2 files changed, 42 insertions, 0 deletions
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index 9bd0d764f8..91edfd45d7 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -88,6 +88,7 @@ from synapse.rest.admin.users import (
     UserByThreePid,
     UserMembershipRestServlet,
     UserRegisterServlet,
+    UserReplaceMasterCrossSigningKeyRestServlet,
     UserRestServletV2,
     UsersRestServletV2,
     UserTokenRestServlet,
@@ -292,6 +293,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListDestinationsRestServlet(hs).register(http_server)
     RoomMessagesRestServlet(hs).register(http_server)
     RoomTimestampToEventRestServlet(hs).register(http_server)
+    UserReplaceMasterCrossSigningKeyRestServlet(hs).register(http_server)
     UserByExternalId(hs).register(http_server)
     UserByThreePid(hs).register(http_server)
 
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 73878dd99d..9900498fbe 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -1270,6 +1270,46 @@ class AccountDataRestServlet(RestServlet):
         }
 
 
+class UserReplaceMasterCrossSigningKeyRestServlet(RestServlet):
+    """Allow a given user to replace their master cross-signing key without UIA.
+
+    This replacement is permitted for a limited period (currently 10 minutes).
+
+    While this is exposed via the admin API, this is intended for use by the
+    Matrix Authentication Service rather than server admins.
+    """
+
+    PATTERNS = admin_patterns(
+        "/users/(?P<user_id>[^/]*)/_allow_cross_signing_replacement_without_uia"
+    )
+    REPLACEMENT_PERIOD_MS = 10 * 60 * 1000  # 10 minutes
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastores().main
+
+    async def on_POST(
+        self,
+        request: SynapseRequest,
+        user_id: str,
+    ) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        if user_id is None:
+            raise NotFoundError("User not found")
+
+        timestamp = (
+            await self._store.allow_master_cross_signing_key_replacement_without_uia(
+                user_id, self.REPLACEMENT_PERIOD_MS
+            )
+        )
+
+        if timestamp is None:
+            raise NotFoundError("User has no master cross-signing key")
+
+        return HTTPStatus.OK, {"updatable_without_uia_before_ms": timestamp}
+
+
 class UserByExternalId(RestServlet):
     """Find a user based on an external ID from an auth provider"""