Limiting access to CloudFront

Introduction

At VRT, we use CloudFront extensively to front most of our websites. CloudFront is the CDN solution provided by Amazon Web Services. It provides the usual benefits of a CDN, but it's also just cheaper per GB than serving straight from EC2, at least for mostly European clients, like we have.

CloudFront allows some extensive configuration of its behaviour, such as routing to different backends (origins), or setting up specific caching behaviours. One of the newer features is Lambda@Edge, which allows you to run custom code on the CloudFront edge server for every request. Lambda@Edge provides 4 hooks for your code to hook in to. Two hooks are run in the request phase: when a request enters CloudFront and when CloudFront sends out a request to the origin. The two remaining hooks are run for the corresponding responses.

Within your Lambda@Edge functions, you are free to inspect and/or modify the request or response you are processing. You can even short-circuit the whole process by generating a response right from the request-hook, bypassing the whole backend.

Problem description

Next to our public websites, most sites have a staging version that should not be publicly accessible. This is challenging with CloudFront being a public service. CloudFront does provide some mechanisms to restrict access, but none of them fit our needs.

Our previous implementation uses Amazon's Web Application Firewall (WAF) to limit access by source IP. This solution works, but filtering on Source IP has its drawbacks. Especially with the rise of colleagues working at home (surfing with their ISP's IP address), scanning proxies (which group lots of customers behind a large block of addresses) and IPv6, it became clear that we wanted to verify access on a "higher" layer.

Preferably, we would like the following properties for the authorisation logic:

We want to verify if the user has access, independent of his current location or network setup.
Access should delegable. When a particular user has access, they should be able to share that access with other parties.
Access should always be limited in time and scope (i.e. which sites are allowed). This limits the impact of any stolen credentials.
Most of the components should be shared across several protected sites.
We want to grant access to non-human users as well: debug-builds of native apps, other backend systems that need staging access, ...
The observable behaviour of protected sites should be changed as little as possible.

Existing solutions

You can configure CloudFront to require that every request is signed, with the signature being either in the URL, or in a Cookie. This approach looks promising, since it moves the authorisation from a network location (the IP you appear to come from) to the user agent (browser) knowing a particular secret (either as URL, or as a cookie), independent of the current network situation.

For our use case, we want to minimise the amount of changes to the sites that are to be protected, so using signed URLs is not an option. Cookies are a good candidate: they can be set once, and are automatically used by the browser, even on cross-domain access from one protected site to another (e.g. images.example.org from www.example.org).

The only thing left to do, is issuing a bunch of cookies to authorized browser sessions, one set per domain that the user is authorized for. This turns out to be quite a challenge... Let me reformulate the problem to see why: We want to track a user across websites using 3rd party cookies. Does that sound like online advertisement to you? Yeah, me too. And browsers don't like it either. The practical behaviour ranges from "users need to explicitly accept the cookie for every domain" (Chrome) to "cookies are, practically speaking, silently ignored" (Safari).

Back to the drawing board...

The only way to set a cookie in a reliable way, is to do so when a user actually accesses that particular domain. This implies that we need to intercept the case when a user visits a protected domain without a cookie, or with an expired cookie. CloudFront issues a 403-Forbidden in this situation, with fairly boring XML content.

HTTP/1.1 403 Forbidden
Server: CloudFront
Date: Mon, 20 Aug 2018 08:08:58 GMT
Content-Type: text/xml
Content-Length: 146
Connection: keep-alive
X-Cache: Error from cloudfront
Via: 1.1 f188ead5b3846af391f02eedab2df3f7.cloudfront.net (CloudFront)
X-Amz-Cf-Id: Qmxuwgo-QrasQfFyQRjsSoWDyTk3eifML6VAw0XHGdqk-hHPhk8dTw==

<?xml version="1.0" encoding="UTF-8"?><Error><Code>MissingKey</Code><Message>Missing Key-Pair-Id query parameter or cookie value</Message></Error>

You can replace 403's with a custom error page, and invoke some custom logic to attempt to issue a cookie. However, this replacement is distribution-wide, also replacing 403's that the backend issued. This would change the observable behaviour of the protected site, which is unacceptable.

