diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 24fc3fd22..f53193556 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1110,6 +1110,8 @@ static int ssl_parse_max_fragment_length_ext( mbedtls_ssl_context *ssl,
         len != 1 ||
         buf[0] != ssl->conf->mfl_code )
     {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching max fragment length extension" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1125,6 +1127,8 @@ static int ssl_parse_truncated_hmac_ext( mbedtls_ssl_context *ssl,
     if( ssl->conf->trunc_hmac == MBEDTLS_SSL_TRUNC_HMAC_DISABLED ||
         len != 0 )
     {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching truncated HMAC extension" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1145,6 +1149,8 @@ static int ssl_parse_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
         ssl->minor_ver == MBEDTLS_SSL_MINOR_VERSION_0 ||
         len != 0 )
     {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching encrypt-then-MAC extension" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1165,6 +1171,8 @@ static int ssl_parse_extended_ms_ext( mbedtls_ssl_context *ssl,
         ssl->minor_ver == MBEDTLS_SSL_MINOR_VERSION_0 ||
         len != 0 )
     {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching extended master secret extension" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1184,6 +1192,8 @@ static int ssl_parse_session_ticket_ext( mbedtls_ssl_context *ssl,
     if( ssl->conf->session_tickets == MBEDTLS_SSL_SESSION_TICKETS_DISABLED ||
         len != 0 )
     {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching session ticket extension" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1208,6 +1218,8 @@ static int ssl_parse_supported_point_formats_ext( mbedtls_ssl_context *ssl,
     if( list_size + 1 != len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1232,6 +1244,7 @@ static int ssl_parse_supported_point_formats_ext( mbedtls_ssl_context *ssl,
     }
 
     MBEDTLS_SSL_DEBUG_MSG( 1, ( "no point format in common" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
     return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
 }
 #endif /* MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C || 
@@ -1260,6 +1273,8 @@ static int ssl_parse_ecjpake_kkpp( mbedtls_ssl_context *ssl,
                                                 buf, len ) ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecjpake_read_round_one", ret );
+        if( ( ret = mbedtls_ssl_send_fatal_handshake_failure( ssl ) ) != 0 )
+            return( ret );
         return( ret );
     }
 
@@ -1276,7 +1291,11 @@ static int ssl_parse_alpn_ext( mbedtls_ssl_context *ssl,
 
     /* If we didn't send it, the server shouldn't send it */
     if( ssl->conf->alpn_list == NULL )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "non-matching ALPN extension" ) );
+        mbedtls_ssl_send_fatal_handshake_failure( ssl );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
 
     /*
      * opaque ProtocolName<1..2^8-1>;
@@ -1290,15 +1309,27 @@ static int ssl_parse_alpn_ext( mbedtls_ssl_context *ssl,
 
     /* Min length is 2 (list_len) + 1 (name_len) + 1 (name) */
     if( len < 4 )
+    {
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
 
     list_len = ( buf[0] << 8 ) | buf[1];
     if( list_len != len - 2 )
+    {
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
 
     name_len = buf[2];
     if( name_len != list_len - 1 )
+    {
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    }
 
     /* Check that the server chosen protocol was in our list and save it */
     for( p = ssl->conf->alpn_list; *p != NULL; p++ )
@@ -1311,6 +1342,8 @@ static int ssl_parse_alpn_ext( mbedtls_ssl_context *ssl,
         }
     }
 
+    MBEDTLS_SSL_DEBUG_MSG( 1, ( "ALPN extension: no matching protocol" ) );
+    mbedtls_ssl_send_fatal_handshake_failure( ssl );
     return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
 }
 #endif /* MBEDTLS_SSL_ALPN */
@@ -1415,6 +1448,7 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
 
     if( ( ret = mbedtls_ssl_read_record( ssl ) ) != 0 )
     {
+        /* No alert on a read error. */
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_read_record", ret );
         return( ret );
     }
@@ -1440,6 +1474,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
 #endif /* MBEDTLS_SSL_RENEGOTIATION */
 
