diff --git a/system/config/security.yaml b/system/config/security.yaml
index 1a00bfb0e..b596d0051 100644
--- a/system/config/security.yaml
+++ b/system/config/security.yaml
@@ -31,6 +31,7 @@ xss_dangerous_tags:
   - bgsound
   - title
   - base
+  - isindex
 uploads_dangerous_extensions:
   - php
   - php2