How US federal agencies can authenticate to AWS with multi-factor authentication

This post is part of a series about how AWS can help your US federal agency meet the requirements of the President’s Executive Order on Improving the Nation’s Cybersecurity. We recognize that government agencies have varying degrees of identity management and cloud maturity and that the requirement to implement multi-factor, risk-based authentication across an entire enterprise is a vast undertaking. This post specifically focuses on how you can use AWS information security practices to help meet the requirement to “establish multi-factor, risk-based authentication and conditional access across the enterprise” as it applies to your AWS environment.

This post focuses on the best-practices for enterprise authentication to AWS – specifically federated access via an existing enterprise identity provider (IdP).

Many federal customers use authentication factors on their Personal Identity Verification (PIV) or Common Access Cards (CAC) to authenticate to an existing enterprise identity service which can support Security Assertion Markup Language (SAML), which is then used to grant user access to AWS. SAML is an industry-standard protocol and most IdPs support a range of authentication methods, so if you’re not using a PIV or CAC, the concepts will still work for your organization’s multi-factor authentication (MFA) requirements.

Accessing AWS with MFA

There are two categories we want to look at for authentication to AWS services:

1. AWS APIs, which include access through the following:
   • The AWS Management Console.
   • The AWS Command Line Interface (AWS CLI).
2. Resources you launch that are running within your AWS VPC, which can include database engines or operating system environments such as Amazon Elastic Compute Cloud (Amazon EC2) instances, Amazon WorkSpaces, or Amazon AppStream 2.0.

There is also a third category of services where authentication occurs in AWS that is beyond the scope of this post: applications that you build on AWS that authenticate internal or external end users to those applications. For this category, multi-factor authentication is still important, but will vary based on the specifics of the application architecture. Workloads that sit behind an AWS Application Load Balancer can use the ALB to authenticate users using either Open ID Connect or SAML IdP that enforce MFA upstream.

MFA for the AWS APIs

AWS recommends that you use SAML and an IdP that enforces MFA as your means of granting users access to AWS. Many government customers achieve AWS federated authentication with Active Directory Federation Services (AD FS). The IdP used by our federal government customers should enforce usage of CAC/PIV to achieve MFA and be the sole means of access to AWS.

Federated authentication uses SAML to assume an AWS Identity and Access Management (IAM) role for access to AWS resources. A role doesn’t have standard long-term credentials such as a password or access keys associated with it. Instead, when you assume a role, it provides you with temporary security credentials for your role session.

AWS accounts in all AWS Regions, including AWS GovCloud (US) Regions, have the same authentication options for IAM roles through identity federation with a SAML IdP. The AWS Single Sign-on (SSO) service is another way to implement federated authentication to the AWS APIs in regions where it is available.

MFA for AWS CLI access

In AWS Regions excluding AWS GovCloud (US), you can consider using the AWS CloudShell service, which is an interactive shell environment that runs in your web browser and uses the same authentication pipeline that you use to access the AWS Management Console—thus inheriting MFA enforcement from your SAML IdP.

If you need to use federated authentication with MFA for the CLI on your own workstation, you’ll need to retrieve and present the SAML assertion token. For information about how you can do this in Windows environments, see the blog post How to Set Up Federated API Access to AWS by Using Windows PowerShell. For information about how to do this with Python, see How to Implement a General Solution for Federated API/CLI Access Using SAML 2.0.

Conditional access

IAM permissions policies support conditional access. Common use cases include allowing certain actions only from a specified, trusted range of IP addresses; granting access only to specified AWS Regions; and granting access only to resources with specific tags. You should create your IAM policies to provide least-privilege access across a number of attributes. For example, you can grant an administrator access to launch or terminate an EC2 instance only if the request originates from a certain IP address and is tagged with an appropriate tag.

You can also implement conditional access controls using SAML session tags provided by their IdP and passed through the SAML assertion to be consumed by AWS. This means two separate users from separate departments can assume the same IAM role but have tailored, dynamic permissions. As an example, the SAML IdP can provide each individual’s cost center as a session tag on the role assertion. IAM policy statements can be written to allow the user from cost center A the ability to administer resources from cost center A, but not resources from cost center B.

Many customers ask about how to limit control plane access to certain IP addresses. AWS supports this, but there is an important caveat to highlight. Some AWS services, such as AWS CloudFormation, perform actions on behalf of an authorized user or role, and execute from within the AWS cloud’s own IP address ranges. See this document for an example of a policy statement using the aws:ViaAWSService condition key to exclude AWS services from your IP address restrictions to avoid unexpected authorization failures.

Multi-factor authentication to resources you launch

You can launch resources such as Amazon WorkSpaces, AppStream 2.0, Redshift, and EC2 instances that you configure to require MFA. The Amazon WorkSpaces Streaming Protocol (WSP) supports CAC/PIV authentication for pre-authentication, and in-session access to the smart card. For more information, see Use smart cards for authentication. To see a short video of it in action, see the blog post Amazon WorkSpaces supports CAC/PIV smart card authentication. Redshift and AppStream 2.0 support SAML 2.0 natively, so you can configure those services to work with your SAML IdP similarly to how you configure AWS Console access and inherit the MFA enforced by the upstream IdP.

MFA access to EC2 instances can occur via the existing methods and enterprise directories used in your on-premises environments. You can, of course, implement other systems that enforce MFA access to an operating system such as RADIUS or other third-party directory or MFA token solutions.

Shell access with Systems Manager Session Manager

An alternative method for MFA for shell access to EC2 instances is to use the Session Manager feature of AWS Systems Manager. Session Manager uses the Systems Manager management agent to provide role-based access to a shell (PowerShell on Windows) on an instance. Users can access Session Manager from the AWS Console or from the command line with the Session Manager AWS CLI plugin. Similar to using CloudShell for CLI access, accessing EC2 hosts via Session Manager uses the same authentication pipeline you use for accessing the AWS control plane. Your interactive session on that host can be configured for audit logging.

Security best practices in IAM

The focus of this blog is on integrating an agency’s existing MFA-enabled enterprise authentication service; but to make it easier for you to view the entire security picture, you might be interested in IAM security best practices. You can enforce these best-practice security configurations with AWS Organizations Service Control Policies.

Conclusion

This post covered methods your federal agency should consider in your efforts to apply the multi-factor authentication (MFA) requirements in the Executive Order on Improving the Nation’s Cybersecurity to your AWS environment. To learn more about how AWS can help you meet the requirements of the executive order, see the other posts in this series:

• How AWS can help your US federal agency meet the executive order on improving the nation’s cybersecurity
• How US federal agencies can use AWS to improve logging and log retention
• How US federal agencies can use AWS to encrypt data at rest and in transit

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