diff --git a/changelog.d/6262.bugfix b/changelog.d/6262.bugfix
new file mode 100644
index 0000000000..32687f0d2b
--- /dev/null
+++ b/changelog.d/6262.bugfix
@@ -0,0 +1 @@
+Improve signature checking on some federation APIs.
diff --git a/synapse/federation/federation_base.py b/synapse/federation/federation_base.py
index 5a1e23a145..223aace0d9 100644
--- a/synapse/federation/federation_base.py
+++ b/synapse/federation/federation_base.py
@@ -278,9 +278,7 @@ def _check_sigs_on_pdus(keyring, room_version, pdus):
             pdu_to_check.sender_domain,
             e.getErrorMessage(),
         )
-        # XX not really sure if these are the right codes, but they are what
-        # we've done for ages
-        raise SynapseError(400, errmsg, Codes.UNAUTHORIZED)
+        raise SynapseError(403, errmsg, Codes.FORBIDDEN)
 
     for p, d in zip(pdus_to_check_sender, more_deferreds):
         d.addErrback(sender_err, p)
@@ -314,8 +312,7 @@ def _check_sigs_on_pdus(keyring, room_version, pdus):
                 "event id %s: unable to verify signature for event id domain: %s"
                 % (pdu_to_check.pdu.event_id, e.getErrorMessage())
             )
-            # XX as above: not really sure if these are the right codes
-            raise SynapseError(400, errmsg, Codes.UNAUTHORIZED)
+            raise SynapseError(403, errmsg, Codes.FORBIDDEN)
 
         for p, d in zip(pdus_to_check_event_id, more_deferreds):
             d.addErrback(event_err, p)
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 21e52c9695..5fc7c1d67b 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -370,6 +370,7 @@ class FederationServer(FederationBase):
         pdu = event_from_pdu_json(content, format_ver)
         origin_host, _ = parse_server_name(origin)
         yield self.check_server_matches_acl(origin_host, pdu.room_id)
+        pdu = yield self._check_sigs_and_hash(room_version, pdu)
         ret_pdu = yield self.handler.on_invite_request(origin, pdu)
         time_now = self._clock.time_msec()
         return {"event": ret_pdu.get_pdu_json(time_now)}
@@ -386,6 +387,9 @@ class FederationServer(FederationBase):
         yield self.check_server_matches_acl(origin_host, pdu.room_id)
 
         logger.debug("on_send_join_request: pdu sigs: %s", pdu.signatures)
+
+        pdu = yield self._check_sigs_and_hash(room_version, pdu)
+
         res_pdus = yield self.handler.on_send_join_request(origin, pdu)
         time_now = self._clock.time_msec()
         return (
@@ -421,6 +425,9 @@ class FederationServer(FederationBase):
         yield self.check_server_matches_acl(origin_host, pdu.room_id)
 
         logger.debug("on_send_leave_request: pdu sigs: %s", pdu.signatures)
+
+        pdu = yield self._check_sigs_and_hash(room_version, pdu)
+
         yield self.handler.on_send_leave_request(origin, pdu)
         return 200, {}
 
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 4b4c6c15f9..488058fe68 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -1222,7 +1222,6 @@ class FederationHandler(BaseHandler):
         Returns:
             Deferred[FrozenEvent]
         """
-
         if get_domain_from_id(user_id) != origin:
             logger.info(
                 "Got /make_join request for user %r from different origin %s, ignoring",
@@ -1280,11 +1279,20 @@ class FederationHandler(BaseHandler):
         event = pdu
 
         logger.debug(
-            "on_send_join_request: Got event: %s, signatures: %s",
+            "on_send_join_request from %s: Got event: %s, signatures: %s",
+            origin,
             event.event_id,
             event.signatures,
         )
 
+        if get_domain_from_id(event.sender) != origin:
+            logger.info(
+                "Got /send_join request for user %r from different origin %s",
+                event.sender,
+                origin,
+            )
+            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)
+
         event.internal_metadata.outlier = False
         # Send this event on behalf of the origin server.
         #
@@ -1503,6 +1511,14 @@ class FederationHandler(BaseHandler):
             event.signatures,
         )
 
+        if get_domain_from_id(event.sender) != origin:
+            logger.info(
+                "Got /send_leave request for user %r from different origin %s",
+                event.sender,
+                origin,
+            )
+            raise SynapseError(403, "User not from origin", Codes.FORBIDDEN)
+
         event.internal_metadata.outlier = False
 
         context = yield self._handle_new_event(origin, event)