From f0422f56d11ab3f4d9cde32fb38ab074bd5c88d7 Mon Sep 17 00:00:00 2001
From: yi chen <94xhn1@gmail.com>
Date: Fri, 10 Jul 2026 08:05:51 +0800
Subject: [PATCH] Reject empty ALPN protocol list in
 mbedtls_ssl_conf_alpn_protocols()

mbedtls_ssl_conf_alpn_protocols() validates that no individual
protocol name is empty or too long, but never checked that the list
itself has at least one entry. Configuring an empty list (a
NULL-terminated array whose first element is already NULL) was
silently accepted, and the list is later used as-is by
ssl_write_alpn_ext() to build the ALPN extension - producing a
protocol_name_list of length 0.

RFC 7301 3.1 requires the ALPN extension's protocol_name_list to
contain at least one entry; a zero-length list is not a valid
encoding. Peers that correctly validate the extension respond with a
fatal decode_error alert, breaking the handshake.

Reject an empty list up front with MBEDTLS_ERR_SSL_BAD_INPUT_DATA,
matching how this function already rejects other invalid inputs
(empty individual protocol names, names or total length exceeding
the configured maximums).

Fixes #10461

Signed-off-by: yi chen <94xhn1@gmail.com>
---
 ChangeLog.d/fix-alpn-empty-list.txt |  7 +++++++
 library/ssl_tls.c                   | 10 ++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 ChangeLog.d/fix-alpn-empty-list.txt

diff --git a/ChangeLog.d/fix-alpn-empty-list.txt b/ChangeLog.d/fix-alpn-empty-list.txt
new file mode 100644
index 0000000000..03b9345686
--- /dev/null
+++ b/ChangeLog.d/fix-alpn-empty-list.txt
@@ -0,0 +1,7 @@
+Bugfix
+   * Fix mbedtls_ssl_conf_alpn_protocols() silently accepting an empty
+     protocol list, which caused an invalid ALPN extension with a
+     zero-length protocol_name_list to be sent during the handshake,
+     violating RFC 7301 and causing conforming peers to reject the
+     connection with a fatal decode error. mbedtls_ssl_conf_alpn_protocols()
+     now returns MBEDTLS_ERR_SSL_BAD_INPUT_DATA for an empty list.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 0195576213..dbebdf9c57 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2526,6 +2526,16 @@ int mbedtls_ssl_conf_alpn_protocols(mbedtls_ssl_config *conf,
     size_t cur_len, tot_len;
     const char *const *p;
 
+    /*
+     * RFC 7301 3.1: the ALPN extension's protocol_name_list must contain
+     * at least one entry - an empty list is not a valid ProtocolNameList
+     * and must not be sent. Reject it here rather than silently sending
+     * an ALPN extension with a zero-length list later.
+     */
+    if (*protos == NULL) {
+        return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
+    }
+
     /*
      * RFC 7301 3.1: "Empty strings MUST NOT be included and byte strings
      * MUST NOT be truncated."