Private application load balancers should drop HTTP headers

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Description

This control checks that private AWS Application Load Balancers (ALBs) are set to discard invalid HTTP headers. The control fails if the setting routing.http.drop_invalid_header_fields.enabled is false, and skips public-facing ALBs as those are covered in a separate finding. By default, ALBs do not drop invalid HTTP header values. Discarding these invalid headers is essential to prevent HTTP desynchronization attacks.

Remediation

From the console

To configure the load balancer to drop invalid header fields:

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.
  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.