Secure Your S3 Buckets: IaC Misconfiguration & Public ACLs

Hey everyone! Let's dive into a common security concern in cloud environments: IaC (Infrastructure as Code) misconfigurations, specifically focusing on AWS S3 buckets and their public access settings. In this article, we'll explore how misconfigurations can lead to vulnerabilities, how to identify them using tools like Checkov, and, most importantly, how to fix them. We'll break down the issue of public ACLs (Access Control Lists) in S3 buckets, which can inadvertently expose your data. Think of it as making sure your digital front door isn't wide open. So, let's get started and secure those S3 buckets!

Understanding the Problem: IaC Misconfiguration and Public ACLs

What is IaC misconfiguration?

IaC (Infrastructure as Code) misconfiguration happens when there are errors or vulnerabilities in the code used to define and manage your cloud infrastructure. This code, written in languages like Terraform or CloudFormation, specifies how your resources – like S3 buckets, virtual machines, and databases – are created and configured. If this code is flawed, it can lead to security issues and operational problems. The source provided on the problem, it points out that this misconfiguration was discovered on Oct 06, 2025 09:22, and the severity is MEDIUM.

Why are S3 Bucket Public ACLs a Risk?

S3 buckets are used to store files and other objects in the cloud. Public ACLs, when enabled, give anyone on the internet permission to access these objects. This can be a huge security risk, as it can lead to unauthorized data access, data breaches, and compliance violations. The goal is to ensure that only authorized individuals or services can access the data. The description states that the IgnorePublicAcls setting causes S3 to ignore all public ACLs on a bucket and any objects that it contains. Enabling this setting does not affect the persistence of any existing ACLs and does not prevent new public ACLs from being set. It also mentions that it will block public access granted by ACLs while still allowing PUT Object calls that include a public ACL.

Identifying the Vulnerability: Checkov and Automated Scanning

The Role of Checkov

Checkov is a great open-source tool for scanning your IaC code to identify security and compliance violations. It analyzes your code (Terraform, CloudFormation, etc.) and flags any misconfigurations. Checkov will then provide the user with detailed information about the issue, including the location of the problem in the code and steps for remediation. The finding name in the information provided is Ensure S3 bucket has ignore public ACLs enabled. In this scenario, Checkov has detected a potential issue: the absence of the ignore_public_acls setting on an S3 bucket.

How Checkov Works

Checkov works by running a series of tests against your IaC code. These tests are based on a set of predefined rules. If your code violates any of these rules, Checkov will flag it. In this case, Checkov is looking for the ignore_public_acls setting. If the setting isn't enabled, Checkov will raise an alert, indicating a potential security risk. The Checkov Rule ID for this specific issue is CKV_AWS_55. The Introduced through: is through checkov_test/config/cloudformation.yml.

Remediation: Fixing the S3 Bucket Configuration

Remediation steps

Fortunately, fixing this misconfiguration is relatively straightforward. The goal is to prevent accidental public access to your S3 bucket. Here's how to enable the ignore_public_acls setting in your IaC code:

Fix - Buildtime

Terraform

• Resource: aws_s3_bucket_public_access_block
• Arguments: ignore_public_acls

resource "aws_s3_bucket_public_access_block" "artifacts" {
  ...
  restrict_public_buckets = true
+ ignore_public_acls=true
}

CloudFormation

• Resource: AWS::S3::Bucket
• Arguments: Properties.PublicAccessBlockConfiguration.IgnorePublicAcls

Type: 'AWS::S3::Bucket'
    Properties:
    ...
    PublicAccessBlockConfiguration:
        ...
+       IgnorePublicAcls: true

By adding the ignore_public_acls=true argument, you're telling S3 to ignore any public ACLs on your bucket. This significantly reduces the risk of unauthorized access. Always test these changes in a non-production environment first to ensure they don't break anything. The best practice is to make these changes in your IaC code and then redeploy your infrastructure. This ensures that the configuration is consistent and repeatable.

Best Practices for S3 Bucket Security

Beyond ignore_public_acls

While enabling ignore_public_acls is a crucial step, it's only one part of a comprehensive S3 bucket security strategy. Here are some more best practices to consider:

• Use Bucket Policies: Define explicit bucket policies to control access to your data. Policies give you fine-grained control over who can access your bucket and what actions they can perform.
• Enable Versioning: Versioning allows you to recover previous versions of objects in case of accidental deletion or corruption. It is very important for data protection.
• Encrypt Your Data: Always encrypt your data at rest and in transit. S3 offers several encryption options, including server-side encryption and client-side encryption.
• Monitor Your Buckets: Implement monitoring and logging to track access to your buckets. CloudTrail can help you identify suspicious activity.
• Regular Audits: Perform regular security audits of your S3 buckets and your IaC code. This helps to identify and fix any misconfigurations.

Conclusion: Securing Your Data

IaC misconfigurations are a significant threat to cloud security, but the good news is that they can be detected and fixed with the right tools and practices. By understanding the risks associated with public ACLs in S3 buckets, using tools like Checkov to identify vulnerabilities, and implementing proper remediation steps, you can significantly improve your cloud security posture. Remember to always prioritize security in your IaC code and to stay informed about the latest security threats and best practices. By following these steps, you can create a much safer and more secure environment for your data in the cloud. Keep your code clean, your configurations secure, and your data safe! Keep learning, keep adapting, and keep your cloud secure!

For further reading and resources on S3 bucket security and IaC best practices, I recommend checking out the official AWS documentation and security blogs. AWS S3 Documentation is a great place to start.