Config option to inhibit 3PID errors on /requestToken

Adds a request_token_inhibit_errors configuration flag (disabled by
default) which, if enabled, change the behaviour of all /requestToken
endpoints so that they return a 200 and a fake sid if the 3PID was/was
not found associated with an account (depending on the endpoint),
instead of an error.

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

3 files changed, 48 insertions, 2 deletions

diff --git a/synapse/config/server.py b/synapse/config/server.py
index 7525765fee..8acf3946eb 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -507,6 +507,17 @@ class ServerConfig(Config):
 
         self.enable_ephemeral_messages = config.get("enable_ephemeral_messages", False)
 
+        # Inhibits the /requestToken endpoints from returning an error that might leak
+        # information about whether an e-mail address is in use or not on this
+        # homeserver, and instead return a 200 with a fake sid if this kind of error is
+        # met, without sending anything.
+        # This is a compromise between sending an email, which could be a spam vector,
+        # and letting the client know which email address is bound to an account and
+        # which one isn't.
+        self.request_token_inhibit_3pid_errors = config.get(
+            "request_token_inhibit_3pid_errors", False,
+        )
+
     def has_tls_listener(self) -> bool:
         return any(l["tls"] for l in self.listeners)
 
@@ -967,6 +978,16 @@ class ServerConfig(Config):
           #  - shortest_max_lifetime: 3d
           #    longest_max_lifetime: 1y
           #    interval: 1d
+
+        # Inhibits the /requestToken endpoints from returning an error that might leak
+        # information about whether an e-mail address is in use or not on this
+        # homeserver.
+        # Note that for some endpoints the error situation is the e-mail already being
+        # used, and for others the error is entering the e-mail being unused.
+        # If this option is enabled, instead of returning an error, these endpoints will
+        # act as if no error happened and return a fake session ID ('sid') to clients.
+        #
+        #request_token_inhibit_3pid_errors: true
         """
             % locals()
         )
diff --git a/synapse/rest/client/v2_alpha/account.py b/synapse/rest/client/v2_alpha/account.py
index 631cc74cb4..e2fdcda655 100644
--- a/synapse/rest/client/v2_alpha/account.py
+++ b/synapse/rest/client/v2_alpha/account.py
@@ -30,7 +30,7 @@ from synapse.http.servlet import (
 )
 from synapse.push.mailer import Mailer, load_jinja2_templates
 from synapse.util.msisdn import phone_number_to_msisdn
-from synapse.util.stringutils import assert_valid_client_secret
+from synapse.util.stringutils import assert_valid_client_secret, random_string
 from synapse.util.threepids import check_3pid_allowed
 
 from ._base import client_patterns, interactive_auth_handler
@@ -100,6 +100,11 @@ class EmailPasswordRequestTokenRestServlet(RestServlet):
         )
 
         if existing_user_id is None:
+            if self.config.request_token_inhibit_3pid_errors:
+                # Make the client think the operation succeeded. See the rationale in the
+                # comments for request_token_inhibit_3pid_errors.
+                return 200, {"sid": random_string(16)}
+
             raise SynapseError(400, "Email not found", Codes.THREEPID_NOT_FOUND)
 
         if self.config.threepid_behaviour_email == ThreepidBehaviour.REMOTE:
@@ -378,6 +383,11 @@ class EmailThreepidRequestTokenRestServlet(RestServlet):
         )
 
         if existing_user_id is not None:
+            if self.config.request_token_inhibit_3pid_errors:
+                # Make the client think the operation succeeded. See the rationale in the
+                # comments for request_token_inhibit_3pid_errors.
+                return 200, {"sid": random_string(16)}
+
             raise SynapseError(400, "Email is already in use", Codes.THREEPID_IN_USE)
 
         if self.config.threepid_behaviour_email == ThreepidBehaviour.REMOTE:
@@ -441,6 +451,11 @@ class MsisdnThreepidRequestTokenRestServlet(RestServlet):
         existing_user_id = await self.store.get_user_id_by_threepid("msisdn", msisdn)
 
         if existing_user_id is not None:
+            if self.hs.config.request_token_inhibit_3pid_errors:
+                # Make the client think the operation succeeded. See the rationale in the
+                # comments for request_token_inhibit_3pid_errors.
+                return 200, {"sid": random_string(16)}
+
             raise SynapseError(400, "MSISDN is already in use", Codes.THREEPID_IN_USE)
 
         if not self.hs.config.account_threepid_delegate_msisdn:
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index a09189b1b4..416489ae52 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -49,7 +49,7 @@ from synapse.http.servlet import (
 from synapse.push.mailer import load_jinja2_templates
 from synapse.util.msisdn import phone_number_to_msisdn
 from synapse.util.ratelimitutils import FederationRateLimiter
-from synapse.util.stringutils import assert_valid_client_secret
+from synapse.util.stringutils import assert_valid_client_secret, random_string
 from synapse.util.threepids import check_3pid_allowed
 
 from ._base import client_patterns, interactive_auth_handler
@@ -135,6 +135,11 @@ class EmailRegisterRequestTokenRestServlet(RestServlet):
         )
 
         if existing_user_id is not None:
+            if self.hs.config.request_token_inhibit_3pid_errors:
+                # Make the client think the operation succeeded. See the rationale in the
+                # comments for request_token_inhibit_3pid_errors.
+                return 200, {"sid": random_string(16)}
+
             raise SynapseError(400, "Email is already in use", Codes.THREEPID_IN_USE)
 
         if self.config.threepid_behaviour_email == ThreepidBehaviour.REMOTE:
@@ -202,6 +207,11 @@ class MsisdnRegisterRequestTokenRestServlet(RestServlet):
         )
 
         if existing_user_id is not None:
+            if self.hs.config.request_token_inhibit_3pid_errors:
+                # Make the client think the operation succeeded. See the rationale in the
+                # comments for request_token_inhibit_3pid_errors.
+                return 200, {"sid": random_string(16)}
+
             raise SynapseError(
                 400, "Phone number is already in use", Codes.THREEPID_IN_USE
             )