Check that the server selected ciphersuite is valid for use with TLSv1.2.

In the legacy stack, ensure that the server selected ciphersuite is valid
for use with TLSv1.2 - this effectively means that it is not a TLSv1.3
ciphersuite. We currently fail the handshake, but at a later stage.

Reported by Tom Gouville from the tlspuffin team.

ok tb@

1 files changed, 10 insertions, 1 deletions

diff --git a/src/lib/libssl/ssl_clnt.c b/src/lib/libssl/ssl_clnt.c
index d9fdfff2a3..4803e35dd2 100644
--- a/src/lib/libssl/ssl_clnt.c
+++ b/src/lib/libssl/ssl_clnt.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssl_clnt.c,v 1.174 2026/07/15 13:56:38 jsing Exp $ */
+/* $OpenBSD: ssl_clnt.c,v 1.175 2026/07/15 15:00:17 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -1010,6 +1010,15 @@ ssl3_get_server_hello(SSL *s)
                 goto fatal_err;
         }
 
+        /* Require a ciphersuite that can be used with TLSv1.2. */
+        if (cipher->algorithm_ssl != SSL_SSLV3 &&
+            cipher->algorithm_ssl != SSL_TLSV1 &&
+            cipher->algorithm_ssl != SSL_TLSV1_2) {
+                al = SSL_AD_ILLEGAL_PARAMETER;
+                SSLerror(s, SSL_R_WRONG_CIPHER_RETURNED);
+                goto fatal_err;
+        }
+
         /*
          * Depending on the session caching (internal/external), the cipher
          * and/or cipher_id values may not be set. Make sure that