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -1466,6 +1502,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
         buf[0] != MBEDTLS_SSL_HS_SERVER_HELLO )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1520,6 +1558,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
     if( n > 32 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1532,6 +1572,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
             ssl->in_hslen != mbedtls_ssl_hs_hdr_len( ssl ) + 40 + n + ext_len )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
         }
     }
@@ -1542,6 +1584,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
     else
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1569,6 +1613,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
 #endif/* MBEDTLS_ZLIB_SUPPORT */
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "server hello, bad compression: %d", comp ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE );
     }
 
@@ -1580,6 +1626,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
     if( ssl->transform_negotiate->ciphersuite_info == NULL )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "ciphersuite info for %04x not found", i ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
     }
 
@@ -1617,6 +1665,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
         if( ( ret = mbedtls_ssl_derive_keys( ssl ) ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_derive_keys", ret );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
             return( ret );
         }
     }
@@ -1636,6 +1686,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
         )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
 
@@ -1647,6 +1699,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
         if( ssl->conf->ciphersuite_list[ssl->minor_ver][i] == 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
         }
 
@@ -1664,6 +1718,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
       )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
     }
     ssl->session_negotiate->compression = comp;
@@ -1682,6 +1738,8 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
         if( ext_size + 4 > ext_len )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
         }
 
@@ -2218,6 +2276,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ( ret = ssl_get_ecdh_params_from_cert( ssl ) ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "ssl_get_ecdh_params_from_cert", ret );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
             return( ret );
         }
 
@@ -2239,6 +2299,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -2256,6 +2318,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         }
 
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -2272,6 +2336,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ssl_parse_server_psk_hint( ssl, &p, end ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
         }
     } /* FALLTROUGH */
@@ -2293,6 +2359,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ssl_parse_server_dh_params( ssl, &p, end ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
         }
     }
@@ -2309,6 +2377,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ssl_parse_server_ecdh_params( ssl, &p, end ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
         }
     }
@@ -2324,6 +2394,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ret != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecjpake_read_round_two", ret );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
         }
     }
@@ -2358,12 +2430,16 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
                                                &md_alg, &pk_alg ) != 0 )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
                 return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
             }
 
             if( pk_alg != mbedtls_ssl_get_ciphersuite_sig_pk_alg( ciphersuite_info ) )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
                 return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
             }
         }
@@ -2395,6 +2471,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( end != p + sig_len )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
         }
 
@@ -2466,6 +2544,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
                                      mbedtls_md_info_from_type( md_alg ), 0 ) ) != 0 )
             {
                 MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_md_setup", ret );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
                 return( ret );
             }
 
@@ -2489,6 +2569,8 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ssl->session_negotiate->peer_cert == NULL )
         {
             MBEDTLS_SSL_DEBUG_MSG( 2, ( "certificate required" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
             return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
         }
 
@@ -2498,12 +2580,16 @@ static int ssl_parse_server_key_exchange( mbedtls_ssl_context *ssl )
         if( ! mbedtls_pk_can_do( &ssl->session_negotiate->peer_cert->pk, pk_alg ) )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
             return( MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH );
         }
 
         if( ( ret = mbedtls_pk_verify( &ssl->session_negotiate->peer_cert->pk,
                                md_alg, hash, hashlen, p, sig_len ) ) != 0 )
         {
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECRYPT_ERROR );
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_pk_verify", ret );
             return( ret );
         }
@@ -2579,6 +2665,8 @@ static int ssl_parse_certificate_request( mbedtls_ssl_context *ssl )
         if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate request message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
             return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
         }
 
@@ -2632,6 +2720,8 @@ static int ssl_parse_certificate_request( mbedtls_ssl_context *ssl )
     if( ssl->in_hslen < mbedtls_ssl_hs_hdr_len( ssl ) + 2 + n )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate request message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_REQUEST );
     }
 
