diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index f5cd97714..7f742ad4c 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -138,6 +138,9 @@
 #define MBEDTLS_SSL_TRANSPORT_DATAGRAM          1   /*!< DTLS     */
 
 #define MBEDTLS_SSL_MAX_HOST_NAME_LEN           255 /*!< Maximum host name defined in RFC 1035 */
+#define MBEDTLS_SSL_MAX_ALPN_NAME_LEN           255 /*!< Maximum size in bytes of a protocol name in alpn ext., RFC 7301 */
+
+#define MBEDTLS_SSL_MAX_ALPN_LIST_LEN           65535 /*!< Maximum size in bytes of list in alpn ext., RFC 7301          */
 
 /* RFC 6066 section 4, see also mfl_code_to_length in ssl_tls.c
  * NONE must be zero so that memset()ing structure to zero works */
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index cbec74fe8..9bc4fa81b 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -7589,7 +7589,9 @@ int mbedtls_ssl_conf_alpn_protocols( mbedtls_ssl_config *conf, const char **prot
         cur_len = strlen( *p );
         tot_len += cur_len;
 
-        if( cur_len == 0 || cur_len > 255 || tot_len > 65535 )
+        if( ( cur_len == 0 ) ||
+            ( cur_len > MBEDTLS_SSL_MAX_ALPN_NAME_LEN ) ||
+            ( tot_len > MBEDTLS_SSL_MAX_ALPN_LIST_LEN ) )
             return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
     }