From 1a7a17e5484b3f634c0341905f474ac148b9adfc Mon Sep 17 00:00:00 2001
From: Angus Gratton <angus@espressif.com>
Date: Wed, 20 Jun 2018 15:43:50 +1000
Subject: [PATCH] Check for invalid short Alert messages

(Short Change Cipher Spec & Handshake messages are already checked for.)
---
 ChangeLog         |  2 ++
 library/ssl_tls.c | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 3b4566064..98ebd2d65 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -39,6 +39,8 @@ Bugfix
      seen when communicating with OpenSSL using TLS 1.0. Reported by @kFYatek
      (#1632) and by Conor Murphy on the forum. Fix contributed by Espressif
      Systems.
+   * Fail when receiving a TLS alert message with an invalid length, or invalid
+     zero-length messages when using TLS 1.2. Contributed by Espressif Systems.
 
 Changes
    * Change the shebang line in Perl scripts to look up perl in the PATH.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index b7c8881fb..f1856e278 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4187,6 +4187,16 @@ int mbedtls_ssl_handle_message_type( mbedtls_ssl_context *ssl )
 
     if( ssl->in_msgtype == MBEDTLS_SSL_MSG_ALERT )
     {
+        if( ssl->in_msglen != 2 )
+        {
+            /* Note: Standard allows for more than one 2 byte alert
+               to be packed in a single message, but Mbed TLS doesn't
+               currently support this. */
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid alert message, len: %d",
+                           ssl->in_msglen ) );
+            return( MBEDTLS_ERR_SSL_INVALID_RECORD );
+        }
+
         MBEDTLS_SSL_DEBUG_MSG( 2, ( "got an alert message, type: [%d:%d]",
                        ssl->in_msg[0], ssl->in_msg[1] ) );