Add a comment about SSLv23_METHOD (#8043)

matrix-org · Aug 7, 2020 · de6f892 · de6f892
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diff --git a/changelog.d/8043.misc b/changelog.d/8043.misc
@@ -0,0 +1 @@
+Add a comment to `ServerContextFactory` about the use of `SSLv23_METHOD`.
diff --git a/synapse/crypto/context_factory.py b/synapse/crypto/context_factory.py
@@ -48,6 +48,14 @@ class ServerContextFactory(ContextFactory):
     connections."""
 
     def __init__(self, config):
+        # TODO: once pyOpenSSL exposes TLS_METHOD and SSL_CTX_set_min_proto_version,
+        # switch to those (see https://github.com/pyca/cryptography/issues/5379).
+        #
+        # note that, despite the confusing name, SSLv23_METHOD does *not* enforce SSLv2
+        # or v3, but is a synonym for TLS_METHOD, which allows the client and server
+        # to negotiate an appropriate version of TLS constrained by the version options
+        # set with context.set_options.
+        #
         self._context = SSL.Context(SSL.SSLv23_METHOD)
         self.configure_context(self._context, config)