Add restrictions by default to open registration in Synapse

changelog.d/12091.misc

@@ -0,0 +1 @@
+Refuse to start if registration is enabled without email or captcha verification unless new config flag `enable_registration_without_verification` is set.

docs/sample_config.yaml

@@ -1212,10 +1212,18 @@ oembed:
 # Registration can be rate-limited using the parameters in the "Ratelimiting"
 # section of this file.
 
-# Enable registration for new users.
+# Enable registration for new users. Defaults to 'false'. It is highly recommended that if you enable registration,
+# you use either captcha or email verification to verify that new users are not bots. In order to enable registration
+# without captcha or email verification, you must also set `enable_registration_without_verification`, found below.
 #
 #enable_registration: false
 
+# Enable registration without email or captcha verification. Note: this option is *not* recommended,
+# as registration without verification is a known vector for spam and abuse. Defaults to false. Has no effect
+# unless `enable_registration` is also enabled.
+#
+#enable_registration_without_verification: true
+
 # Time that a user's session remains valid for, after they log in.
 #
 # Note that this is not currently compatible with guest logins.

synapse/app/homeserver.py

@@ -348,6 +348,22 @@ def setup(config_options: List[str]) -> SynapseHomeServer:
     if config.server.gc_seconds:
         synapse.metrics.MIN_TIME_BETWEEN_GCS = config.server.gc_seconds
 
+    if (
+        config.registration.enable_registration
+        and not config.registration.enable_registration_without_verification
+    ):
+        if (
+            not config.captcha.enable_registration_captcha
+            and not config.registration.registrations_require_3pid
+        ):
+
+            raise ConfigError(
+                "You have enabled open registration without any verification. This is a known vector for"
+                "spam and abuse. If you have enabled registration, please add email or capcha verification, or"
+                "use the config option `enable_registration_without_verification` in conjunction with `enable_verification."
+            )
+            sys.exit(1)
+
     hs = SynapseHomeServer(
         config.server.server_name,
         config=config,

synapse/config/registration.py

@@ -33,6 +33,10 @@ def read_config(self, config, **kwargs):
                 str(config["disable_registration"])
             )
 
+        self.enable_registration_without_verification = strtobool(
+            str(config.get("enable_registration_without_verification", False))
+        )
+
         self.registrations_require_3pid = config.get("registrations_require_3pid", [])
         self.allowed_local_3pids = config.get("allowed_local_3pids", [])
         self.enable_3pid_lookup = config.get("enable_3pid_lookup", True)
@@ -207,10 +211,18 @@ def generate_config_section(self, generate_secrets=False, **kwargs):
         # Registration can be rate-limited using the parameters in the "Ratelimiting"
         # section of this file.
 
-        # Enable registration for new users.
+        # Enable registration for new users. Defaults to 'false'. It is highly recommended that if you enable registration,
+        # you use either captcha or email verification to verify that new users are not bots. In order to enable registration
+        # without captcha or email verification, you must also set `enable_registration_without_verification`, found below.
         #
         #enable_registration: false
 
+        # Enable registration without email or captcha verification. Note: this option is *not* recommended,
+        # as registration without verification is a known vector for spam and abuse. Defaults to false. Has no effect
+        # unless `enable_registration` is also enabled.
+        #
+        #enable_registration_without_verification: true
+
         # Time that a user's session remains valid for, after they log in.
         #
         # Note that this is not currently compatible with guest logins.

tests/config/test_registration_config.py

@@ -11,14 +11,16 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+import synapse.app.homeserver
 from synapse.config import ConfigError
 from synapse.config.homeserver import HomeServerConfig
 
-from tests.unittest import TestCase
+from tests.config.utils import ConfigFileTestCase
 from tests.utils import default_config
 
 
-class RegistrationConfigTestCase(TestCase):
+class RegistrationConfigTestCase(ConfigFileTestCase):
     def test_session_lifetime_must_not_be_exceeded_by_smaller_lifetimes(self):
         """
         session_lifetime should logically be larger than, or at least as large as,
@@ -76,3 +78,22 @@ def test_session_lifetime_must_not_be_exceeded_by_smaller_lifetimes(self):
         HomeServerConfig().parse_config_dict(
             {"session_lifetime": "31m", "refresh_token_lifetime": "31m", **config_dict}
         )
+
+    def test_refuse_to_start_if_open_registration_and_no_verification(self):
+        self.generate_config()
+        self.add_lines_to_config(
+            [
+                " ",
+                "enable_registration: true",
+                "registrations_require_3pid: false",
+                "enable_registration_captcha: false",
+            ]
+        )
+
+        # Test that allowing open registration without verification raises an error
+        with self.assertRaises(ConfigError):
+            synapse.app.homeserver.setup(["-c", self.config_file])
+
+        # Test that setting `enable_registration_without_verification` to true overrides config error
+        self.add_lines_to_config(["enable_registration_without_verification: true"])
+        synapse.app.homeserver.setup(["-c", self.config_file])