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

synapse/handlers/identity.py

@@ -26,7 +26,6 @@ from synapse.api.errors import (
     SynapseError,
 )
 from synapse.api.ratelimiting import Ratelimiter
-from synapse.config.emailconfig import ThreepidBehaviour
 from synapse.http import RequestTimedOutError
 from synapse.http.client import SimpleHttpClient
 from synapse.http.site import SynapseRequest
@@ -434,48 +433,6 @@ class IdentityHandler:
 
         return session_id
 
-    async def requestEmailToken(
-        self,
-        id_server: str,
-        email: str,
-        client_secret: str,
-        send_attempt: int,
-        next_link: Optional[str] = None,
-    ) -> JsonDict:
-        """
-        Request an external server send an email on our behalf for the purposes of threepid
-        validation.
-
-        Args:
-            id_server: The identity server to proxy to
-            email: The email to send the message to
-            client_secret: The unique client_secret sends by the user
-            send_attempt: Which attempt this is
-            next_link: A link to redirect the user to once they submit the token
-
-        Returns:
-            The json response body from the server
-        """
-        params = {
-            "email": email,
-            "client_secret": client_secret,
-            "send_attempt": send_attempt,
-        }
-        if next_link:
-            params["next_link"] = next_link
-
-        try:
-            data = await self.http_client.post_json_get_json(
-                id_server + "/_matrix/identity/api/v1/validate/email/requestToken",
-                params,
-            )
-            return data
-        except HttpResponseException as e:
-            logger.info("Proxied requestToken failed: %r", e)
-            raise e.to_synapse_error()
-        except RequestTimedOutError:
-            raise SynapseError(500, "Timed out contacting identity server")
-
     async def requestMsisdnToken(
         self,
         id_server: str,
@@ -549,18 +506,7 @@ class IdentityHandler:
         validation_session = None
 
         # Try to validate as email
-        if self.hs.config.email.threepid_behaviour_email == ThreepidBehaviour.REMOTE:
-            # Remote emails will only be used if a valid identity server is provided.
-            assert (
-                self.hs.config.registration.account_threepid_delegate_email is not None
-            )
-
-            # Ask our delegated email identity server
-            validation_session = await self.threepid_from_creds(
-                self.hs.config.registration.account_threepid_delegate_email,
-                threepid_creds,
-            )
-        elif self.hs.config.email.threepid_behaviour_email == ThreepidBehaviour.LOCAL:
+        if self.hs.config.email.can_verify_email:
             # Get a validated session matching these details
             validation_session = await self.store.get_threepid_validation_session(
                 "email", client_secret, sid=sid, validated=True