[Openvpn-devel,S] Change in openvpn[master]: Disable TLS 1.3 support with mbed TLS

Message ID 388d6164aee3eb863e780b1cfe3d53c50de9f1a4-HTML@gerrit.openvpn.net
State Superseded
Headers show
Series [Openvpn-devel,S] Change in openvpn[master]: Disable TLS 1.3 support with mbed TLS | expand

Commit Message

flichtenheld (Code Review) Nov. 10, 2023, 3:35 p.m. UTC
Attention is currently required from: flichtenheld, plaisthos.

Hello plaisthos, flichtenheld,

I'd like you to do a code review.
Please visit


to review the following change.

Change subject: Disable TLS 1.3 support with mbed TLS

Disable TLS 1.3 support with mbed TLS

As of version 3.5.0 the TLS-Exporter function is not yet implemented in
mbed TLS, and the exporter_master_secret is not exposed to the
application either. Falling back to an older PRF when claiming to use
TLS1.3 seems like false advertising.

Change-Id: If4e1c4af9831eb1090ccb3a3c4d3e76b413f0708
Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
M README.mbedtls
M src/openvpn/ssl_mbedtls.c
2 files changed, 6 insertions(+), 29 deletions(-)

  git pull ssh://gerrit.openvpn.net:29418/openvpn refs/changes/03/403/1


diff --git a/README.mbedtls b/README.mbedtls
index 9b75c2b..ed9d369 100644
--- a/README.mbedtls
+++ b/README.mbedtls
@@ -43,19 +43,5 @@ 
-Mbed TLS 3 supports the TLS 1.3 protocol, but the implementation is not yet
-complete. Therefore, using TLS 1.3 in the mbed TLS build of OpenVPN is not yet
-Nevertheless, here are some pointers to make it work with mbed TLS 3.5.0:
- * The stock configuration of mbed TLS does not support TLS 1.3. To enable it,
-   uncomment `#define MBEDTLS_SSL_PROTO_TLS1_3` in your mbedtls_config.h before
-   compiling the library.
- * An OpenVPN client with mbed TLS cannot connect to a server with OpenSSL
-   using TLS 1.3.
- * An OpenVPN client with OpenSSL *can* connect to a server using mbed TLS with
-   TLS 1.3, but *only* if `#define MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE` has
-   been uncommented in mbedtls_config.h.
-Note that none of these limitations apply to TLS 1.2.
+Mbed TLS 3 has implemented (parts of) the TLS 1.3 protocol, but we have disabled
+support in OpenVPN because the TLS-Exporter function is not yet implemented.
diff --git a/src/openvpn/ssl_mbedtls.c b/src/openvpn/ssl_mbedtls.c
index 09559be..b940aa4 100644
--- a/src/openvpn/ssl_mbedtls.c
+++ b/src/openvpn/ssl_mbedtls.c
@@ -1013,17 +1013,15 @@ 
-#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
-    return TLS_VER_1_3;
-#elif defined(MBEDTLS_SSL_PROTO_TLS1_2)
+#if defined(MBEDTLS_SSL_PROTO_TLS1_2)
     return TLS_VER_1_2;
 #elif defined(MBEDTLS_SSL_PROTO_TLS1_1)
     return TLS_VER_1_1;
 #elif defined(MBEDTLS_SSL_PROTO_TLS1)
     return TLS_VER_1_0;
-#else /* if defined(MBEDTLS_SSL_PROTO_TLS1_3) */
-    #error "mbedtls is compiled without support for any version of TLS."
+#else /* defined(MBEDTLS_SSL_PROTO_TLS1_2) */
+    #error "mbedtls is compiled without support for TLS 1.0, 1.1 and 1.2."
+#endif /* defined(MBEDTLS_SSL_PROTO_TLS1_2) */
@@ -1065,13 +1063,6 @@ 
-#if defined(MBEDTLS_SSL_PROTO_TLS1_3)
-        case TLS_VER_1_3:
-            *major = MBEDTLS_SSL_MAJOR_VERSION_3;
-            *minor = MBEDTLS_SSL_MINOR_VERSION_4;
-            break;
             msg(M_FATAL, "%s: invalid or unsupported TLS version %d", __func__, tls_ver);