@@ -2656,6 +2746,8 @@ static int ssl_parse_certificate_request( mbedtls_ssl_context *ssl )
         if( ssl->in_hslen < mbedtls_ssl_hs_hdr_len( ssl ) + 2 + n )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate request message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_REQUEST );
         }
     }
@@ -2669,6 +2761,8 @@ static int ssl_parse_certificate_request( mbedtls_ssl_context *ssl )
     if( ssl->in_hslen != mbedtls_ssl_hs_hdr_len( ssl ) + 3 + n )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate request message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_REQUEST );
     }
 
@@ -2710,6 +2804,8 @@ static int ssl_parse_server_hello_done( mbedtls_ssl_context *ssl )
         ssl->in_msg[0] != MBEDTLS_SSL_HS_SERVER_HELLO_DONE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server hello done message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO_DONE );
     }
 
@@ -3190,6 +3286,8 @@ static int ssl_parse_new_session_ticket( mbedtls_ssl_context *ssl )
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad new session ticket message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -3207,6 +3305,8 @@ static int ssl_parse_new_session_ticket( mbedtls_ssl_context *ssl )
         ssl->in_hslen < 6 + mbedtls_ssl_hs_hdr_len( ssl ) )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad new session ticket message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_NEW_SESSION_TICKET );
     }
 
@@ -3220,6 +3320,8 @@ static int ssl_parse_new_session_ticket( mbedtls_ssl_context *ssl )
     if( ticket_len + 6 + mbedtls_ssl_hs_hdr_len( ssl ) != ssl->in_hslen )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad new session ticket message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_NEW_SESSION_TICKET );
     }
 
@@ -3245,6 +3347,8 @@ static int ssl_parse_new_session_ticket( mbedtls_ssl_context *ssl )
     if( ( ticket = mbedtls_calloc( 1, ticket_len ) ) == NULL )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "ticket alloc failed" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
         return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
     }
 
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index c9051a4d9..451370c6a 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -101,6 +101,8 @@ static int ssl_parse_servername_ext( mbedtls_ssl_context *ssl,
     if( servername_list_size + 2 != len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -111,6 +113,8 @@ static int ssl_parse_servername_ext( mbedtls_ssl_context *ssl,
         if( hostname_len + 3 > servername_list_size )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
         }
 
@@ -135,6 +139,8 @@ static int ssl_parse_servername_ext( mbedtls_ssl_context *ssl,
     if( servername_list_size != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -201,6 +207,8 @@ static int ssl_parse_signature_algorithms_ext( mbedtls_ssl_context *ssl,
         sig_alg_list_size % 2 != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -247,6 +255,8 @@ static int ssl_parse_supported_elliptic_curves( mbedtls_ssl_context *ssl,
         list_size % 2 != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -254,6 +264,8 @@ static int ssl_parse_supported_elliptic_curves( mbedtls_ssl_context *ssl,
     if( ssl->handshake->curves != NULL )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -264,7 +276,11 @@ static int ssl_parse_supported_elliptic_curves( mbedtls_ssl_context *ssl,
         our_size = MBEDTLS_ECP_DP_MAX;
 
     if( ( curves = mbedtls_calloc( our_size, sizeof( *curves ) ) ) == NULL )
+    {
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
         return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
+    }
 
     ssl->handshake->curves = curves;
 
@@ -297,6 +313,8 @@ static int ssl_parse_supported_point_formats( mbedtls_ssl_context *ssl,
     if( list_size + 1 != len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -342,6 +360,8 @@ static int ssl_parse_ecjpake_kkpp( mbedtls_ssl_context *ssl,
                                                 buf, len ) ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ecjpake_read_round_one", ret );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( ret );
     }
 
