From 6e7bf5fa66be4abcc8ca39445c65d04688f9ae8c Mon Sep 17 00:00:00 2001
From: Juliusz Sosinowicz <juliusz@wolfssl.com>
Date: Mon, 1 Jun 2026 18:31:58 +0200
Subject: [PATCH] F-5629: reject ServerHello ALPN response with multiple
 protocols

A response ProtocolNameList was passed whole to ALPN_find_match, which
accepts the first configured match, so a server could return more than
one protocol. Per RFC 7301 Section 3.1 the ServerHello list must contain
exactly one name; enforce that the single name spans the whole list and
reject otherwise with a fatal illegal_parameter alert.
---
 src/tls.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/tls.c b/src/tls.c
index bf58e2ee2d..c23f3f7a49 100644
--- a/src/tls.c
+++ b/src/tls.c
@@ -2065,6 +2065,15 @@ static int TLSX_ALPN_ParseAndSet(WOLFSSL *ssl, const byte *input, word16 length,
         byte sel_len = 0;
         TLSX *extension = NULL;
 
+        /* RFC 7301 Section 3.1: a ServerHello ALPN extension MUST contain
+         * exactly one protocol name. The first name's length byte plus its
+         * payload must therefore span the whole list. */
+        if ((word16)(input[offset] + OPAQUE8_LEN) != size) {
+            SendAlert(ssl, alert_fatal, illegal_parameter);
+            WOLFSSL_ERROR_VERBOSE(BUFFER_ERROR);
+            return BUFFER_ERROR;
+        }
+
         r = ALPN_find_match(ssl, &extension, &sel, &sel_len, input + offset, size);
         if (r != 0)
             return r;