SNI: added restriction for TLSv1.3 cross-SNI session resumption. #493
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In OpenSSL, session resumption always happens in the default SSL context, prior to invoking the SNI callback. Further, unlike in TLSv1.2 and older protocols, SSL_get_servername() returns values received in the resumption handshake, which may be different from the value in the initial handshake. Notably, this makes the restriction added in b720f65 insufficient for sessions resumed with different SNI server name. Considering the example from b720f65, previously, a client was able to request example.org by presenting a certificate for example.org, then to resume and request example.com. The fix is to reject handshakes resumed with a different server name, if verification of client certificates is enabled in a corresponding server configuration.
arut
approved these changes
Feb 5, 2025
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In OpenSSL, session resumption always happens in the default SSL context, prior to invoking the SNI callback. Further, unlike in TLSv1.2 and older protocols, SSL_get_servername() returns values received in the resumption handshake, which may be different from the value in the initial handshake. Notably, this makes the restriction added in b720f65 insufficient for sessions resumed with different SNI server name.
Considering the example from b720f65, previously, a client was able to request example.org by presenting a certificate for example.org, then to resume and request example.com.
The fix is to reject handshakes resumed with a different server name, if verification of client certificates is enabled in a corresponding server configuration.
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