@@ -360,6 +380,8 @@ static int ssl_parse_max_fragment_length_ext( mbedtls_ssl_context *ssl,
     if( len != 1 || buf[0] >= MBEDTLS_SSL_MAX_FRAG_LEN_INVALID )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -377,6 +399,8 @@ static int ssl_parse_truncated_hmac_ext( mbedtls_ssl_context *ssl,
     if( len != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -397,6 +421,8 @@ static int ssl_parse_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
     if( len != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -420,6 +446,8 @@ static int ssl_parse_extended_ms_ext( mbedtls_ssl_context *ssl,
     if( len != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -531,11 +559,19 @@ static int ssl_parse_alpn_ext( mbedtls_ssl_context *ssl,
 
     /* Min length is 2 (list_len) + 1 (name_len) + 1 (name) */
     if( len < 4 )
+    {
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
 
     list_len = ( buf[0] << 8 ) | buf[1];
     if( list_len != len - 2 )
+    {
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
 
     /*
      * Use our order of preference
@@ -549,13 +585,21 @@ static int ssl_parse_alpn_ext( mbedtls_ssl_context *ssl,
         {
             /* If the list is well formed, we should get equality first */
             if( theirs > end )
+            {
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+            }
 
             cur_len = *theirs++;
 
             /* Empty strings MUST NOT be included */
             if( cur_len == 0 )
+            {
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+            }
 
             if( cur_len == ours_len &&
                 memcmp( theirs, *ours, cur_len ) == 0 )
@@ -1064,6 +1108,9 @@ have_ciphersuite_v2:
 }
 #endif /* MBEDTLS_SSL_SRV_SUPPORT_SSLV2_CLIENT_HELLO */
 
+/* This function doesn't alert on errors that happen early during
+   ClientHello parsing because they might indicate that the client is
+   not talking SSL/TLS at all and would not understand our alert. */
 static int ssl_parse_client_hello( mbedtls_ssl_context *ssl )
 {
     int ret, got_common_suite;
@@ -1098,6 +1145,7 @@ read_record_header:
     {
         if( ( ret = mbedtls_ssl_fetch_input( ssl, 5 ) ) != 0 )
         {
+            /* No alert on a read error. */
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_fetch_input", ret );
             return( ret );
         }
@@ -1349,10 +1397,8 @@ read_record_header:
                             " [%d:%d] < [%d:%d]",
                             ssl->major_ver, ssl->minor_ver,
                             ssl->conf->min_major_ver, ssl->conf->min_minor_ver ) );
-
         mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
                                      MBEDTLS_SSL_ALERT_MSG_PROTOCOL_VERSION );
-
         return( MBEDTLS_ERR_SSL_BAD_HS_PROTOCOL_VERSION );
     }
 
@@ -1380,6 +1426,8 @@ read_record_header:
         sess_len + 34 + 2 > msg_len ) /* 2 for cipherlist length field */
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -1403,6 +1451,8 @@ read_record_header:
         if( cookie_offset + 1 + cookie_len + 2 > msg_len )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_PROTOCOL_VERSION );
             return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
         }
 
@@ -1435,6 +1485,7 @@ read_record_header:
             /* We know we didn't send a cookie, so it should be empty */
             if( cookie_len != 0 )
             {
+                /* This may be an attacker's probe, so don't send an alert */
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
             }
@@ -1459,6 +1510,8 @@ read_record_header:
         ( ciph_len % 2 ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -1477,6 +1530,8 @@ read_record_header:
         comp_len + comp_offset + 1 > msg_len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
     }
 
@@ -1515,6 +1570,8 @@ read_record_header:
             if( msg_len < ext_offset + 2 )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
             }
 
