Update the TLS cipher string and provide configurability for TLS on outgoing federation (#5550)

2 files changed, 64 insertions, 7 deletions

diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index 8fcf801418..ca508a224f 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -23,7 +23,7 @@ import six
 
 from unpaddedbase64 import encode_base64
 
-from OpenSSL import crypto
+from OpenSSL import SSL, crypto
 from twisted.internet._sslverify import Certificate, trustRootFromCertificates
 
 from synapse.config._base import Config, ConfigError
@@ -81,6 +81,27 @@ class TlsConfig(Config):
             "federation_verify_certificates", True
         )
 
+        # Minimum TLS version to use for outbound federation traffic
+        self.federation_client_minimum_tls_version = str(
+            config.get("federation_client_minimum_tls_version", 1)
+        )
+
+        if self.federation_client_minimum_tls_version not in ["1", "1.1", "1.2", "1.3"]:
+            raise ConfigError(
+                "federation_client_minimum_tls_version must be one of: 1, 1.1, 1.2, 1.3"
+            )
+
+        # Prevent people shooting themselves in the foot here by setting it to
+        # the biggest number blindly
+        if self.federation_client_minimum_tls_version == "1.3":
+            if getattr(SSL, "OP_NO_TLSv1_3", None) is None:
+                raise ConfigError(
+                    (
+                        "federation_client_minimum_tls_version cannot be 1.3, "
+                        "your OpenSSL does not support it"
+                    )
+                )
+
         # Whitelist of domains to not verify certificates for
         fed_whitelist_entries = config.get(
             "federation_certificate_verification_whitelist", []
@@ -261,6 +282,15 @@ class TlsConfig(Config):
         #
         #federation_verify_certificates: false
 
+        # The minimum TLS version that will be used for outbound federation requests.
+        #
+        # Defaults to `1`. Configurable to `1`, `1.1`, `1.2`, or `1.3`. Note
+        # that setting this value higher than `1.2` will prevent federation to most
+        # of the public Matrix network: only configure it to `1.3` if you have an
+        # entirely private federation setup and you can ensure TLS 1.3 support.
+        #
+        #federation_client_minimum_tls_version: 1.2
+
         # Skip federation certificate verification on the following whitelist
         # of domains.
         #
diff --git a/synapse/crypto/context_factory.py b/synapse/crypto/context_factory.py
index 2bc5cc3807..4f48e8e88d 100644
--- a/synapse/crypto/context_factory.py
+++ b/synapse/crypto/context_factory.py
@@ -24,12 +24,25 @@ from OpenSSL import SSL, crypto
 from twisted.internet._sslverify import _defaultCurveName
 from twisted.internet.abstract import isIPAddress, isIPv6Address
 from twisted.internet.interfaces import IOpenSSLClientConnectionCreator
-from twisted.internet.ssl import CertificateOptions, ContextFactory, platformTrust
+from twisted.internet.ssl import (
+    CertificateOptions,
+    ContextFactory,
+    TLSVersion,
+    platformTrust,
+)
 from twisted.python.failure import Failure
 
 logger = logging.getLogger(__name__)
 
 
+_TLS_VERSION_MAP = {
+    "1": TLSVersion.TLSv1_0,
+    "1.1": TLSVersion.TLSv1_1,
+    "1.2": TLSVersion.TLSv1_2,
+    "1.3": TLSVersion.TLSv1_3,
+}
+
+
 class ServerContextFactory(ContextFactory):
     """Factory for PyOpenSSL SSL contexts that are used to handle incoming
     connections."""
@@ -43,16 +56,18 @@ class ServerContextFactory(ContextFactory):
         try:
             _ecCurve = crypto.get_elliptic_curve(_defaultCurveName)
             context.set_tmp_ecdh(_ecCurve)
-
         except Exception:
             logger.exception("Failed to enable elliptic curve for TLS")
-        context.set_options(SSL.OP_NO_SSLv2 | SSL.OP_NO_SSLv3)
+
+        context.set_options(
+            SSL.OP_NO_SSLv2 | SSL.OP_NO_SSLv3 | SSL.OP_NO_TLSv1 | SSL.OP_NO_TLSv1_1
+        )
         context.use_certificate_chain_file(config.tls_certificate_file)
         context.use_privatekey(config.tls_private_key)
 
         # https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
         context.set_cipher_list(
-            "ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES256:ECDH+AES128:!aNULL:!SHA1"
+            "ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES256:ECDH+AES128:!aNULL:!SHA1:!AESCCM"
         )
 
     def getContext(self):
@@ -79,10 +94,22 @@ class ClientTLSOptionsFactory(object):
             # Use CA root certs provided by OpenSSL
             trust_root = platformTrust()
 
-        self._verify_ssl_context = CertificateOptions(trustRoot=trust_root).getContext()
+        # "insecurelyLowerMinimumTo" is the argument that will go lower than
+        # Twisted's default, which is why it is marked as "insecure" (since
+        # Twisted's defaults are reasonably secure). But, since Twisted is
+        # moving to TLS 1.2 by default, we want to respect the config option if
+        # it is set to 1.0 (which the alternate option, raiseMinimumTo, will not
+        # let us do).
+        minTLS = _TLS_VERSION_MAP[config.federation_client_minimum_tls_version]
+
+        self._verify_ssl = CertificateOptions(
+            trustRoot=trust_root, insecurelyLowerMinimumTo=minTLS
+        )
+        self._verify_ssl_context = self._verify_ssl.getContext()
         self._verify_ssl_context.set_info_callback(self._context_info_cb)
 
-        self._no_verify_ssl_context = CertificateOptions().getContext()
+        self._no_verify_ssl = CertificateOptions(insecurelyLowerMinimumTo=minTLS)
+        self._no_verify_ssl_context = self._no_verify_ssl.getContext()
         self._no_verify_ssl_context.set_info_callback(self._context_info_cb)
 
     def get_options(self, host):