diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 06860e2..6295b18 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -492,6 +492,7 @@
 #define MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_EXT      110  /* 0x6E */
 #define MBEDTLS_SSL_ALERT_MSG_UNRECOGNIZED_NAME    112  /* 0x70 */
 #define MBEDTLS_SSL_ALERT_MSG_UNKNOWN_PSK_IDENTITY 115  /* 0x73 */
+#define MBEDTLS_SSL_ALERT_MSG_CERT_REQUIRED        116  /* 0x74 */
 #define MBEDTLS_SSL_ALERT_MSG_NO_APPLICATION_PROTOCOL 120 /* 0x78 */
 
 #define MBEDTLS_SSL_HS_HELLO_REQUEST            0
diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 9fcb2b2..9912d6c 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -509,6 +509,12 @@
     uint8_t sni_authmode;               /*!< authmode from SNI callback     */
 #endif
 
+#if defined(MBEDTLS_SSL_SRV_C)
+    /* Flag indicating if a CertificateRequest message has been sent
+     * to the client or not. */
+    uint8_t certificate_request_sent;
+#endif /* MBEDTLS_SSL_SRV_C */
+
 #if defined(MBEDTLS_SSL_SESSION_TICKETS)
     uint8_t new_session_ticket;         /*!< use NewSessionTicket?    */
 #endif /* MBEDTLS_SSL_SESSION_TICKETS */
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index e69fd7b..239be53 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -367,14 +367,10 @@
 
 /*
  *
- * STATE HANDLING: Incoming Certificate, client-side only currently.
+ * STATE HANDLING: Incoming Certificate.
  *
  */
 
-/*
- * Implementation
- */
-
 #if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)
 #if defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
 /*
@@ -439,6 +435,13 @@
         mbedtls_free( ssl->session_negotiate->peer_cert );
     }
 
+    if( certificate_list_len == 0 )
+    {
+        ssl->session_negotiate->peer_cert = NULL;
+        ret = 0;
+        goto exit;
+    }
+
     if( ( ssl->session_negotiate->peer_cert =
           mbedtls_calloc( 1, sizeof( mbedtls_x509_crt ) ) ) == NULL )
     {
@@ -515,6 +518,7 @@
         p += extensions_len;
     }
 
+exit:
     /* Check that all the message is consumed. */
     if( p != end )
     {
@@ -547,10 +551,58 @@
 static int ssl_tls13_validate_certificate( mbedtls_ssl_context *ssl )
 {
     int ret = 0;
+    int authmode = MBEDTLS_SSL_VERIFY_REQUIRED;
     mbedtls_x509_crt *ca_chain;
     mbedtls_x509_crl *ca_crl;
     uint32_t verify_result = 0;
 
+    /* If SNI was used, overwrite authentication mode
+     * from the configuration. */
+#if defined(MBEDTLS_SSL_SRV_C)
+    if( ssl->conf->endpoint == MBEDTLS_SSL_IS_SERVER )
+        authmode = ssl->conf->authmode;
+#endif
+
+    /*
+     * If the peer hasn't sent a certificate ( i.e. it sent
+     * an empty certificate chain ), this is reflected in the peer CRT
+     * structure being unset.
+     * Check for that and handle it depending on the
+     * authentication mode.
+     */
+    if( ssl->session_negotiate->peer_cert == NULL )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "peer has not sent a certificate" ) );
+
+#if defined(MBEDTLS_SSL_SRV_C)
+        if( ssl->conf->endpoint == MBEDTLS_SSL_IS_SERVER )
+        {
+            /* The client was asked for a certificate but didn't send
+             * one. The client should know what's going on, so we
+             * don't send an alert.
+             */
+            ssl->session_negotiate->verify_result = MBEDTLS_X509_BADCERT_MISSING;
+            if( authmode == MBEDTLS_SSL_VERIFY_OPTIONAL )
+                return( 0 );
+            else
+            {
+                MBEDTLS_SSL_PEND_FATAL_ALERT( MBEDTLS_SSL_ALERT_MSG_NO_CERT,
+                                              MBEDTLS_ERR_SSL_NO_CLIENT_CERTIFICATE  );
+                return( MBEDTLS_ERR_SSL_NO_CLIENT_CERTIFICATE );
+            }
+        }
+#endif /* MBEDTLS_SSL_SRV_C */
+
+#if defined(MBEDTLS_SSL_CLI_C)
+        if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
+        {
+            MBEDTLS_SSL_PEND_FATAL_ALERT( MBEDTLS_SSL_ALERT_MSG_NO_CERT,
+                                          MBEDTLS_ERR_SSL_FATAL_ALERT_MESSAGE );
+            return( MBEDTLS_ERR_SSL_FATAL_ALERT_MESSAGE );
+        }
+#endif /* MBEDTLS_SSL_CLI_C */
+    }
+
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
     if( ssl->handshake->sni_ca_chain != NULL )
     {
@@ -593,8 +645,20 @@
             ret = MBEDTLS_ERR_SSL_BAD_CERTIFICATE;
     }
 
