AWS CodeBuild Access Denied to S3

AWS CodeBuild is a fully managed build service that compiles source code, runs tests, and produces software packages that are ready to deploy. Amazon S3 (Simple Storage Service) is an object storage service that offers industry-leading scalability, data availability, security, and performance. It's common for CodeBuild projects to interact with S3 buckets, for example, to store build artifacts, access source code, or retrieve configuration files. However, one of the frequent issues developers encounter is the Access Denied error when CodeBuild tries to access an S3 bucket. This blog post will explore the core concepts, typical usage scenarios, common practices, and best practices to resolve and prevent such access - denied issues.

Table of Contents#

  1. Core Concepts
    • AWS CodeBuild Basics
    • Amazon S3 Basics
    • IAM and Permissions
  2. Typical Usage Scenarios
    • Storing Build Artifacts in S3
    • Retrieving Source Code from S3
    • Accessing Configuration Files in S3
  3. Common Reasons for Access Denied
    • Incorrect IAM Role Permissions
    • Bucket Policy Restrictions
    • S3 Block Public Access Settings
    • Cross - Account Access Issues
  4. Common Practices to Resolve Access Denied
    • Review and Update IAM Roles
    • Check and Modify Bucket Policies
    • Adjust S3 Block Public Access Settings
    • Handle Cross - Account Access Properly
  5. Best Practices to Prevent Access Denied
    • Least Privilege Principle
    • Regular Permission Audits
    • Use Tags for Permission Management
  6. Conclusion
  7. FAQ
  8. References

Article#

Core Concepts#

AWS CodeBuild Basics#

AWS CodeBuild is a cloud - based build service that automates the process of building, testing, and packaging software. It supports multiple programming languages and build tools. CodeBuild projects are configured with a build specification file that defines the steps to be executed during the build process.

Amazon S3 Basics#

Amazon S3 is an object storage service that stores data as objects within buckets. Each object has a unique key within the bucket. S3 provides features like versioning, lifecycle management, and access control. Buckets can be configured with various permissions and policies to control who can access the objects stored in them.

IAM and Permissions#

AWS Identity and Access Management (IAM) is used to manage access to AWS services and resources. IAM roles are used to grant permissions to AWS services, including CodeBuild. When a CodeBuild project tries to access an S3 bucket, it uses the permissions associated with the IAM role assigned to the project. Bucket policies can also be used to control access to S3 buckets at the bucket level.

Typical Usage Scenarios#

Storing Build Artifacts in S3#

After a successful build, CodeBuild can store the build artifacts in an S3 bucket. These artifacts can include compiled binaries, test reports, and deployment packages. This allows for easy retrieval and sharing of the build results.

Retrieving Source Code from S3#

If the source code for the build is stored in an S3 bucket, CodeBuild needs to access the bucket to retrieve the code. This is useful when the source code is large or when it needs to be shared across multiple projects.

Accessing Configuration Files in S3#

CodeBuild may need to access configuration files stored in an S3 bucket, such as environment variables, build scripts, or dependency lists. These files can be used to customize the build process.

Common Reasons for Access Denied#

Incorrect IAM Role Permissions#

The IAM role assigned to the CodeBuild project may not have the necessary permissions to access the S3 bucket. For example, if the role does not have the s3:GetObject permission, CodeBuild will not be able to retrieve objects from the bucket.

Bucket Policy Restrictions#

The bucket policy may be configured in such a way that it restricts access from the CodeBuild project. For example, the policy may only allow access from specific IP addresses or AWS accounts.

S3 Block Public Access Settings#

S3 Block Public Access settings can prevent CodeBuild from accessing the bucket if they are too restrictive. These settings are designed to prevent accidental public exposure of S3 buckets.

Cross - Account Access Issues#

If the CodeBuild project and the S3 bucket belong to different AWS accounts, cross - account access needs to be properly configured. Without the correct permissions and trust relationships, access will be denied.

Common Practices to Resolve Access Denied#

Review and Update IAM Roles#

Check the IAM role assigned to the CodeBuild project and ensure that it has the necessary S3 permissions. The following is an example of an IAM policy that allows CodeBuild to access an S3 bucket:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetObject",
                "s3:PutObject"
            ],
            "Resource": "arn:aws:s3:::your - bucket - name/*"
        }
    ]
}

Check and Modify Bucket Policies#

Review the bucket policy and make sure it allows access from the CodeBuild project. If necessary, update the policy to include the IAM role of the CodeBuild project.

Adjust S3 Block Public Access Settings#

If S3 Block Public Access settings are too restrictive, adjust them to allow access from the CodeBuild project. However, be careful not to expose the bucket to the public accidentally.

Handle Cross - Account Access Properly#

If cross - account access is required, create a trust relationship between the AWS accounts and configure the necessary IAM roles and policies in both accounts.

Best Practices to Prevent Access Denied#

Least Privilege Principle#

Follow the least privilege principle when assigning permissions to the IAM role of the CodeBuild project. Only grant the minimum permissions necessary for the project to function correctly.

Regular Permission Audits#

Regularly audit the IAM roles and bucket policies to ensure that they are up - to - date and do not have any unnecessary permissions.

Use Tags for Permission Management#

Use AWS tags to manage permissions more effectively. For example, you can tag S3 buckets and IAM roles and use tag - based conditions in your policies.

Conclusion#

The "Access Denied" error when AWS CodeBuild tries to access an S3 bucket can be caused by various factors, including incorrect IAM role permissions, bucket policy restrictions, S3 Block Public Access settings, and cross - account access issues. By understanding the core concepts, typical usage scenarios, and following common practices and best practices, software engineers can resolve and prevent these issues, ensuring smooth and efficient build processes.

FAQ#

Q: How do I know which IAM role is assigned to my CodeBuild project? A: You can find the IAM role assigned to a CodeBuild project in the AWS Management Console. Navigate to the CodeBuild project, and in the project settings, look for the "Service role" field.

Q: Can I use a bucket policy to allow access from multiple CodeBuild projects? A: Yes, you can include multiple IAM roles in a bucket policy to allow access from multiple CodeBuild projects. You need to specify the ARNs of the IAM roles in the policy.

Q: What should I do if I accidentally set too restrictive S3 Block Public Access settings? A: You can adjust the S3 Block Public Access settings in the AWS Management Console. Navigate to the S3 bucket, click on the "Permissions" tab, and modify the Block Public Access settings as needed.

References#

2025-10