Changes/CloudEC2IMDSv2Only

Register EC2 Cloud Images with IMDSv2-only AMI flag

This is a proposed Change for Fedora Linux.
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.

Summary

In November 2019, AWS launched IMDSv2 (Instance Meta-Data Store version 2 - see https://aws.amazon.com/blogs/security/defense-in-depth-open-firewalls-reverse-proxies-ssrf-vulnerabilities-ec2-instance-metadata-service/ ) which provides "belt and suspenders" protections for four types of vulnerabilities that could be used to try to access the Instance Meta-Data Store available to EC2 instances. In that announcement, AWS recommended adopting IMDSv2 and restricting access to IMDSv2 only for added security. This can be done at instance launch time, or (more recently in October 2022) by providing a flag when registering an AMI to indicate that the AMI should by default launch with IMDSv1 disabled, and thus require IMDSv2.

By enabling this flag for Fedora, we provide a better security posture for Fedora users running in EC2.

When an AMI is registered for IMDSv2 it is still possible to launch instances with IMDSv1 enabled by providing the right option to the RunInstances EC2 API call. The flag merely switches the default.

Owner

• Name: Stewart Smith David Duncan
• Email: trawets@amazon.com

Current status

• Targeted release: Fedora Linux 39
• Last updated: 2023-03-20
• devel thread
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

Attached locally to every EC2 instance, the Instance Meta-Data Service runs on a special "link local" IP address of 169.254.169.254 that means only software running on the instance can access it. For applications with access to IMDS, it makes available metadata about the instance, its network, and its storage. The IMDS also makes the AWS credentials available for any IAM role that is attached to the instance.

IMDS is the primary data source for cloud-init on EC2, and various other utilities will also access it.

The IMDSv2 announcement gives more details as to the "belt and suspenders" protections it brings for four types of vulnerabilities that could be used to try to access the IMDS.

By default, registering and then launching an AMI will launch an EC2 instance where both IMDSv1 and IMDSv2 is enabled. A recent addition to the EC2 API is the ability to register an AMI with a flag that indicates that the default behavior when launching an instance should be to have IMDSv2 enabled, and disable IMDSv1.

The proposal is to (starting with Fedora 39), register EC2 AMIs with this flag set as documented in the EC2 User Guide.

Feedback

While Amazon Linux 2023 (then called AL2022) was in Tech Preview, its AMIs have been registered with this flag the flag was announced in October 2022.

During this time, we have not received any negative feedback about this change. The only user of IMDSv1 calls that we have so far had to migrate to IMDSv2 calls has been some internal test cases run by a service team.

Benefit to Fedora

This change will provide Fedora users on EC2 with an enhanced security posture by default.

Scope

• Proposal owners:

Modify AMI registration to include the flag. No other technical work is required.

• Other developers:

Any remaining code that talks to IMDS that does not use IMDSv2 will need to be adapted to continue to work by default.

• Release engineering: #Releng issue number

• Policies and guidelines: N/A (not needed for this Change)

• Trademark approval: N/A (not needed for this Change)

• Alignment with Objectives:

Upgrade/compatibility impact

No impact for existing EC2 Instances. The AMI flag only affects new instance launches.

How To Test

Testing will not change from any regular Fedora EC2 AMI. The only additional check will need to be that the parameter is set correctly.

User Experience

This change should be transparent to users.

Dependencies

No dependencies.

Contingency Plan

• Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change)
• Contingency deadline: N/A (not a System Wide Change)
• Blocks release? N/A (not a System Wide Change), Yes/No

Documentation

• https://aws.amazon.com/blogs/security/defense-in-depth-open-firewalls-reverse-proxies-ssrf-vulnerabilities-ec2-instance-metadata-service/
• https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-IMDS-new-instances.html#configure-IMDS-new-instances-ami-configuration
• https://aws.amazon.com/about-aws/whats-new/2022/10/amazon-machine-images-support-instance-metadata-service-version-2-default/
• https://docs.aws.amazon.com/linux/al2023/ug/compare-with-al2.html#imdsv2

N/A (not a System Wide Change)