+    /* mbedtls_x509_crt_verify_with_profile is supposed to report a
+     * verification failure through MBEDTLS_ERR_X509_CERT_VERIFY_FAILED,
+     * with details encoded in the verification flags. All other kinds
+     * of error codes, including those from the user provided f_vrfy
+     * functions, are treated as fatal and lead to a failure of
+     * ssl_tls13_parse_certificate even if verification was optional. */
+    if( authmode == MBEDTLS_SSL_VERIFY_OPTIONAL &&
+        ( ret == MBEDTLS_ERR_X509_CERT_VERIFY_FAILED ||
+          ret == MBEDTLS_ERR_SSL_BAD_CERTIFICATE ) )
+    {
+        ret = 0;
+    }
 
-    if( ca_chain == NULL )
+    if( ca_chain == NULL && authmode == MBEDTLS_SSL_VERIFY_REQUIRED )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "got no CA chain" ) );
         ret = MBEDTLS_ERR_SSL_CA_CHAIN_REQUIRED;
@@ -663,7 +727,8 @@
                           &buf, &buf_len ) );
 
     /* Parse the certificate chain sent by the peer. */
-    MBEDTLS_SSL_PROC_CHK( ssl_tls13_parse_certificate( ssl, buf, buf + buf_len ) );
+    MBEDTLS_SSL_PROC_CHK( ssl_tls13_parse_certificate( ssl, buf,
+                                                       buf + buf_len ) );
     /* Validate the certificate chain and set the verification results. */
     MBEDTLS_SSL_PROC_CHK( ssl_tls13_validate_certificate( ssl ) );
 
@@ -671,12 +736,9 @@
                                         buf, buf_len );
 
 cleanup:
+#endif /* MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED */
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= parse certificate" ) );
-#else
-    MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
-    ret = MBEDTLS_ERR_SSL_INTERNAL_ERROR;
-#endif /* MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED */
     return( ret );
 }
 #if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
diff --git a/library/ssl_tls13_server.c b/library/ssl_tls13_server.c
index 8c8eb2a..f3843b1 100644
--- a/library/ssl_tls13_server.c
+++ b/library/ssl_tls13_server.c
@@ -1342,6 +1342,8 @@
     if( authmode == MBEDTLS_SSL_VERIFY_NONE )
         return( SSL_CERTIFICATE_REQUEST_SKIP );
 
+    ssl->handshake->certificate_request_sent = 1;
+
     return( SSL_CERTIFICATE_REQUEST_SEND_REQUEST );
 }
 
