Terraform unable to assume roles with MFA enabled

Question

I'm having a terrible time getting Terraform to assume an IAM role with another account with MFA required. Here's my setup

AWS Config

[default]
region = us-west-2
output = json

[profile GEHC-000]
region = us-west-2
output = json

....

[profile GEHC-056]
source_profile = GEHC-000
role_arn = arn:aws:iam::~069:role/hc/hc-master
mfa_serial = arn:aws:iam::~183:mfa/username
external_id = ~069

AWS Credentials

[default]
aws_access_key_id = xxx
aws_secret_access_key = xxx


[GEHC-000]
aws_access_key_id = same as above
aws_secret_access_key = same as above

Policies assigned to IAM user

STS Policy

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AssumeRole",
            "Effect": "Allow",
            "Action": [
                "sts:AssumeRole"
            ],
            "Resource": [
                "arn:aws:iam::*:role/hc/hc-master"
            ]
        }
    ]
}

User Policy

{
    "Statement": [
        {
            "Action": [
                "iam:*AccessKey*",
                "iam:*MFA*",
                "iam:*SigningCertificate*",
                "iam:UpdateLoginProfile*",
                "iam:RemoveUserFromGroup*"
            ],
            "Effect": "Allow",
            "Resource": [
                "arn:aws:iam::~183:mfa/${aws:username}",
                "arn:aws:iam::~183:mfa/*/${aws:username}",
                "arn:aws:iam::~183:mfa/*/*/${aws:username}",
                "arn:aws:iam::~183:mfa/*/*/*${aws:username}",
                "arn:aws:iam::~183:user/${aws:username}",
                "arn:aws:iam::~183:user/*/${aws:username}",
                "arn:aws:iam::~183:user/*/*/${aws:username}",
                "arn:aws:iam::~183:user/*/*/*${aws:username}"
            ],
            "Sid": "Write"
        },
        {
            "Action": [
                "iam:*Get*",
                "iam:*List*"
            ],
            "Effect": "Allow",
            "Resource": [
                "*"
            ],
            "Sid": "Read"
        },
        {
            "Action": [
                "iam:CreateUser*",
                "iam:UpdateUser*",
                "iam:AddUserToGroup"
            ],
            "Effect": "Allow",
            "Resource": [
                "*"
            ],
            "Sid": "CreateUser"
        }
    ],
    "Version": "2012-10-17"
}

Force MFA Policy

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "BlockAnyAccessOtherThanAboveUnlessSignedInWithMFA",
            "Effect": "Deny",
            "NotAction": "iam:*",
            "Resource": "*",
            "Condition": {
                "BoolIfExists": {
                    "aws:MultiFactorAuthPresent": "false"
                }
            }
        }
    ]
}

main.tf

provider "aws" {
  profile                 = "GEHC-056"
  shared_credentials_file = "${pathexpand("~/.aws/config")}"
  region                  = "${var.region}"
}

data "aws_iam_policy_document" "test" {
  statement {
    sid    = "TestAssumeRole"
    effect = "Allow"

    actions = [
      "sts:AssumeRole",
    ]

    principals = {
      type = "AWS"

      identifiers = [
        "arn:aws:iam::~183:role/hc-devops",
      ]
    }

    sid    = "BuUserTrustDocument"
    effect = "Allow"

    principals = {
      type = "Federated"

      identifiers = [
        "arn:aws:iam::~875:saml-provider/ge-saml-for-aws",
      ]
    }

    condition {
      test     = "StringEquals"
      variable = "SAML:aud"
      values   = ["https://signin.aws.amazon.com/saml"]
    }
  }
}

resource "aws_iam_role" "test_role" {
  name               = "test_role"
  path               = "/"
  assume_role_policy = "${data.aws_iam_policy_document.test.json}"
}

Get Caller Identity

bash-4.4$ aws --profile GEHC-056 sts get-caller-identity
Enter MFA code for arn:aws:iam::772660252183:mfa/503072343:
{
  "UserId": "AROAIWCCLC2BGRPQMJC7U:botocore-session-1537474244",
  "Account": "730993910069",
  "Arn": "arn:aws:sts::730993910069:assumed-role/hc-master/botocore-session-1537474244"
}

And the error:

bash-4.4$ terraform plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.


