diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index c6f50fd7b9..cf7ee60d3a 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -57,6 +57,9 @@ class RoomVersion(object):
     state_res = attr.ib()  # int; one of the StateResolutionVersions
     enforce_key_validity = attr.ib()  # bool
 
+    # bool: before MSC2260, anyone was allowed to send an aliases event
+    special_case_aliases_auth = attr.ib(type=bool, default=False)
+
 
 class RoomVersions(object):
     V1 = RoomVersion(
@@ -65,6 +68,7 @@ class RoomVersions(object):
         EventFormatVersions.V1,
         StateResolutionVersions.V1,
         enforce_key_validity=False,
+        special_case_aliases_auth=True,
     )
     V2 = RoomVersion(
         "2",
@@ -72,6 +76,7 @@ class RoomVersions(object):
         EventFormatVersions.V1,
         StateResolutionVersions.V2,
         enforce_key_validity=False,
+        special_case_aliases_auth=True,
     )
     V3 = RoomVersion(
         "3",
@@ -79,6 +84,7 @@ class RoomVersions(object):
         EventFormatVersions.V2,
         StateResolutionVersions.V2,
         enforce_key_validity=False,
+        special_case_aliases_auth=True,
     )
     V4 = RoomVersion(
         "4",
@@ -86,6 +92,7 @@ class RoomVersions(object):
         EventFormatVersions.V3,
         StateResolutionVersions.V2,
         enforce_key_validity=False,
+        special_case_aliases_auth=True,
     )
     V5 = RoomVersion(
         "5",
@@ -93,6 +100,14 @@ class RoomVersions(object):
         EventFormatVersions.V3,
         StateResolutionVersions.V2,
         enforce_key_validity=True,
+        special_case_aliases_auth=True,
+    )
+    MSC2260_DEV = RoomVersion(
+        "org.matrix.msc2260",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.V3,
+        StateResolutionVersions.V2,
+        enforce_key_validity=True,
     )
 
 
@@ -104,5 +119,6 @@ KNOWN_ROOM_VERSIONS = {
         RoomVersions.V3,
         RoomVersions.V4,
         RoomVersions.V5,
+        RoomVersions.MSC2260_DEV,
     )
 }  # type: Dict[str, RoomVersion]
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index 016d5678e5..3240e8a7b2 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -100,7 +100,12 @@ def check(
             if not event.signatures.get(event_id_domain):
                 raise AuthError(403, "Event not signed by sending server")
 
+    # Implementation of https://matrix.org/docs/spec/rooms/v1#authorization-rules
+    #
+    # 1. If type is m.room.create:
     if event.type == EventTypes.Create:
+        # 1b. If the domain of the room_id does not match the domain of the sender,
+        # reject.
         sender_domain = get_domain_from_id(event.sender)
         room_id_domain = get_domain_from_id(event.room_id)
         if room_id_domain != sender_domain:
@@ -108,40 +113,49 @@ def check(
                 403, "Creation event's room_id domain does not match sender's"
             )
 
+        # 1c. If content.room_version is present and is not a recognised version, reject
         room_version_prop = event.content.get("room_version", "1")
         if room_version_prop not in KNOWN_ROOM_VERSIONS:
             raise AuthError(
                 403,
                 "room appears to have unsupported version %s" % (room_version_prop,),
             )
-        # FIXME
+
         logger.debug("Allowing! %s", event)
         return
 
+    # 3. If event does not have a m.room.create in its auth_events, reject.
     creation_event = auth_events.get((EventTypes.Create, ""), None)
-
     if not creation_event:
         raise AuthError(403, "No create event in auth events")
 
+    # additional check for m.federate
     creating_domain = get_domain_from_id(event.room_id)
     originating_domain = get_domain_from_id(event.sender)
     if creating_domain != originating_domain:
         if not _can_federate(event, auth_events):
             raise AuthError(403, "This room has been marked as unfederatable.")
 
-    # FIXME: Temp hack
+    # 4. If type is m.room.aliases
     if event.type == EventTypes.Aliases:
+        # 4a. If event has no state_key, reject
         if not event.is_state():
             raise AuthError(403, "Alias event must be a state event")
         if not event.state_key:
             raise AuthError(403, "Alias event must have non-empty state_key")
+
+        # 4b. If sender's domain doesn't matches [sic] state_key, reject
         sender_domain = get_domain_from_id(event.sender)
         if event.state_key != sender_domain:
             raise AuthError(
                 403, "Alias event's state_key does not match sender's domain"
             )
-        logger.debug("Allowing! %s", event)
-        return
+
+        # 4c. Otherwise, allow.
+        # This is removed by https://github.com/matrix-org/matrix-doc/pull/2260
+        if room_version.special_case_aliases_auth:
+            logger.debug("Allowing! %s", event)
+            return
 
     if logger.isEnabledFor(logging.DEBUG):
         logger.debug("Auth events: %s", [a.event_id for a in auth_events.values()])