diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 6840ca0..906d6ac 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2731,6 +2731,15 @@
         if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
         {
             /* Make room for the additional DTLS fields */
+            if( MBEDTLS_SSL_MAX_CONTENT_LEN - ssl->out_msglen < 8 )
+            {
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "DTLS handshake message too large: "
+                              "size %u, maximum %u",
+                               (unsigned) ( ssl->in_hslen - 4 ),
+                               (unsigned) ( MBEDTLS_SSL_MAX_CONTENT_LEN - 12 ) ) );
+                return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+            }
+
             memmove( ssl->out_msg + 12, ssl->out_msg + 4, len - 4 );
             ssl->out_msglen += 8;
             len += 8;