How to use AWS command line (CLI) with Google Authenticator as virtual MFA device

Written by Claudio Kuenzler - 0 comments

Published on November 22nd 2021 - Listed in AWS Security Linux Cloud

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When MFA (multi factor authentication) is enabled on your IAM user in AWS, the login in the browser asks for an additional input after the username/password credentials:

The MFA code is a temporary combination or number, also called a "token". This token can be retrieved from the device or software, such as Google Authenticator.

In the browser, logging into the "AWS Console", it is pretty much self explaining where to enter the current MFA token. Once the token is verified, the user is logged in and that's it.

But how does that apply when using the aws command line (cli)?

Note: See this article how to configure and use the aws cli using an AWS Access Key ID and Secret Access Key.

aws cli: UnauthorizedOperation

When using your AWS API credentials and your IAM user has MFA enabled, the following error message will appear when using the aws command:

ck@mintp ~ $ aws ec2 describe-instances

An error occurred (UnauthorizedOperation) when calling the DescribeInstances operation: You are not authorized to perform this operation.

Obviously AWS has detected that MFA has been enabled for this IAM user and rejects access.

Retrieve the MFA device ID

To use aws cli with multi factor authentication enabled, a new "temporary login" must be created. This login includes the MFA device (Google Authenticator in my case) and the current token from it.

The command to create the temporary login is the following:

aws sts get-session-token --serial-number <DEVICE-ID> --token-code <MFA TOKEN>

Obviously the <DEVICE-ID> and <MFA TOKEN> placeholders must be replaced with the real data. To find the device ID of the MFA device assigned to your own user, log into AWS Console -> go to IAM -> select your user -> switch to the "Security credentials" tab. You should find the device ID in the row "Assigned MFA device".

The device ID is basically arn:aws:iam::AWS-USER-ID:mfa/AWS-USER.

Copy this part as this is your MFA device ID.

Create new MFA AWS session with aws cli

Now that the MFA device ID of the virtual device (Google Authenticator) is known, a new AWS session can be launched. Use the current code which is shown in Google Authenticator as token code.

If the login was successful, the response shows a JSON output with a set of credentials:

ck@mintp ~ $ aws sts get-session-token --serial-number arn:aws:iam::440XXXXXXXXX:mfa/me@example.com --token-code 976154
{
    "Credentials": {
        "AccessKeyId": "ASIXXXXXXXXXXXXO2WUU",
        "SecretAccessKey": "cRXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX3wR",
        "SessionToken": "FwoGZXIvYXdzEHAaDBZ2SekE/BrZbtxgZSKGAYVebDRsr9BEx1KI565HlkCxyJt0oMPwcoxo88L2MQeavSjwnmgpM/ksNVl/P6ycn3SICL/rSSaoEnuE6CWy2SRB0fcO/5dN+eIt2+AcBuI7Puu6pW5LVPrRQHgAMNOrJ2KggOoAj/6dm2Ix71w1hnVpYfSNeNyjL/TzvIba639wHWsWKD3KKIbv7IwGMihcI+/Cc2V9JDx8Kra9ZWFfkxFVWmealZAJ6CISCrbK63eQsKU5cIrq",
        "Expiration": "2021-11-22T18:24:06Z"
    }
}

The output also shows the expiration of this session, which by defaults is set to 12 hours.

The credentials from the output must now be used (exported) as environment variables. On a Linux machine:

ck@mintp ~ $ export AWS_ACCESS_KEY_ID=ASIXXXXXXXXXXXXO2WUU
ck@mintp ~ $ export AWS_SECRET_ACCESS_KEY=cRXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX3wR
ck@mintp ~ $ export AWS_SESSION_TOKEN=FwoGZXIvYXdzEHAaDBZ2SekE/BrZbtxgZSKGAYVebDRsr9BEx1KI565HlkCxyJt0oMPwcoxo88L2MQeavSjwnmgpM/ksNVl/P6ycn3SICL/rSSaoEnuE6CWy2SRB0fcO/5dN+eIt2+AcBuI7Puu6pW5LVPrRQHgAMNOrJ2KggOoAj/6dm2Ix71w1hnVpYfSNeNyjL/TzvIba639wHWsWKD3KKIbv7IwGMihcI+/Cc2V9JDx8Kra9ZWFfkxFVWmealZAJ6CISCrbK63eQsKU5cIrq

With these environment variables defined, the aws command line can now be used:

ck@mintp ~ $ aws ec2 describe-instances | head
{
    "Reservations": [
        {
            "Groups": [],
            "Instances": [
                {
                    "AmiLaunchIndex": 0,
                    "ImageId": "ami-d7f32ab8",
                    "InstanceId": "i-06XXXXXXXXXX29fbd",
                    "InstanceType": "t3.large",

No authentication error this time. The aws command line works again - at least until the MFA session expires in 12 hours.

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