diff --git a/src/calibre/srv/render_book.py b/src/calibre/srv/render_book.py
index 6c565c46ec..3e123ef796 100644
--- a/src/calibre/srv/render_book.py
+++ b/src/calibre/srv/render_book.py
@@ -211,6 +211,7 @@ class Container(ContainerBase):
                         a.set('data-' + link_uid, json.dumps({'name':parts[0], 'frag':parts[1]}, ensure_ascii=False))
                     else:
                         a.set('target', '_blank')
+                        a.set('rel', 'noopener')
                     changed.add(name)
             elif mt == 'image/svg+xml':
                 self.virtualized_names.add(name)