Control: 4 Auto Scaling group launch configuration should not have metadata response hop limit greater than 1

Description

This control checks the number of network hops that a metadata token can travel. The control fails if the metadata response hop limit is greater than 1.

The Instance Metadata Service (IMDS) provides metadata information about an Amazon EC2 instance and is useful for application configuration. Restricting the HTTP PUT response for the metadata service to only the EC2 instance protects the IMDS from unauthorized use.

The Time To Live (TTL) field in the IP packet is reduced by one on every hop. This reduction can be used to ensure that the packet does not travel outside EC2. IMDSv2 protects EC2 instances that may have been misconfigured as open routers, layer 3 firewalls, VPNs, tunnels, or NAT devices, which prevents unauthorized users from retrieving metadata. With IMDSv2, the PUT response that contains the secret token cannot travel outside the instance because the default metadata response hop limit is set to 1. However, if this value is greater than 1, the token can leave the EC2 instance.

Remediation

For detailed instructions on how to modify the metadata response hop limit for an existing launch configuration, see Modify instance metadata options for existing instances in the Amazon EC2 User Guide for Linux Instances.