You can now set an EC2 Amazon Machine Image (AMI) to use Instance Metadata Service Version 2 (IMDSv2) by default. IMDSv2 is an enhancement to instance metadata access that requires session-oriented requests to add defense in depth against unauthorized metadata access. IMDSv2 requires a PUT request to initiate a session to the instance metadata service and retrieve a token. To set your instances as IMDSv2-only, you previously had to configure Instance Metadata Options during instance launch or update your instance after launch using the ModifyInstanceMetadataOptions API.
Source:: Amazon AWS