handshake.

Steffen Ullrich
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