Add checking int the client to check the magic values which are

set by a 1.3 server when it downgrades to tls 1.2 or 1.1 as per
RFC 8446 section 4.1.3

ok jsing@

1 files changed, 17 insertions, 1 deletions

diff --git a/src/lib/libssl/tls13_client.c b/src/lib/libssl/tls13_client.c
index cab113b8c3..477cca2e04 100644
--- a/src/lib/libssl/tls13_client.c
+++ b/src/lib/libssl/tls13_client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls13_client.c,v 1.30 2020/01/23 06:15:44 beck Exp $ */
+/* $OpenBSD: tls13_client.c,v 1.31 2020/01/23 07:30:55 beck Exp $ */
 /*
  * Copyright (c) 2018, 2019 Joel Sing <jsing@openbsd.org>
  *
@@ -285,6 +285,22 @@ tls13_server_hello_process(struct tls13_ctx *ctx, CBS *cbs)
                 goto err;
 
         if (tls13_server_hello_is_legacy(cbs)) {
+                /*
+                 * RFC 8446 section 4.1.3, We must not downgrade if
+                 * the server random value contains the TLS 1.2 or 1.1
+                 * magical value.
+                 */
+                if (!CBS_skip(&server_random, CBS_len(&server_random) -
+                    sizeof(tls13_downgrade_12)))
+                        goto err;
+                if (CBS_mem_equal(&server_random, tls13_downgrade_12,
+                    sizeof(tls13_downgrade_12)) ||
+                    CBS_mem_equal(&server_random, tls13_downgrade_11,
+                    sizeof(tls13_downgrade_11))) {
+                        ctx->alert = SSL_AD_ILLEGAL_PARAMETER;
+                        goto err;
+                }
+
                 if (!CBS_skip(cbs, CBS_len(cbs)))
                         goto err;
                 return tls13_use_legacy_client(ctx);