Use ECR images in EKS from another account

Question

I've two accounts: Account A and Account B. I would like to run an image from ECR at Account A on EKS on Account B.

I'm a bit confused on how to give the EKS the permissions. At first I thought of creating a docker-registry in the EKS with User role. But, as I read more I understood that it's not the way.

Have anyone tried it before?

Answer 1

I asked and answered this on re:Post, but I will summarize things here to help the next lost soul who is trying to solve this.

First, let's use an example account setup like this:

1. artifacts: the AWS account where ECR lives
2. production: the AWS account where EKS lives

In the artifacts account, we have a repository that we will call example-repo, and in the production account, we have an IAM role which is in use by the EKS nodes named prod-eks-node-role. To make a problem statement: we need EKS in production to be able to pull images from the ECR example-repo in artifacts.

When EKS in production, via the prod-eks-node-role, attempts to access the example-repo ECR repository in the artifacts account, the auth workflow looks like this:

Step 1: Generate ECR Authorization Token in the Same Account

prod-eks-node-role calls the equivalent of aws ecr get-login-password within its own account in production. To do this, prod-eks-node-role must have a policy attached to it like this:

{
  "Statement": [
    {
      "Action": "ecr:GetAuthorizationToken",
      "Effect": "Allow",
      "Resource": "*",
      "Sid": "AllowGetAuthToken"
    }
  ],
  "Version": "2012-10-17"
}

At first glance, this seems odd: why are we generating an authorization token within the production account in order to access the artifacts ECR registry?

The reason is that ECR authorization tokens (Docker login passwords) uniquely identify the given IAM principal regardless of AWS account. When a password is generated, AWS correlates the IAM principal (in our case, prod-eks-node-role in production) with the password. Then, when prod-eks-node-role attempts to perform actions on the ECR repository (i.e. pull/push), the generated password tells ECR who is accessing it.

Now, when EKS in production attempts to pull an image from the ECR example-repo repository in artifacts, it now checks the ECR repository policy. We can think of this first step as authentication or identifying the user, and this step is effectively global across all AWS accounts.

Step 2: Attempt to Perform Actions Against ECR in the Other Account

Next, IAM in the artifacts account checks the ECR example-repo repository policy for whether the authenticated/identified IAM principal can perform actions. This step is authorization.

In our case, we simply want to allow EKS to pull images from our ECR repository named example-repo. To do this, we need to define an ECR repository policy which allows the IAM principal (prod-eks-node-role) to perform the related pull actions. Define this on the example-repo:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowPull",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::987654321098:role/prod-ecr-pull-role"
      },
      "Action": [
        "ecr:BatchCheckLayerAvailability",
        "ecr:BatchGetImage",
        "ecr:DescribeRepositories",
        "ecr:GetDownloadUrlForLayer",
        "ecr:ListImages"
      ]
    }
  ]
}

Replace the ARN above with the ARN of your role.

After defining this, EKS should be able to:

• ✅ authenticate with ECR globally to identify itself
• ✅ authorize with the ECR repository example-repo in the artifacts account

And we're full circle now.

Example

For an example of how this workflow will work, we can do it with the AWS CLI. We will assume that we have an AWS CLI profile named production for the account where EKS lives, and a profile named artifacts for the account where ECR lives.

Start by discovering what IAM principal you're using:

aws --profile production sts get-caller-identity | jq -rS '.Arn'

The output from this command must be allowed in the ECR repository policy. Do that first, and then continue.

Next, let's get a password for ECR:

DOCKER_PASSWORD="$(aws --profile production ecr get-login-password)"

Note that the command does not identify a target account; it runs with the credentials in the production profile against ECR globally.

If the above step fails, ensure that your IAM principal (user/role) has the ability to ecr:GetAuthorizationToken.

Next, we will use the password to authenticate our Docker daemon with the specific registry in the artifacts account:

ARTIFACTS_ACCOUNT_ID=123456789012
ARTIFACTS_ECR_REGION=us-east-1

docker login -u AWS --password "$DOCKER_PASSWORD" \
  "${ARTIFACTS_ACCOUNT_ID}.dkr.ecr.${ARTIFACTS_ECR_REGION}.amazonaws.com"

If you obtained a valid password, the above step should not fail in any case, as the authentication is global across all AWS accounts.

Finally, let's try pulling an image:

docker pull "${ARTIFACTS_ACCOUNT_ID}.dkr.ecr.${ARTIFACTS_ECR_REGION}/example-repo:latest"

If this doesn't work, ensure that the repository exists, the Docker image tag latest exists, and the ECR repository policy allows your IAM principal to use the IAM actions required for pulling.

Summary

To allow EKS in the artifacts account to pull images from an ECR repository in the production account, we must:

1. Attach a policy to the EKS node IAM role that allows the role to obtain credentials that uniquely identify it globally. The permission is ecr:GetAuthorizationToken and must be granted on "Resource": ["*"] in the production account.
2. Create an ECR repository policy on the target repository which allows the EKS node IAM role to perform the actions it needs to pull (or push) images.

Answer 2

It pays to remember that AWS applies an Explicit Deny access policy by default. This translates to "Anything coming IN or going OUT needs to be explicitly allowed".

If you want to provide access to a specific ECR repository it can be achieved in the following manner:

1. Create a resource policy on Account A

Note the Principal field granting access to the role in Account B.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowPull",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::<ACCOUNT_B_ID>:role/<ROLE_NAME>"
      },
      "Action": [
        "ecr:GetAuthorizationToken",
        "ecr:BatchCheckLayerAvailability",
        "ecr:GetDownloadUrlForLayer",
        "ecr:GetRepositoryPolicy",
        "ecr:DescribeRepositories",
        "ecr:ListImages",
        "ecr:DescribeImages",
        "ecr:BatchGetImage",
        "ecr:GetLifecyclePolicy",
        "ecr:GetLifecyclePolicyPreview",
        "ecr:ListTagsForResource",
        "ecr:DescribeImageScanFindings" 
      ]
    }
  ]
}

2. Create a permission policy in Account B

For the ROLE_NAME specified above the following permission policy needs to be added otherwise on an access attempt from Account B you will get a 403 Forbidden. This policy satisfies the outbound access constraint for Account B.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowPullFromECRFooRepo"
            "Effect": "Allow",
            "Action": [
                 "ecr:GetAuthorizationToken",
                "ecr:BatchCheckLayerAvailability",
                "ecr:GetDownloadUrlForLayer",
                "ecr:GetRepositoryPolicy",
                "ecr:DescribeRepositories",
                "ecr:ListImages",
                "ecr:DescribeImages",
                "ecr:BatchGetImage",
                "ecr:GetLifecyclePolicy",
                "ecr:GetLifecyclePolicyPreview",
                "ecr:ListTagsForResource",
                "ecr:DescribeImageScanFindings" 
            ],
            "Resource": "<ACCOUNT_A_ID>.dkr.ecr.<REGION>.amazonaws.com/foo/*"

        }
    ]
}

Answer 3

First, your EKS needs to have IAM permissions to do these operations as if they were performed agains ECR in the same account.

Second, you need to allow the other account to access the ECR repository. You can do this by logging into management console of the account that hosts the ECR. Go to ECR -> click on the repository that you want to make accessible by the other account -> on the left panel, click on permissions -> click edit -> click add statement -> fill in AWS account IDs - optional field with the account number of your second account, leave the rest untouched -> click save