sslutil: require TLS 1.1+ when supported

Currently, Mercurial will use TLS 1.0 or newer when connecting to
remote servers, selecting the highest TLS version supported by both
peers. On older Pythons, only TLS 1.0 is available. On newer Pythons,
TLS 1.1 and 1.2 should be available.

Security professionals recommend avoiding TLS 1.0 if possible.
PCI DSS 3.1 "strongly encourages" the use of TLS 1.2.

Known attacks like BEAST and POODLE exist against TLS 1.0 (although
mitigations are available and properly configured servers aren't
vulnerable).

I asked Eric Rescorla - Mozilla's resident crypto expert - whether
Mercurial should drop support for TLS 1.0. His response was
"if you can get away with it." Essentially, a number of servers on
the Internet don't support TLS 1.1+. This is why web browsers
continue to support TLS 1.0 despite desires from security experts.

This patch changes Mercurial's default behavior on modern Python
versions to require TLS 1.1+, thus avoiding known security issues
with TLS 1.0 and making Mercurial more secure by default. Rather
than drop TLS 1.0 support wholesale, we still allow TLS 1.0 to be
used if configured. This is a compromise solution - ideally we'd
disallow TLS 1.0. However, since we're not sure how many Mercurial
servers don't support TLS 1.1+ and we're not sure how much user
inconvenience this change will bring, I think it is prudent to ship
an escape hatch that still allows usage of TLS 1.0. In the default
case our users get better security. In the worst case, they are no
worse off than before this patch.

This patch has no effect when running on Python versions that don't
support TLS 1.1+.

As the added test shows, connecting to a server that doesn't
support TLS 1.1+ will display a warning message with a link to
our wiki, where we can guide people to configure their client to
allow less secure connections.

mercurial/help/config.txt

@@ -1008,10 +1008,18 b' The following options control default be'
 ``minimumprotocol``
     Defines the minimum channel encryption protocol to use.
 
-    By default, the highest version of TLS - 1.0 or greater - supported by
-    both client and server is used.
-
-    Allowed values are: ``tls1.0`` (the default), ``tls1.1``, ``tls1.2``.
+    By default, the highest version of TLS supported by both client and server
+    is used.
+
+    Allowed values are: ``tls1.0``, ``tls1.1``, ``tls1.2``.
+
+    When running on an old Python version, only ``tls1.0`` is allowed since
+    old versions of Python only support up to TLS 1.0.
+
+    When running a Python that supports modern TLS versions, the default is
+    ``tls1.1``. ``tls1.0`` can still be used to allow TLS 1.0. However, this
+    weakens security and should only be used as a feature of last resort if
+    a server does not support TLS 1.1+.
 
 Options in the ``[hostsecurity]`` section can have the form
 ``hostname``:``setting``. This allows multiple settings to be defined on a

mercurial/sslutil.py

@@ -154,9 +154,17 b' def _hostsettings(ui, hostname):'
                 hint=_('valid protocols: %s') %
                      ' '.join(sorted(configprotocols)))
 
+    # Legacy Python can only do TLS 1.0. We default to TLS 1.1+ where we
+    # can because TLS 1.0 has known vulnerabilities (like BEAST and POODLE).
+    # We allow users to downgrade to TLS 1.0+ via config options in case a
+    # legacy server is encountered.
+    if modernssl:
+        defaultprotocol = 'tls1.1'
+    else:
+        defaultprotocol = 'tls1.0'
+
     key = 'minimumprotocol'
-    # Default to TLS 1.0+ as that is what browsers are currently doing.
-    protocol = ui.config('hostsecurity', key, 'tls1.0')
+    protocol = ui.config('hostsecurity', key, defaultprotocol)
     validateprotocol(protocol, key)
 
     key = '%s:minimumprotocol' % hostname

tests/test-https.t

@@ -377,6 +377,11 b' Clients talking same TLS versions work'
 
 Clients requiring newer TLS version than what server supports fail
 
+  $ P="$CERTSDIR" hg id https://localhost:$HGPORT/
+  (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
+  abort: error: *unsupported protocol* (glob)
+  [255]
+
   $ P="$CERTSDIR" hg --config hostsecurity.minimumprotocol=tls1.1 id https://localhost:$HGPORT/
   (could not negotiate a common protocol; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this error)
   abort: error: *unsupported protocol* (glob)