Lambda@Edge

So we replaced the built-in CloudFront logic to check the Cookie with our own implementation. When a valid cookie is present, the HTTP request is passed through, just as before. But when no cookie (or an expired cookie) is present, the browser is redirected to the authorizer domain to check the "master"-Cookie.

In order to do this Cookie-QueryParameter-Cookie dance, we need to hijack at least one URL from the protected domain. This does change the observable behaviour, but by choosing /auth-89CE3FEF-FCF6-43B3-9DBA-7C410CAAE220/set-cookie, it's unlikely to actually impact the site.

Authorizing users

To manage the "master"-Cookie on the authorizer domain, we implemented a very basic workflow, implemented entirely with Lambda functions behind an API-gateway. Authentication happens via Cognito, which leverages our existing ADFS infrastructure behind the scenes to authenticate users.

Once a user is authenticated, they can either authorize themselves (automatically), or choose to delegate their access to a third party. This delegation works by selecting which protected domains they want to give access to, and by specifying an expiration time. The result is an URL with a signed copy of the requested parameters. This URL represents the given access, and can be passed on to third parties.

Visiting this URL sets the "master"-Cookie, which is checked for when accessing a protected site.

Implementation details

The solution is deployed via CloudFormation, using the Serverless framework. It consists of four stacks:

The authoriser API-gateway and the Lambda functions to implement it, the parameter to store the JWT-secret and the associated KMS key and policy, the Cognito User Pool and Client.
The Lambda@Edge validator function. This is a separate template, since Lambda@Edge needs to be deployed in the us-east-1 region, while we normally deploy in eu-west-1.
A helper template to gather the required information from the two previous stacks in one place. (not depicted in the diagram)
An example CloudFront distribution to document the needed settings. It imports the needed information from the helper-stack.

Tokens

The used tokens are JSON Web Tokens. This implies that the data is "visible" (albeit base64-encoded) to the client. For our use case, this is not a problem, since it only contains non-secret information:

The list of domains that you have access to
The expiration time of the access
The username of the person that generated the token
A free-text field describing the granted access

Since the party issuing tokens, and the party verifying tokens are the same, we choose the faster and lighter HS256 algorithm. The secret for the JWTs is stored as a SecureString in the Parameter Store.

The authorisation cookie is stripped from the request by the Lambda@Edge function. This is done for multiple reasons. Obviously, it's a security enhancement: the Cookie contains "secret" information that is no longer needed after this step. But it also reduces the observable changes in behaviour in case the distribution is configured to use this Cookie as part of its Cache-key.

Configuration

Normally, you can configure a generic Lambda function via Environment Variables. Lambda@Edge is tricky in this regard, since it does not support Environment Variables. We worked around this issue by abusing the Tags.

The configuration itself is stored in an S3 bucket as JSON files. The name of this S3 bucket is passed as a regular environment variable to the Python Lambda functions, but as a Tag to the Lambda@Edge function. The Lambda@Edge function performs a lambda:GetFunction on itself (it can find out its own name from the context object at runtime) to read its own Tags.

Code

The code for our solution can be found on our GitHub account. The repository consists of three main parts:

The source code of the Python Lambda functions, located in the src directory. The corresponding build-script (build-python.sh) bundles these files into a ZIP file, ready to deploy to AWS Lambda. The build-script optionally outputs a hash of the ZIPed content. We use this hash in the S3 key, which allows CloudFormation to be smart when updating the resources. Additionally, tests are located in the test directory, and can be run with the standard pytest command (make sure to add src to your $PYTHONPATH, if your IDE doesn't do this for you).
The source code of the Lambda@Edge function, located in the λ@E directory. The corresponding build-script (build-nodejs.sh) is similar to the Python variant described above.
The CloudFormation templates, located in the templates directory. The corresponding build-script (build-cfn.sh) converts these to JSONs in the output directory.

Conclusion

The authoriser accomplishes most of what we wanted to achieve. It prevents unauthorised access, while providing a self-service way for authorised users to delegate their permissions to third parties (suppliers, native apps, …). CloudFront distributions that want to use the authoriser only require minimal changes to make use of the provided service. Unfortunately, the observable behaviour of protected websites is slightly changed: a particular (but configurable) path-prefix is no longer available for the site to use.

vrtdev/aws-cloudfront-authorizer