@@ -1525,6 +1582,8 @@ read_record_header:
                 msg_len != ext_offset + 2 + ext_len )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
             }
         }
@@ -1544,6 +1603,8 @@ read_record_header:
             if( ext_size + 4 > ext_len )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
             }
             switch( ext_id )
@@ -1689,6 +1750,8 @@ read_record_header:
             if( ext_len > 0 && ext_len < 4 )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+                mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                                MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
                 return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
             }
         }
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index fb68a1cdc..cf7ad4e81 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4030,6 +4030,7 @@ int mbedtls_ssl_send_alert_message( mbedtls_ssl_context *ssl,
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> send alert message" ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "send alert level=%u message=%u", level, message ));
 
     ssl->out_msgtype = MBEDTLS_SSL_MSG_ALERT;
     ssl->out_msglen = 2;
@@ -4041,7 +4042,6 @@ int mbedtls_ssl_send_alert_message( mbedtls_ssl_context *ssl,
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_write_record", ret );
         return( ret );
     }
-
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= send alert message" ) );
 
     return( 0 );
@@ -4262,6 +4262,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
 
     if( ( ret = mbedtls_ssl_read_record( ssl ) ) != 0 )
     {
+        /* mbedtls_ssl_read_record may have sent an alert already. We
+           let it decide whether to alert. */
         MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_read_record", ret );
         return( ret );
     }
@@ -4283,6 +4285,9 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "SSLv3 client has no certificate" ) );
 
+            /* 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 );
@@ -4304,6 +4309,9 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "TLSv1 client has no certificate" ) );
 
+            /* 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 );
@@ -4318,6 +4326,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -4325,6 +4335,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         ssl->in_hslen < mbedtls_ssl_hs_hdr_len( ssl ) + 3 + 3 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
     }
 
@@ -4339,6 +4351,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         ssl->in_hslen != n + 3 + mbedtls_ssl_hs_hdr_len( ssl ) )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
     }
 
@@ -4354,6 +4368,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%d bytes) failed",
                        sizeof( mbedtls_x509_crt ) ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
         return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
     }
 
@@ -4366,6 +4382,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         if( ssl->in_msg[i] != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
 
@@ -4376,13 +4394,35 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         if( n < 128 || i + n > ssl->in_hslen )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate message" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
 
         ret = mbedtls_x509_crt_parse_der( ssl->session_negotiate->peer_cert,
                                   ssl->in_msg + i, n );
-        if( 0 != ret && ( MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + MBEDTLS_ERR_OID_NOT_FOUND ) != ret )
+        switch( ret )
         {
+        uint8_t alert;
+
+        case 0: /*ok*/
+        case MBEDTLS_ERR_X509_UNKNOWN_SIG_ALG + MBEDTLS_ERR_OID_NOT_FOUND:
+            /* Ignore certificate with an unknown algorithm: maybe a
+               prior certificate was already trusted. */
+            break;
+
+        case MBEDTLS_ERR_X509_ALLOC_FAILED:
+            alert = MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR;
+            goto crt_parse_der_failed;
+
+        case MBEDTLS_ERR_X509_UNKNOWN_VERSION:
+            alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            goto crt_parse_der_failed;
+
+        default:
+            alert = MBEDTLS_SSL_ALERT_MSG_BAD_CERT;
+        crt_parse_der_failed:
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL, alert );
             MBEDTLS_SSL_DEBUG_RET( 1, " mbedtls_x509_crt_parse_der", ret );
             return( ret );
         }
@@ -4403,6 +4443,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         if( ssl->session->peer_cert == NULL )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "new server cert during renegotiation" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
 
@@ -4413,6 +4455,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
                     ssl->session->peer_cert->raw.len ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "server cert changed during renegotiation" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED );
             return( MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE );
         }
     }
@@ -4439,6 +4483,8 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
         if( ca_chain == NULL )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "got no CA chain" ) );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_BAD_CERT );
             return( MBEDTLS_ERR_SSL_CA_CHAIN_REQUIRED );
         }
 
