DVA-C02: How to Fix 403 Forbidden Error on CloudFront Distribution with Signed Cookies

Learn how to troubleshoot and solve a common issue where a CloudFront distribution with signed cookies returns a 403 Forbidden error when accessing the login page. Explore different solutions and choose the best one for your scenario.

Question

A developer is testing a new file storage application that uses an Amazon CloudFront distribution to serve content from an Amazon S3 bucket. The distribution accesses the S3 bucket by using an origin access identity (OAI). The S3 bucket’s permissions explicitly deny access to all other users.

The application prompts users to authenticate on a login page and then uses signed cookies to allow users to access their personal storage directories. The developer has configured the distribution to use its default cache behavior with restricted viewer access and has set the origin to point to the S3 bucket.

However, when the developer tries to navigate to the login page, the developer receives a 403 Forbidden error.

The developer needs to implement a solution to allow unauthenticated access to the login page. The solution also must keep all private content secure.

Which solution will meet these requirements?

A. Add a second cache behavior to the distribution with the same origin as the default cache behavior. Set the path pattern for the second cache behavior to the path of the login page, and make viewer access unrestricted. Keep the default cache behavior’s settings unchanged.

B. Add a second cache behavior to the distribution with the same origin as the default cache behavior. Set the path pattern for the second cache behavior to *, and make viewer access restricted. Change the default cache behavior’s path pattern to the path of the login page, and make viewer access unrestricted.

C. Add a second origin as a failover origin to the default cache behavior. Point the failover origin to the S3 bucket. Set the path pattern for the primary origin to *, and make viewer access restricted. Set the path pattern for the failover origin to the path of the login page, and make viewer access unrestricted.

D. Add a bucket policy to the S3 bucket to allow read access. Set the resource on the policy to the Amazon Resource Name (ARN) of the login page object in the S3 bucket. Add a CloudFront function to the default cache behavior to redirect unauthorized requests to the login page’s S3 URL.

Answer

A. Add a second cache behavior to the distribution with the same origin as the default cache behavior. Set the path pattern for the second cache behavior to the path of the login page, and make viewer access unrestricted. Keep the default cache behavior’s settings unchanged.

Explanation

The correct answer is A. Add a second cache behavior to the distribution with the same origin as the default cache behavior. Set the path pattern for the second cache behavior to the path of the login page, and make viewer access unrestricted. Keep the default cache behavior’s settings unchanged.

Here is an explanation of why this solution will meet the requirements:

• The problem is that the login page is not accessible to unauthenticated users because the distribution uses restricted viewer access and an OAI to access the S3 bucket. This means that only requests that have valid signed cookies can access the content in the bucket.
• To allow unauthenticated access to the login page, a second cache behavior is needed with a different viewer access setting. A cache behavior defines how CloudFront handles requests for a specific path pattern. By adding a second cache behavior with the same origin as the default one, the distribution can still access the S3 bucket using the OAI, but with different permissions for different paths.
• The path pattern for the second cache behavior should be set to the path of the login page, such as /login.html. This way, CloudFront will apply this cache behavior only to requests that match this path. The viewer access for this cache behavior should be unrestricted, which means that CloudFront will not require signed cookies to access this content. This will allow unauthenticated users to access the login page without getting a 403 Forbidden error.
• The default cache behavior should remain unchanged, with its path pattern set to * (which matches all requests that do not match any other cache behavior) and its viewer access set to restricted. This will ensure that all private content in the S3 bucket is still secure and can only be accessed by authenticated users who have valid signed cookies.
• This solution will not affect the performance or cost of the distribution, as CloudFront will still cache the content according to the cache behavior settings and origin response headers. It will also not require any changes to the S3 bucket permissions or policies, as the OAI will still be used to access the bucket.

Therefore, option A is the correct solution that will meet the requirements. Option B is incorrect because it will make all private content accessible to unauthenticated users by changing the default cache behavior’s viewer access to unrestricted. Option C is incorrect because it will not work, as CloudFront does not support failover origins for S3 buckets. Option D is incorrect because it will not work, as adding a bucket policy to allow read access to the login page object will conflict with the explicit deny policy on the bucket, and CloudFront functions cannot redirect requests to S3 URLs.

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