Bref Doesn't Support Multiple Value Headers in ApiGatewayFormatV2

Impacted Resources

bref/src/Event/Http/HttpResponse.php:61-90

Description

When Bref is used in combination with an API Gateway with the v2 format, it does not handle multiple values headers.

Precisely, if PHP generates a response with two headers having the same key but different values only the latest one is kept.

Impact

If an application relies on multiple headers with the same key being set for security reasons, then Bref would lower the application security.

For example, if an application sets multiple Content-Security-Policy headers, then Bref would just reflect the latest one.

PoC

1. Create a new Bref project.
2. Create an index.php file with the following content:

<?php
header("Content-Security-Policy: script-src 'none'", false);
header("Content-Security-Policy: img-src 'self'", false);
?>
<script>alert(document.domain)</script>
<img src="https://bref.sh/favicon-32x32.png">

3. Use the following serverless.yml to deploy the Lambda:

service: app

provider:
    name: aws
    region: eu-central-1

plugins:
    - ./vendor/bref/bref

functions:
    api:
        handler: index.php
        description: ''
        runtime: php-81-fpm
        timeout: 28 # in seconds (API Gateway has a timeout of 29 seconds)
        events:
            -   httpApi: '*'

# Exclude files from deployment
package:
    patterns:
        - '!node_modules/**'
        - '!tests/**'

4. Browse the Lambda URL.
5. Notice that the JavaScript code is executed as the Content-Security-Policy: script-src 'none' header has been removed.
6. Notice that the external image has not been loaded as the Content-Security-Policy: img-src 'self' header has been kept.
7. Start a PHP server inside the project directory (e.g. php -S 127.0.0.1:8090).
8. Browse the index.php script through the PHP server (e.g. http://127.0.0.1:8090/index.php).
9. Notice that the JavaScript code is not executed as the Content-Security-Policy: script-src 'none' header has been kept.
10. Notice that the external image has not been loaded as the Content-Security-Policy: img-src 'self' header has been kept.

Suggested Remediation

Concatenate all the multiple value headers' values with a comma (,) as separator and return a single header with all the values to the API Gateway.

References

• https://www.rfc-editor.org/rfc/rfc9110.html#section-5.2