@@ -4489,6 +4535,36 @@ int mbedtls_ssl_parse_certificate( mbedtls_ssl_context *ssl )
 
         if( authmode == MBEDTLS_SSL_VERIFY_OPTIONAL )
             ret = 0;
+
+        if( ret != 0 )
+        {
+            /* The certificate may have been rejected for several reasons.
+               Pick one and send the corresponding alert. Which alert to send
+               may be a subject of debate in some cases. */
+            uint8_t alert = MBEDTLS_SSL_ALERT_MSG_CERT_UNKNOWN;
+            if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_OTHER )
+                alert = MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_CN_MISMATCH )
+                alert = MBEDTLS_SSL_ALERT_MSG_BAD_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_KEY_USAGE )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_EXT_KEY_USAGE )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_NS_CERT_TYPE )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_BAD_PK )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_BAD_KEY )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_EXPIRED )
+                alert = MBEDTLS_SSL_ALERT_MSG_CERT_EXPIRED;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_REVOKED )
+                alert = MBEDTLS_SSL_ALERT_MSG_CERT_REVOKED;
+            else if( ssl->session_negotiate->verify_result & MBEDTLS_X509_BADCERT_NOT_TRUSTED )
+                alert = MBEDTLS_SSL_ALERT_MSG_UNKNOWN_CA;
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            alert );
+        }
     }
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= parse certificate" ) );
@@ -4541,12 +4617,16 @@ int mbedtls_ssl_parse_change_cipher_spec( mbedtls_ssl_context *ssl )
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_CHANGE_CIPHER_SPEC )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad change cipher spec message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
     if( ssl->in_msglen != 1 || ssl->in_msg[0] != 1 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad change cipher spec message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_CHANGE_CIPHER_SPEC );
     }
 
@@ -4569,6 +4649,8 @@ int mbedtls_ssl_parse_change_cipher_spec( mbedtls_ssl_context *ssl )
         if( ++ssl->in_epoch == 0 )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "DTLS epoch would wrap" ) );
+            /* This is highly unlikely to happen for legitimate reasons, so
+               treat it as an attack and don't send an alert. */
             return( MBEDTLS_ERR_SSL_COUNTER_WRAPPING );
         }
     }
@@ -4593,6 +4675,8 @@ int mbedtls_ssl_parse_change_cipher_spec( mbedtls_ssl_context *ssl )
         if( ( ret = mbedtls_ssl_hw_record_activate( ssl, MBEDTLS_SSL_CHANNEL_INBOUND ) ) != 0 )
         {
             MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_hw_record_activate", ret );
+            mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                            MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR );
             return( MBEDTLS_ERR_SSL_HW_ACCEL_FAILED );
         }
     }
@@ -5171,6 +5255,8 @@ int mbedtls_ssl_parse_finished( mbedtls_ssl_context *ssl )
     if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE );
         return( MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
     }
 
@@ -5186,6 +5272,8 @@ int mbedtls_ssl_parse_finished( mbedtls_ssl_context *ssl )
         ssl->in_hslen  != mbedtls_ssl_hs_hdr_len( ssl ) + hash_len )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_FINISHED );
     }
 
@@ -5193,6 +5281,8 @@ int mbedtls_ssl_parse_finished( mbedtls_ssl_context *ssl )
                       buf, hash_len ) != 0 )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad finished message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR );
         return( MBEDTLS_ERR_SSL_BAD_HS_FINISHED );
     }
 
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 4f633fc96..f7b43f6b7 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1840,8 +1840,12 @@ run_test    "Authentication: client badcert, server required" \
             -s "x509_verify_cert() returned" \
             -s "! The certificate is not correctly signed by the trusted CA" \
             -s "! mbedtls_ssl_handshake returned" \
+            -s "send alert level=2 message=48" \
             -c "! mbedtls_ssl_handshake returned" \
             -s "X509 - Certificate verification failed"
+# We don't check that the client receives the alert because it might
+# detect that its write end of the connection is closed and abort
+# before reading the alert message.
 
 run_test    "Authentication: client badcert, server optional" \
             "$P_SRV debug_level=3 auth_mode=optional" \