SNI: added restriction for TLSv1.3 cross-SNI session resumption

Signed-off-by: Nic <[email protected]>

Author: nic-6443

patch/1.21.4/nginx-sni_restriction.patch

@@ -0,0 +1,72 @@
+From 823e886851263a8ce84fd22aeead4c3aa819bce1 Mon Sep 17 00:00:00 2001
+From: Sergey Kandaurov <[email protected]>
+Date: Wed, 22 Jan 2025 18:55:44 +0400
+Subject: [PATCH] SNI: added restriction for TLSv1.3 cross-SNI session
+ resumption.
+
+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 b720f650b insufficient for
+sessions resumed with different SNI server name.
+
+Considering the example from b720f650b, 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.
+---
+ src/http/ngx_http_request.c | 27 +++++++++++++++++++++++++--
+ 1 file changed, 25 insertions(+), 2 deletions(-)
+
+diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
+index 013b7158e..d5ac3d415 100644
+--- a/src/http/ngx_http_request.c
++++ b/src/http/ngx_http_request.c
+@@ -909,6 +909,31 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
+         goto done;
+     }
+ 
++    sscf = ngx_http_get_module_srv_conf(cscf->ctx, ngx_http_ssl_module);
++
++#if (defined TLS1_3_VERSION                                                   \
++     && !defined LIBRESSL_VERSION_NUMBER && !defined OPENSSL_IS_BORINGSSL)
++
++    /*
++     * SSL_SESSION_get0_hostname() is only available in OpenSSL 1.1.1+,
++     * but servername being negotiated in every TLSv1.3 handshake
++     * is only returned in OpenSSL 1.1.1+ as well
++     */
++
++    if (sscf->verify) {
++        const char  *hostname;
++
++        hostname = SSL_SESSION_get0_hostname(SSL_get0_session(ssl_conn));
++
++        if (hostname != NULL && ngx_strcmp(hostname, servername) != 0) {
++            c->ssl->handshake_rejected = 1;
++            *ad = SSL_AD_ACCESS_DENIED;
++            return SSL_TLSEXT_ERR_ALERT_FATAL;
++        }
++    }
++
++#endif
++
+     hc->ssl_servername = ngx_palloc(c->pool, sizeof(ngx_str_t));
+     if (hc->ssl_servername == NULL) {
+         goto error;
+@@ -922,8 +947,6 @@ ngx_http_ssl_servername(ngx_ssl_conn_t *ssl_conn, int *ad, void *arg)
+ 
+     ngx_set_connection_log(c, clcf->error_log);
+ 
+-    sscf = ngx_http_get_module_srv_conf(hc->conf_ctx, ngx_http_ssl_module);
+-
+     c->ssl->buffer_size = sscf->buffer_size;
+ 
+     if (sscf->ssl.ctx) {
+-- 
+2.43.0
+