Set TLS 1.2 as minimum by default

Do not accept handshakes with peers trying to negotiate TLS lower than 1.2.

TLS 1.1 and 1.0 are not recommended and therefore we will, by default,
allow TLS 1.2 as minimum version.

The minimum allowed version can still be controlled via
'--tls-version-min'.

At the same time automatically set '--tls-version-min' to 1.0 if the
user requires compatibility with versions onlder than 2.3.7, as that was
the only version supported back then.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210913192929.26391-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22838.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

Changes.rst

@@ -71,6 +71,11 @@ Deprecated features
     This option mainly served a role as debug option when NCP was first
     introduced. It should now no longer be necessary.
 
+TLS 1.0 and 1.1 are deprecated
+    ``tls-version-min`` is set to 1.2 by default.  OpenVPN 2.6.0 defaults
+    to a minimum TLS version of 1.2 as TLS 1.0 and 1.1 should be generally
+    avoided. Note that OpenVPN versions older than 2.3.7 use TLS 1.0 only.
+
 ``--cipher`` argument is no longer appended to ``--data-ciphers``
     by default. Data cipher negotiation has been introduced in 2.4.0
     and been significantly improved in 2.5.0. The implicit fallback

doc/man-sections/generic-options.rst

@@ -70,6 +70,8 @@ which mode OpenVPN is configured as.
     ``--data-ciphers``
   - 2.3.x or lower: ``--data-cipher-fallback`` is automatically added with
     the same cipher as ``--cipher``
+  - 2.3.6 or lower: ``--tls-version-min 1.0`` is added to the configuration
+    when ``--tls-version-min`` is not explicitly set.
 
 --config file
   Load additional config options from ``file`` where each line corresponds

doc/man-sections/tls-options.rst

@@ -545,8 +545,8 @@ certificates and keys: https://github.com/OpenVPN/easy-rsa
   them.
 
 --tls-version-min args
-  Sets the minimum TLS version we will accept from the peer (default is
-  "1.0").
+  Sets the minimum TLS version we will accept from the peer (default in
+  2.6.0 and later is "1.2").
 
   Valid syntax:
   ::

src/openvpn/options.c

@@ -3164,6 +3164,21 @@ need_compatibility_before(const struct options *o, unsigned int version)
 static void
 options_set_backwards_compatible_options(struct options *o)
 {
+    /* TLS min version is not set */
+    if ((o->ssl_flags & SSLF_TLS_VERSION_MIN_MASK) == 0)
+    {
+        if (need_compatibility_before(o, 20307))
+        {
+            /* 2.3.6 and earlier have TLS 1.0 only, set minimum to TLS 1.0 */
+            o->ssl_flags = (TLS_VER_1_0 << SSLF_TLS_VERSION_MIN_SHIFT);
+        }
+        else
+        {
+            /* Use TLS 1.2 as proper default */
+            o->ssl_flags = (TLS_VER_1_2 << SSLF_TLS_VERSION_MIN_SHIFT);
+        }
+    }
+
     /* Versions < 2.5.0 do need --cipher in the list of accepted ciphers.
      * Version 2.4 might probably does not need it but NCP was not so
      * good with 2.4 and ncp-disable might be more common on 2.4 peers.