Bref enable serverless PHP on AWS Lambda. When Bref is used in combination with an API Gateway with the v2 format, it does not handle multiple values headers. If PHP generates a response with two headers having the same key but different values only the latest one is kept. 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. This vulnerability is patched in 2.1.13.

Affected: Packagist: bref/bref

## 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 <?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: ```yaml 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…