Application Load Balancers should be configured to drop HTTP headers


This control checks AWS Application Load Balancers are set to discard invalid HTTP headers. The control fails if the setting routing.http.drop_invalid_header_fields.enabled is false. By default, Application Load Balancers do not drop invalid HTTP header values. Discarding these invalid headers is essential to prevent HTTP desynchronization attacks.


From the console

To configure the load balancer to drop invalid header fields:

  1. Open the Amazon EC2 console at
  2. In the navigation pane, choose Load balancers.
  3. Select an Application Load Balancer.
  4. From Actions, choose Edit attributes.
  5. Under Drop Invalid Header Fields, choose Enable.
  6. Click Save.