Understanding AWS CloudFront Origin Access Identity (OAI)

As a Solutions Architect, understanding how to securely serve content from your AWS origins is crucial. AWS CloudFront Origin Access Identity (OAI) is a special CloudFront user that you can use to grant your CloudFront distribution permission to read your private content from an Amazon S3 bucket.

What is Origin Access Identity (OAI)?

Origin Access Identity (OAI) is a unique CloudFront user that is associated with your CloudFront distribution. It allows you to restrict access to your S3 bucket so that only your CloudFront distribution can access the content. This is a key security measure to prevent direct access to your S3 bucket, ensuring that content is only served through CloudFront.

OAI secures your S3 content by granting CloudFront exclusive read access.

When you create an OAI, CloudFront generates a unique canonical user ID. You then configure your S3 bucket policy to grant this canonical user ID read permissions. This ensures that requests for your content must go through CloudFront, which can then validate them.

Without OAI, if your S3 bucket is publicly accessible, anyone could potentially access your content directly by knowing the S3 object URL, bypassing CloudFront's caching and security features. By using OAI, you can make your S3 bucket private and only allow access via CloudFront distributions that are configured with the corresponding OAI. This is essential for protecting sensitive data or ensuring that content is delivered through your CDN's optimized path.

How OAI Works with S3

When you set up CloudFront to use an S3 bucket as an origin, you have two main options for access: public or private. Using OAI is the recommended method for private S3 buckets.

Feature	Public S3 Bucket	Private S3 Bucket with OAI
Access Control	Anyone with the S3 object URL	Only CloudFront distribution with OAI
Security	Lower (direct access possible)	Higher (controlled access via CDN)
Use Case	Publicly available assets	Private content, paid content, sensitive files
Configuration	Bucket policy allows public read	Bucket policy grants OAI read access; OAI created in CloudFront

Steps to Implement OAI

Implementing OAI involves a few key steps within the AWS console.

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Create an Origin Access Identity (OAI): In the CloudFront console, navigate to 'Origin Access' and create a new OAI. This generates a unique canonical user ID.

Update S3 Bucket Policy: Go to your S3 bucket's permissions and edit the bucket policy. You'll add a statement that grants
code
```
s3:GetObject
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permission to the canonical user ID of your OAI. CloudFront often provides a pre-formatted policy that you can copy and paste.

Configure CloudFront Origin: When setting up or editing your CloudFront distribution, select your S3 bucket as the origin. Under 'Origin Access Identity', choose 'Yes, use OAI' and select the OAI you just created. You'll also need to specify whether to 'Grant Read Permissions on Bucket' (which automatically updates the bucket policy) or 'Yes, update bucket policy' if you're managing it manually.

Test Access: After the CloudFront distribution updates, test accessing your content via the CloudFront domain name. Direct access to the S3 object URL should now be denied.

OAI is specifically for S3 origins. For other origins (like EC2 instances or on-premises servers), you would use other methods like custom headers or IP whitelisting to control access.

Key Considerations and Best Practices

When working with OAI, keep these points in mind for optimal security and performance.

What is the primary benefit of using OAI for an S3 origin?

It prevents direct access to S3 content, ensuring it's only served through CloudFront.

Bucket Policy Management: While CloudFront can update your bucket policy, it's good practice to understand the policy structure. Ensure no other policies grant public read access.

OAI vs. Origin Access Control (OAC): AWS is transitioning to Origin Access Control (OAC) as a more secure and flexible successor to OAI. OAC supports more origin types and offers enhanced security features. For new distributions, consider using OAC.

Cache Invalidation: Remember that changes to your S3 bucket content won't immediately reflect on CloudFront unless you invalidate the cache.

Private Content Delivery: OAI is fundamental for delivering private content, such as video-on-demand, software downloads, or any sensitive files that should only be accessible through your application's authenticated sessions managed by CloudFront.

Summary

Origin Access Identity (OAI) is a critical security feature for CloudFront distributions using S3 origins. It allows you to lock down your S3 bucket, ensuring that content is only accessible via your CloudFront CDN. This enhances security, controls access, and leverages CloudFront's performance benefits. As you progress in your AWS Solutions Architect journey, mastering OAI and its successor, OAC, will be essential for building secure and efficient content delivery solutions.

Learning Resources

What is Origin Access Identity?(documentation)

The official AWS documentation explaining the purpose and functionality of Origin Access Identity for restricting access to S3 buckets.

Creating a Distribution with an S3 Origin(documentation)

Step-by-step guide from AWS on how to create a CloudFront distribution, specifically detailing the configuration for S3 origins with OAI.

Controlling Access to Amazon S3 Content(documentation)

Detailed explanation of how to use OAI and bucket policies to control access to your S3 content when using CloudFront.

AWS CloudFront Origin Access Control (OAC)(documentation)

Information on the newer, more secure Origin Access Control (OAC) which is recommended over OAI for new distributions.

Securing Content with CloudFront and S3(blog)

A blog post from AWS that walks through best practices for securing content delivery using CloudFront and S3, including OAI.

CloudFront OAI vs OAC: What's the difference?(video)

A video tutorial explaining the differences between OAI and the newer OAC, and when to use each.

AWS CloudFront Tutorial: Securely Serving Private Content(video)

A practical video demonstration of setting up CloudFront with an S3 origin and OAI to serve private content.

AWS CloudFront Origin Access Identity Explained(blog)

An article that breaks down the concept of OAI, its importance, and how it works with S3 and CloudFront.

Amazon S3 Bucket Policies(documentation)

Reference for S3 bucket policies, including examples that can be adapted for OAI configurations.

CloudFront Developer Guide(documentation)

The comprehensive developer guide for AWS CloudFront, covering all aspects of its functionality and configuration.