Content Security Policy

Contao 5.3 introduced support for Content Security Policies (MDN Web Docs) for its front end. Implementing CSP directives is an important tool to secure your website against XSS attacks and unwanted external resources. At the same time we want to specifically allow scripts and resources generated or added by your application. In this article we will show you how to use Contao’s CSP framework in order to adjust the generated Content-Security-Policy response header from within your templates, controllers and services.

The CspHandler

Content Security Policies are applied via Contao’s Response Context concept. When CSP is enabled in the settings of a website root a CspHandler instance will be added to the response context. You can access the CspHandler in your services like this for example:

// src/ExampleService.php
namespace App;

use Contao\CoreBundle\Routing\ResponseContext\Csp\CspHandler;
use Contao\CoreBundle\Routing\ResponseContext\ResponseContextAccessor;

class ExampleService
{
    public function __construct(private readonly ResponseContextAccessor $responseContextAccessor)
    {
    }

    public function __invoke(): void
    {
        $responseContext = $this->responseContextAccessor->getResponseContext();

        if ($responseContext?->has(CspHandler::class)) {
            /** @var CspHandler $csp */
            $cspHandler = $responseContext->get(CspHandler::class);

            // Retrieve nonces, add sources, …
        }
    }
}

The following describes the most important methods of the CspHandler. In many cases you will want to make these adjustments directly from your templates though, rather than from within your controller for example. This way you can provide a lot more flexibility to your users - if e.g. a part of the template is not output, it also does not need any CSP information for it. Consequently Contao provides different template helper methods that you can use to manage CSP nonces, sources and hashes.

Adding Sources

Typically when employing CSP directives you want to only allow resources that come from the website itself, i.e. from the same domain. This can be achieved via default-src 'self' for example. If the website would then contain references to an external source - e.g. via an <iframe> or <script src="…"> etc. - that resource will be blocked by the browser. However, your application might deliberately want to include external resources and thus it is necessary to allow this resource specifically (or at least the domain of that resource). This can be done via the addSource method:

$cspHandler->addSource('frame-src', 'https://www.youtube.com/embed/foobar123');

{% do csp_source('frame-src', 'https://www.youtube.com/embed/foobar123') %}

<?php $this->addCspSource('frame-src', 'https://www.youtube.com/embed/foobar123') ?>

Retrieving a Nonce

When employing a CSP directive that affects JavaScript or CSS (default-src, script-src, style-src) inline scripts will not be allowed anymore as it is recommended to only integrate scripts via files. However, if your application still wants to use inline script or style elements it can still be allowed through the usage of nonces, without having to resort to allowing 'unsafe-inline' in your directives. Contao’s default JavaScript templates for instance still use inline JavaScript and thus will add a nonce when used. You can retrieve a nonce for script-src or style-src like this:

$nonce = $cspHandler->getNonce('script-src');

<script{{ attrs().setIfExists('nonce', csp_nonce('script-src')) }}>

<script<?= $this->attr()->setIfExists('nonce', $this->nonce('script-src')) ?>>

This nonce can then be added as a nonce="…" attribute to your <script> or <style> tag respectively.

Adding Hashes

As mentioned in the previous section, any directives that affect JavaScript or CSS will automatically disallow any inline scripts or inline styles. This also includes inline scripts via HTML attributes, e.g. onclick="…" or style="…". It is highly recommended to move these styles and scripts into files instead. However, if for some reason this is not possible such inline scripts can still be allowed through hashes. A hash can be generated and added to a directive for a specific inline script or style like this:

$cspHandler->addHash('style-src', 'display:none');

{% do csp_hash('display:none') %}
<div style="display:none">

<div style="<?= $this->cspInlineStyle('display:none') ?>">

The WysiwygStyleProcessor

Contao uses the WYSIWYG Editor TinyMCE which creates inline styles for certain formatting options. In Contao’s default templates these styles are automatically processed. The styles are extracted and filtered by the WysiwygStyleProcessor and then their hashes are added via the CspHandler. The allowed inline style properties can be configured via the contao.csp.allowed_inline_styles bundle configuration.

The 'unsafe-hashes' directive will also be added automatically by the template helpers for CSP Level 3 compliance.

// src/ExampleService.php
namespace App;

use Contao\CoreBundle\Csp\WysiwygStyleProcessor;
use Contao\CoreBundle\Routing\ResponseContext\Csp\CspHandler;
use Contao\CoreBundle\Routing\ResponseContext\ResponseContextAccessor;

class ExampleService
{
    public function __construct(
        private readonly ResponseContextAccessor $responseContextAccessor,
        private readonly WysiwygStyleProcessor $wysiwygProcessor,
    ) {
    }

    public function processInlineStyles(string $html): void
    {
        $responseContext = $this->responseContextAccessor->getResponseContext();

        if (!$responseContext?->has(CspHandler::class)) {
            return;
        }

        if (!$styles = $this->wysiwygProcessor->extractStyles($html)) {
            return;
        }

        /** @var CspHandler $csp */
        $csp = $responseContext->get(CspHandler::class);

        foreach ($styles as $style) {
            $csp->addHash('style-src', $style);
        }

        $csp->addSource('style-src', 'unsafe-hashes');
    }
}

{{ text|csp_inline_styles|insert_tag|encode_email|raw }}

<?= $this->cspInlineStyles($this->text) ?>