1 files changed, 20 insertions, 0 deletions
diff --git a/tests/rest/client/test_third_party_rules.py b/tests/rest/client/test_third_party_rules.py
index d404550800..0048bea54a 100644
--- a/tests/rest/client/test_third_party_rules.py
+++ b/tests/rest/client/test_third_party_rules.py
@@ -114,6 +114,26 @@ class ThirdPartyRulesTestCase(unittest.HomeserverTestCase):
         self.render(request)
         self.assertEquals(channel.result["code"], b"403", channel.result)
 
+    def test_cannot_modify_event(self):
+        """cannot accidentally modify an event before it is persisted"""
+
+        # first patch the event checker so that it will try to modify the event
+        async def check(ev: EventBase, state):
+            ev.content = {"x": "y"}
+            return True
+
+        current_rules_module().check_event_allowed = check
+
+        # now send the event
+        request, channel = self.make_request(
+            "PUT",
+            "/_matrix/client/r0/rooms/%s/send/modifyme/1" % self.room_id,
+            {"x": "x"},
+            access_token=self.tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.result["code"], b"500", channel.result)
+
     def test_modify_event(self):
         """The module can return a modified version of the event"""
         # first patch the event checker so that it will modify the event