Tighten the rate limit of creating new devices.

changelog.d/15135.misc

@@ -0,0 +1 @@
+Tighten the login ratelimit defaults.

docs/usage/configuration/config_documentation.md

@@ -1518,11 +1518,11 @@ rc_registration_token_validity:
 
 This option specifies several limits for login:
 * `address` ratelimits login requests based on the client's IP
-      address. Defaults to `per_second: 0.17`, `burst_count: 3`.
+      address. Defaults to `per_second: 0.003`, `burst_count: 5`.
 
 * `account` ratelimits login requests based on the account the
-  client is attempting to log into. Defaults to `per_second: 0.17`,
-  `burst_count: 3`.
+  client is attempting to log into. Defaults to `per_second: 0.03`,
+  `burst_count: 5`.
 
 * `failed_attempts` ratelimits login requests based on the account the
   client is attempting to log into, based on the amount of failed login

synapse/config/ratelimiting.py

@@ -87,9 +87,18 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             defaults={"per_second": 0.1, "burst_count": 5},
         )
 
+        # It is reasonable to login with a bunch of devices at once (i.e. when
+        # setting up an account), but it is *not* valid to continually be
+        # logging into new devices.
         rc_login_config = config.get("rc_login", {})
-        self.rc_login_address = RatelimitSettings(rc_login_config.get("address", {}))
-        self.rc_login_account = RatelimitSettings(rc_login_config.get("account", {}))
+        self.rc_login_address = RatelimitSettings(
+            rc_login_config.get("address", {}),
+            defaults={"per_second": 0.003, "burst_count": 5},
+        )
+        self.rc_login_account = RatelimitSettings(
+            rc_login_config.get("account", {}),
+            defaults={"per_second": 0.003, "burst_count": 5},
+        )
         self.rc_login_failed_attempts = RatelimitSettings(
             rc_login_config.get("failed_attempts", {})
         )