Add restrictions by default to open registration in Synapse (#12091)

7 files changed, 67 insertions, 4 deletions

diff --git a/changelog.d/12091.misc b/changelog.d/12091.misc
new file mode 100644
index 0000000000..def44987b4
--- /dev/null
+++ b/changelog.d/12091.misc
@@ -0,0 +1 @@
+Refuse to start if registration is enabled without email, captcha, or token-based verification unless new config flag `enable_registration_without_verification` is set.
diff --git a/demo/start.sh b/demo/start.sh
index 55e69685e3..5a9972d24c 100755
--- a/demo/start.sh
+++ b/demo/start.sh
@@ -38,6 +38,7 @@ for port in 8080 8081 8082; do
             printf '\n\n# Customisation made by demo/start.sh\n\n'
             echo "public_baseurl: http://localhost:$port/"
             echo 'enable_registration: true'
+            echo 'enable_registration_without_verification: true'
             echo ''
 
 			# Warning, this heredoc depends on the interaction of tabs and spaces.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 9c2359ed8e..a21b48ab2e 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -1218,10 +1218,18 @@ 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, email, or token-based verification to verify that new users are not bots. In order to enable registration
+# without any 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.
diff --git a/docs/upgrade.md b/docs/upgrade.md
index f039710520..062e823333 100644
--- a/docs/upgrade.md
+++ b/docs/upgrade.md
@@ -108,6 +108,12 @@ for more information and instructions on how to fix a database with incorrect va
 
 # Upgrading to v1.55.0
 
+## Open registration without verification is now disabled by default
+
+Synapse will refuse to start if registration is enabled without email, captcha, or token-based verification unless the new config 
+flag `enable_registration_without_verification` is set to "true".
+
+
 ## `synctl` script has been moved
 
 The `synctl` script
diff --git a/synapse/app/homeserver.py b/synapse/app/homeserver.py
index ad2b7c9515..0f75e7b9d4 100644
--- a/synapse/app/homeserver.py
+++ b/synapse/app/homeserver.py
@@ -351,6 +351,23 @@ 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
+            and not config.registration.registration_requires_token
+        ):
+
+            raise ConfigError(
+                "You have enabled open registration without any verification. This is a known vector for "
+                "spam and abuse. If you would like to allow public registration, please consider adding email, "
+                "captcha, or token-based verification. Otherwise this check can be removed by setting the "
+                "`enable_registration_without_verification` config option to `true`."
+            )
+
     hs = SynapseHomeServer(
         config.server.server_name,
         config=config,
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index ea9b50fe97..40fb329a7f 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -33,6 +33,10 @@ class RegistrationConfig(Config):
                 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 @@ class RegistrationConfig(Config):
         # 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, email, or token-based verification to verify that new users are not bots. In order to enable registration
+        # without any 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.
diff --git a/tests/config/test_registration_config.py b/tests/config/test_registration_config.py
index 17a84d20d8..2acdb6ac61 100644
--- a/tests/config/test_registration_config.py
+++ b/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 +78,19 @@ class RegistrationConfigTestCase(TestCase):
         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: []",
+                "enable_registration_captcha: false",
+                "registration_requires_token: false",
+            ]
+        )
+
+        # Test that allowing open registration without verification raises an error
+        with self.assertRaises(ConfigError):
+            synapse.app.homeserver.setup(["-c", self.config_file])