awsmfa

awsmfa helps AWS users use two factor authentication (MFA) with their ~/.aws/credentials file. awsmfa also makes it easy to rotate your access keys and to use role assumption. MFA, key rotation, and role assumption are three best practices to use when securing your AWS environment from attacks.

awsmfa is ideal for engineering teams that have long-lived AWS access keys sitting on developer machines. If you are looking for an easy way to improve your team's security, this might be a good starting point for you.

When do I want to use MFA?

Most teams give their engineers an AWS Access Key pair and they are written to an ~/.aws/credentials file, or stuffed in a .bashrc, and forgotten about. This is a security problem because those credentials are stored in plaintext and can be exfiltrated by malware, copied off of stolen laptops, etc.

One simple solution to this is to configure your AWS account so that those credentials are not useful unless the user has also asserted their identity by way of two-factor authentication.

IAM, S3, and other types of AWS Policies allow you to require that the user has recently verified their identity using two-factor authentication (MFA). This criteria is specified in the policy documents using a Condition block, like this:

"Condition": {
    "Bool": {
        "aws:MultiFactorAuthPresent": "true"
    }
}

Typical deployments involve attaching policies to your users that create two levels of access.

1. The first level allows very few privileges, such as only those necessary to identify themselves (iam:GetUser), enable an MFA device (iam:EnableMFADevice), list or resync their MFA devices (iam:ListMFADevices, iam:ResyncMFADevice), and to acquire temporary credentials (sts:GetSessionToken). These are available to the user just by merely proving they have the AWS Access Key ID and the AWS Secret Access Key.

2. The second level grants more privileges, but requires that the user has recently authenticated themselves using two-factor authentication. In small teams, the privileges may be as generous as full administrative access of the AWS account. In more secure environments, the policies can be arbitrarily fine.

You can read more about using MFA with AWS Policies in the AWS documentation.

Policies

If you have pre-existing IAM policies, the easiest way to get started is to just attach the sts:MultiFactorAuthPresent condition to any of your existing policies that you want to secure.

If you don't have any IAM policies yet, consider using the starter kit of basic IAM policies included with awsmfa. Most teams -- even teams of one -- will be able to use that template as-is and as a starting point for more customized policies. You can install these policies using the CloudFormation Console or using the command line examples from the template.

Getting Started

To use awsmfa, you'll need:

1. an IAM account (or a root account)

2. an MFA device attached to that account

3. to be allowed to call iam:GetUser, iam:ListMfaDevices, and iam:GetSessionToken.

4. your ~/.aws/credentials file configured with your long-lived AWS Access Key pair. If you don't have this, create one with:

   $ pip install --user awscli
   $ aws configure --profile identity
   

Installing awsmfa is as easy as installing a pip package:

$ pip install --user awsmfa
...

Once installed, running awsmfa will verify your MFA devices and then prompt you for the MFA code. Once AWS verifies your MFA code, awsmfa will write out your new temporary credentials to the [default] section of your ~/.aws/credentials file. The default section is read by most AWS SDKs automatically. Here's an example:

$ awsmfa
MFA Token Code: 123456
Temporary credentials will expire in 5:59:59.746453.

By default, the long-lived credentials (access key and secret key) are read from a profile called [identity] and the temporary credentials (access key, secret key, session token) are written to [default]. These are configurable with the --identity-profile and --target-profile flags, or AWS_MFA_IDENTITY_PROFILE and AWS_MFA_TARGET_PROFILE environment variables, respectively.

You can also provide the code from the command line:

$ awsmfa --token-code 123456
Temporary credentials will expire in 5:59:59.746453.

Most awsmfa behaviors are controlled by command line flags or environment variables. Run with --help for more details.

$ awsmfa --help
usage: awsmfa [-h] [--version] [--aws-credentials AWS_CREDENTIALS]
              [-d DURATION] [-i IDENTITY_PROFILE]
              [--serial-number SERIAL_NUMBER] [-t TARGET_PROFILE]
              [--role-session-name ROLE_SESSION_NAME] [-c TOKEN_CODE]
              [--rotate-identity-keys]
              [role-to-assume]
...

Skipping MFA

By default, awsmfa sends your MFA token code to AWS when acquiring temporary credentials. This is optional behavior. If you don't want to use MFA, pass -c skip or set the AWS_MFA_TOKEN_CODE environment variable to skip. Example:

$ awsmfa -c skip

Note that the temporary credentials obtained in this way will not satisfy the sts::MultiFactorAuthPresent condition variable.

Role Assumption

awsmfa can also help you with role assumption. If you'd like to assume a role, pass the full ARN of the role as the first parameter to awsmfa. Example:

$ awsmfa arn:aws:iam::123456789012:role/s3access

You can also customize the role session name:

$ awsmfa --role-session-name ingest arn:aws:iam::123456789012:role/s3access

Key Rotation

Rotating your access keys regularly is a good security practice. If your IAM user is allowed to call iam:ListAccessKeys, iam:DeleteAccessKeys, and iam:CreateAccessKey, awsmfa can also automatically rotate your access keys automatically when you acquire temporary credentials. Example:

$ awsmfa --rotate-identity-keys
MFA Token Code:
Temporary credentials will expire in 5:59:59.677774.
Rotating from AKIAIM55UP4UAQDYGNHA to AKIAJCB6F3RJ3GJFIUGQ.
work-eng profile updated.

If you want to rotate your identity keys every time you acquire temporary credentials, you can set the AWS_MFA_ROTATE_IDENTITY_KEYS environment variable. Example:

$ echo AWS_MFA_ROTATE_IDENTITY_KEYS=True >> ~/.bashrc

Setting Environment Variables

Some AWS tools can only read credentials from environment variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN). awsmfa --env will print shell commands to define those variables. Example:

$ awsmfa --env
Enter MFA Code: 123456
Temporary credentials will expire in 5:59:59.945582.
AWS_ACCESS_KEY_ID=ASIAIYM...; export AWS_ACCESS_KEY_ID;
AWS_SECRET_ACCESS_KEY=uyug...; export AWS_SECRET_ACCESS_KEY;
AWS_SESSION_TOKEN=FQoDY...; export AWS_SESSION_TOKEN;
AWS_SECURITY_TOKEN=FQoDY...; export AWS_SECURITY_TOKEN;

The prompt and expiration notice are written to stderr, and the environment variables are written to stdout, so you can also eval the output:

$ eval $(awsmfa --env)
Enter MFA Code: 123456
Temporary credentials will expire in 5:59:59.945582.
$ echo ${AWS_ACCESS_KEY_ID}
ASIA...

Note: both AWS_SESSION_TOKEN (the new standard name for the session token used in multiple SDKs) and AWS_SECURITY_TOKEN (used by older versions of boto) are emitted for backwards compatibility.