Error: Error refreshing state: 1 error(s) occurred:

* provider.aws: Error creating AWS session: AssumeRoleTokenProviderNotSetError: assume role with MFA enabled, but AssumeRoleTokenProvider session option not set.

Answer 1

One other way is to use credential_process in order to generate the credentials with a local script and cache the tokens in a new profile (let's call it tf_temp)

This script would :

• check if the token is still valid for the profile tf_temp

• if token is valid, extract the token from existing config using aws configure get xxx --profile tf_temp

• if token is not valid, prompt use to enter mfa token

• generate the session token with aws assume-role --token-code xxxx ... --profile your_profile

• set the temporary profile token tf_temp using aws configure set xxx --profile tf_temp

You would have:

~/.aws/credentials

[prod]
aws_secret_access_key = redacted
aws_access_key_id = redacted

[tf_temp]

[tf]
credential_process = sh -c 'mfa.sh arn:aws:iam::{account_id}:role/{role} arn:aws:iam::{account_id}:mfa/{mfa_entry} prod 2> $(tty)'

mfa.sh

gist

move this script in /bin/mfa.sh or /usr/local/bin/mfa.sh :

#!/bin/sh
set -e

role=$1
mfa_arn=$2
profile=$3
temp_profile=tf_temp

if [ -z $role ]; then echo "no role specified"; exit 1; fi
if [ -z $mfa_arn ]; then echo "no mfa arn specified"; exit 1; fi
if [ -z $profile ]; then echo "no profile specified"; exit 1; fi

resp=$(aws sts get-caller-identity --profile $temp_profile | jq '.UserId')

if [ ! -z $resp ]; then
    echo '{
        "Version": 1,
        "AccessKeyId": "'"$(aws configure get aws_access_key_id --profile $temp_profile)"'",
        "SecretAccessKey": "'"$(aws configure get aws_secret_access_key --profile $temp_profile)"'",
        "SessionToken": "'"$(aws configure get aws_session_token --profile $temp_profile)"'",
        "Expiration": "'"$(aws configure get expiration --profile $temp_profile)"'"
    }'
    exit 0
fi
read -p "Enter MFA token: " mfa_token

if [ -z $mfa_token ]; then echo "MFA token can't be empty"; exit 1; fi

data=$(aws sts assume-role --role-arn $role \
                    --profile $profile \
                    --role-session-name "$(tr -dc A-Za-z0-9 </dev/urandom | head -c 20)" \
                    --serial-number $mfa_arn \
                    --token-code $mfa_token | jq '.Credentials')

aws_access_key_id=$(echo $data | jq -r '.AccessKeyId')
aws_secret_access_key=$(echo $data | jq -r '.SecretAccessKey')
aws_session_token=$(echo $data | jq -r '.SessionToken')
expiration=$(echo $data | jq -r '.Expiration')

aws configure set aws_access_key_id $aws_access_key_id --profile $temp_profile
aws configure set aws_secret_access_key $aws_secret_access_key --profile $temp_profile
aws configure set aws_session_token $aws_session_token --profile $temp_profile
aws configure set expiration $expiration --profile $temp_profile

echo '{
  "Version": 1,
  "AccessKeyId": "'"$aws_access_key_id"'",
  "SecretAccessKey": "'"$aws_secret_access_key"'",
  "SessionToken": "'"$aws_session_token"'",
  "Expiration": "'"$expiration"'"
}'

Use the tf profile in provider settings. The first time, you will be prompted mfa token :

# terraform apply
Enter MFA token: 428313

This solution works fine with terraform and/or terragrunt