@@ -1495,7 +1497,16 @@
                 MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE );
         return( ret );
     }
-    mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_CLIENT_FINISHED );
+
+    if( ssl->handshake->certificate_request_sent )
+    {
+        mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_CLIENT_CERTIFICATE );
+
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "Switch to handshake keys for inbound traffic" ) );
+        mbedtls_ssl_set_inbound_transform( ssl, ssl->handshake->transform_handshake );
+    }
+    else
+        mbedtls_ssl_handshake_set_state( ssl, MBEDTLS_SSL_CLIENT_FINISHED );
     return( 0 );
 }
 
@@ -1506,10 +1517,12 @@
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
-    MBEDTLS_SSL_DEBUG_MSG( 1,
-                  ( "Switch to handshake traffic keys for inbound traffic" ) );
-    mbedtls_ssl_set_inbound_transform( ssl, ssl->handshake->transform_handshake );
-
+    if( ! ssl->handshake->certificate_request_sent )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1,
+            ( "Switch to handshake traffic keys for inbound traffic" ) );
+        mbedtls_ssl_set_inbound_transform( ssl, ssl->handshake->transform_handshake );
+    }
     ret = mbedtls_ssl_tls13_process_finished_message( ssl );
     if( ret != 0 )
         return( ret );
@@ -1613,6 +1626,27 @@
             ret = ssl_tls13_handshake_wrapup( ssl );
             break;
 
+        case MBEDTLS_SSL_CLIENT_CERTIFICATE:
+            ret = mbedtls_ssl_tls13_process_certificate( ssl );
+            if( ret == 0 && ssl->session_negotiate->peer_cert != NULL )
+            {
+                mbedtls_ssl_handshake_set_state(
+                    ssl, MBEDTLS_SSL_CLIENT_CERTIFICATE_VERIFY );
+            }
+            else
+                mbedtls_ssl_handshake_set_state(
+                    ssl, MBEDTLS_SSL_CLIENT_FINISHED );
+            break;
+
+        case MBEDTLS_SSL_CLIENT_CERTIFICATE_VERIFY:
+            ret = mbedtls_ssl_tls13_process_certificate_verify( ssl );
+            if( ret == 0 )
+            {
+                mbedtls_ssl_handshake_set_state(
+                    ssl, MBEDTLS_SSL_CLIENT_FINISHED );
+            }
+            break;
+
         default:
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid state %d", ssl->state ) );
             return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 6eaa534..21d17f9 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -11241,10 +11241,6 @@
             -s "tls13 server state: MBEDTLS_SSL_CLIENT_FINISHED" \
             -s "tls13 server state: MBEDTLS_SSL_HANDSHAKE_WRAPUP"
 
-# Skip this test before openssl exit code issue fixed
-# On fail, openssl return different exit code on OpenCI and internal CI for
-# this test.
-skip_next_test
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
 requires_config_enabled MBEDTLS_DEBUG_C
 requires_config_enabled MBEDTLS_SSL_SRV_C
@@ -11252,7 +11248,7 @@
 run_test    "TLS 1.3: Server side check - openssl with client authentication" \
             "$P_SRV debug_level=4 auth_mode=required crt_file=data_files/server5.crt key_file=data_files/server5.key force_version=tls13 tickets=0" \
             "$O_NEXT_CLI -msg -debug -cert data_files/server5.crt -key data_files/server5.key -tls1_3 -no_middlebox" \
-            1 \
+            0 \
             -s "tls13 server state: MBEDTLS_SSL_CLIENT_HELLO" \
             -s "tls13 server state: MBEDTLS_SSL_SERVER_HELLO" \
             -s "tls13 server state: MBEDTLS_SSL_ENCRYPTED_EXTENSIONS" \
@@ -11291,7 +11287,7 @@
 run_test    "TLS 1.3: Server side check - gnutls with client authentication" \
             "$P_SRV debug_level=4 auth_mode=required crt_file=data_files/server5.crt key_file=data_files/server5.key force_version=tls13 tickets=0" \
             "$G_NEXT_CLI localhost -d 4 --x509certfile data_files/server5.crt --x509keyfile data_files/server5.key --priority=NORMAL:-VERS-ALL:+VERS-TLS1.3:%NO_TICKETS:%DISABLE_TLS13_COMPAT_MODE -V" \
-            1 \
+            0 \
             -s "tls13 server state: MBEDTLS_SSL_CLIENT_HELLO" \
             -s "tls13 server state: MBEDTLS_SSL_SERVER_HELLO" \
             -s "tls13 server state: MBEDTLS_SSL_ENCRYPTED_EXTENSIONS" \
@@ -11329,7 +11325,7 @@
 run_test    "TLS 1.3: Server side check - mbedtls with client authentication" \
             "$P_SRV debug_level=4 auth_mode=required crt_file=data_files/server5.crt key_file=data_files/server5.key force_version=tls13 tickets=0" \
             "$P_CLI debug_level=4 crt_file=data_files/server5.crt key_file=data_files/server5.key force_version=tls13" \
-            1 \
+            0 \
             -s "tls13 server state: MBEDTLS_SSL_CLIENT_HELLO" \
             -s "tls13 server state: MBEDTLS_SSL_SERVER_HELLO" \
             -s "tls13 server state: MBEDTLS_SSL_ENCRYPTED_EXTENSIONS" \
@@ -11339,6 +11335,40 @@
             -s "=> parse client hello" \
             -s "<= parse client hello"
 
