diff --git a/event.c b/event.c
index cd268211..2def5560 100644
--- a/event.c
+++ b/event.c
@@ -294,6 +294,10 @@ event_handle_configurerequest(void *data __attribute__ ((unused)),
         if (ev->value_mask & XCB_CONFIG_WINDOW_BORDER_WIDTH)
             client_setborder(c, ev->border_width);
 
+        /* Clients are not allowed to directly mess with stacking parameters. */
+        ev->value_mask &= ~(XCB_CONFIG_WINDOW_SIBLING |
+                                            XCB_CONFIG_WINDOW_STACK_MODE);
+
         if(c->isbanned)
         {
             /* We'll be sending protocol geometry, so don't readd borders and titlebar. */