Drop support for delegating email validation (#13192)

* Drop support for delegating email validation

Delegating email validation to an IS is insecure (since it allows the owner of
the IS to do a password reset on your HS), and has long been deprecated. It
will now cause a config error at startup.

* Update unit test which checks for email verification

Give it an `email` config instead of a threepid delegate

* Remove unused method `requestEmailToken`

* Simplify config handling for email verification

Rather than an enum and a boolean, all we need here is a single bool, which
says whether we are or are not doing email verification.

* update docs

* changelog

* upgrade.md: fix typo

* update version number

this will be in 1.64, not 1.63

* update version number

this one too

2 files changed, 15 insertions, 41 deletions

diff --git a/synapse/config/emailconfig.py b/synapse/config/emailconfig.py
index 6e11fbdb9a..3ead80d985 100644
--- a/synapse/config/emailconfig.py
+++ b/synapse/config/emailconfig.py
@@ -18,7 +18,6 @@
 import email.utils
 import logging
 import os
-from enum import Enum
 from typing import Any
 
 import attr
@@ -131,41 +130,22 @@ class EmailConfig(Config):
 
         self.email_enable_notifs = email_config.get("enable_notifs", False)
 
-        self.threepid_behaviour_email = (
-            # Have Synapse handle the email sending if account_threepid_delegates.email
-            # is not defined
-            # msisdn is currently always remote while Synapse does not support any method of
-            # sending SMS messages
-            ThreepidBehaviour.REMOTE
-            if self.root.registration.account_threepid_delegate_email
-            else ThreepidBehaviour.LOCAL
-        )
-
         if config.get("trust_identity_server_for_password_resets"):
             raise ConfigError(
                 'The config option "trust_identity_server_for_password_resets" '
-                'has been replaced by "account_threepid_delegate". '
-                "Please consult the configuration manual at docs/usage/configuration/config_documentation.md for "
-                "details and update your config file."
+                "is no longer supported. Please remove it from the config file."
             )
 
-        self.local_threepid_handling_disabled_due_to_email_config = False
-        if (
-            self.threepid_behaviour_email == ThreepidBehaviour.LOCAL
-            and email_config == {}
-        ):
-            # We cannot warn the user this has happened here
-            # Instead do so when a user attempts to reset their password
-            self.local_threepid_handling_disabled_due_to_email_config = True
-
-            self.threepid_behaviour_email = ThreepidBehaviour.OFF
+        # If we have email config settings, assume that we can verify ownership of
+        # email addresses.
+        self.can_verify_email = email_config != {}
 
         # Get lifetime of a validation token in milliseconds
         self.email_validation_token_lifetime = self.parse_duration(
             email_config.get("validation_token_lifetime", "1h")
         )
 
-        if self.threepid_behaviour_email == ThreepidBehaviour.LOCAL:
+        if self.can_verify_email:
             missing = []
             if not self.email_notif_from:
                 missing.append("email.notif_from")
@@ -356,18 +336,3 @@ class EmailConfig(Config):
                     "Config option email.invite_client_location must be a http or https URL",
                     path=("email", "invite_client_location"),
                 )
-
-
-class ThreepidBehaviour(Enum):
-    """
-    Enum to define the behaviour of Synapse with regards to when it contacts an identity
-    server for 3pid registration and password resets
-
-    REMOTE = use an external server to send tokens
-    LOCAL = send tokens ourselves
-    OFF = disable registration via 3pid and password resets
-    """
-
-    REMOTE = "remote"
-    LOCAL = "local"
-    OFF = "off"
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index fcf99be092..685a0423c5 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -20,6 +20,13 @@ from synapse.config._base import Config, ConfigError
 from synapse.types import JsonDict, RoomAlias, UserID
 from synapse.util.stringutils import random_string_with_symbols, strtobool
 
+NO_EMAIL_DELEGATE_ERROR = """\
+Delegation of email verification to an identity server is no longer supported. To
+continue to allow users to add email addresses to their accounts, and use them for
+password resets, configure Synapse with an SMTP server via the `email` setting, and
+remove `account_threepid_delegates.email`.
+"""
+
 
 class RegistrationConfig(Config):
     section = "registration"
@@ -51,7 +58,9 @@ class RegistrationConfig(Config):
         self.bcrypt_rounds = config.get("bcrypt_rounds", 12)
 
         account_threepid_delegates = config.get("account_threepid_delegates") or {}
-        self.account_threepid_delegate_email = account_threepid_delegates.get("email")
+        if "email" in account_threepid_delegates:
+            raise ConfigError(NO_EMAIL_DELEGATE_ERROR)
+        # self.account_threepid_delegate_email = account_threepid_delegates.get("email")
         self.account_threepid_delegate_msisdn = account_threepid_delegates.get("msisdn")
         self.default_identity_server = config.get("default_identity_server")
         self.allow_guest_access = config.get("allow_guest_access", False)