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_config_enabled MBEDTLS_DEBUG_C
+requires_config_enabled MBEDTLS_SSL_SRV_C
+requires_config_enabled MBEDTLS_SSL_CLI_C
+run_test    "TLS 1.3: Server side check - mbedtls with client empty certificate" \
+            "$P_SRV debug_level=4 auth_mode=required crt_file=data_files/server5.crt key_file=data_files/server5.key force_version=tls13 tickets=0" \
+            "$P_CLI debug_level=4 crt_file=none key_file=none force_version=tls13" \
+            1 \
+            -s "tls13 server state: MBEDTLS_SSL_CLIENT_HELLO" \
+            -s "tls13 server state: MBEDTLS_SSL_SERVER_HELLO" \
+            -s "tls13 server state: MBEDTLS_SSL_ENCRYPTED_EXTENSIONS" \
+            -s "tls13 server state: MBEDTLS_SSL_SERVER_CERTIFICATE" \
+            -s "=> write certificate request" \
+            -s "SSL - No client certification received from the client, but required by the authentication mode" \
+            -c "client state: MBEDTLS_SSL_CERTIFICATE_REQUEST" \
+            -s "=> parse client hello" \
+            -s "<= parse client hello"
+
+requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_3
+requires_config_enabled MBEDTLS_DEBUG_C
+requires_config_enabled MBEDTLS_SSL_SRV_C
+requires_config_enabled MBEDTLS_SSL_CLI_C
+run_test    "TLS 1.3: Server side check - mbedtls with optional client authentication" \
+            "$P_SRV debug_level=4 auth_mode=optional crt_file=data_files/server5.crt key_file=data_files/server5.key force_version=tls13 tickets=0" \
+            "$P_CLI debug_level=4 force_version=tls13 crt_file=none key_file=none" \
+            0 \
+            -s "tls13 server state: MBEDTLS_SSL_CLIENT_HELLO" \
+            -s "tls13 server state: MBEDTLS_SSL_SERVER_HELLO" \
+            -s "tls13 server state: MBEDTLS_SSL_ENCRYPTED_EXTENSIONS" \
+            -s "tls13 server state: MBEDTLS_SSL_SERVER_CERTIFICATE" \
+            -s "=> write certificate request" \
+            -c "client state: MBEDTLS_SSL_CERTIFICATE_REQUEST" \
+            -s "=> parse client hello" \
+            -s "<= parse client hello"
 
 requires_config_enabled MBEDTLS_DEBUG_C
 requires_config_enabled MBEDTLS_SSL_CLI_C