diff --git a/src/snek/system/markdown.py b/src/snek/system/markdown.py
index b708666..6d90d1c 100644
--- a/src/snek/system/markdown.py
+++ b/src/snek/system/markdown.py
@@ -14,7 +14,7 @@ from pygments.lexers import get_lexer_by_name
 
 class MarkdownRenderer(HTMLRenderer):
 
-    _allow_harmful_protocols = True
+    _allow_harmful_protocols = False
 
     def __init__(self, app, template):
         super().__init__(False, True)