Content Security Policies

Content Security Policies (CSP) are a powerful tool to mitigate against Cross Site Scripting (XSS) and related attacks, including card skimmers, session hijacking, clickjacking, and more. Web servers send CSPs in response HTTP headers (namely Content-Security-Policy and Content-Security-Policy-Report-Only) to browsers that whitelist the origins of scripts, styles, and other resources. Together, CSPs and built-in browser features help prevent:

  • Loading a malicious script from an attacker’s website
  • A malicious inline script from sending credit card info to an attacker’s website
  • Loading a malicious style that will make users click on an element that wasn’t supposed to be on a page

See Content Security Policy (CSP) and Content-Security-Policy to learn more about CSP and each individual policy.

Magento and CSP

As of version 2.3.5, Magento supports CSP headers and provides ways to configure them. (This functionality is defined in the Magento_Csp module.) Magento also provides default configurations at the application level and for individual core modules that require extra configuration. Policies can be configured for adminhtml and storefront areas separately to accommodate different use cases. Magento also permits configuring unique CSPs for specific pages.

CSP can work in two modes:

  • report-only - In this mode, Magento reports policy violations but does not interfere. This mode isuseful for debugging. By default, CSP violations are written to the browser console, but they can be configured to be reported to an endpoint as an HTTP request to collect logs. There are a number of services that will collect, store, and sort your store’s CSP violations reports for you.

  • restrict mode - In this mode, Magento acts on any policy violations.

Default configuration

By default, CSP is configured in report-only mode, which allows merchants and developers to configure policies to work according to their custom code. After the policies have been configured, switch the mode to restrict.

Once configured, Magento can enforce policies like these:

  • Any resource, such as .js, .css, .jpg, or .ttf files, can only be loaded from the store’s domain
  • Iframes can only include pages from the store itself
  • AJAX requests can only be sent to the store
  • Forms can only be sent to the store

For more details check the Magento/Csp/etc/config.xml file. Some domains have already been whitelistedfor modules that require it. For instance if the Magento_Paypal module is installed, www.paypal.com is already whitelisted for the script-src policy.

The following features are allowed:

  • Inline JavaScript (JavaScript inside <script> tags and on<event> HTML tags)
  • JavaScript eval() usage
  • Inline styles (CSS inside <style> tags and style HTML attributes)

Some of these features will be disabled by default for Magento 2.4.

Configure a module’s CSP mode

You can set the CSP mode in a custom module by editing the module’s etc/config.xml file. To set the mode to restrict, change the value of the default/csp/mode/admin/report_only and/or the default/csp/mode/storefront/report_only element to 0. To enable report-only mode, set the values to 1.

Example config.xml:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <csp>
            <mode>
                <storefront>
                    <report_only>0</report_only>
                </storefront>
                <admin>
                    <report_only>0</report_only>
                </admin>
            </mode>
        </csp>
    </default>
</config>

You can use the etc/config.xml file in the Magento_Csp module as a reference. Create your component file structure describes how to create a module.

Configure CSPs for your custom code/extension/theme

Magento provides multiple ways to add whitelisted resources to your custom code, extension, or theme. Be sure to add resources only in modules that require it. For example, adding a domain to a default-src policy when you only need to load a .js file from it is not recommended. Add the domain to script-src instead.

The following table describes each type of CSP:

Policy name Description
default-src The default policy.
base-uri Defines which URLs can appear in a page’s <base> element.
child-src Defines the sources for workers and embedded frame contents.
connect-src Defines the sources that can be loaded using script interfaces.
font-src Defines which sources can serve fonts.
form-action Defines valid endpoints for submission from <form> tags.
frame-ancestors Defines the sources that can embed the current page.
frame-src Defines the sources for elements such as <frame> and <iframe>.
img-src Defines the sources from which images can be loaded.
manifest-src Defines the allowable contents of web app manifests.
media-src Defines the sources from which images can be loaded.
object-src Defines the sources for the <object>, <embed>, and <applet> elements.
script-src Defines the sources for JavaScript <script> elements.
style-src Defines the sources for stylesheets.

Add a domain to the whitelist

You can add a domain to the whitelist for a policy (like script-src, style-src, font-src and others) by adding a csp_whitelist.xml to your custom module’s etc folder.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
<?xml version="1.0"?>
<csp_whitelist xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Csp:etc/csp_whitelist.xsd">
    <policies>
        <policy id="script-src">
            <values>
                <value id="devdocs-base" type="host">https://devdocs.magento.com</value>
                <value id="magento" type="host">https://magento.com</value>
            </values>
        </policy>
        <policy id="connect-src">
            <values>
                <value id="devdocs" type="host">https://devdocs.magento.com</value>
            </values>
        </policy>
    </policies>
</csp_whitelist>

In case you include a script/css file from a .phtml template you can also use the dynamic whitelisting utility available as $csp, which is an instance of Magento\Csp\Api\InlineUtilInterface:

1
2
<div>This is my page!!!</div>
<?= /* @noEscape */ $csp->renderTag('script', ['src' => 'http://my.magento.com/static/script.js']); ?>

Whitelist an inline script or style

Although Magento 2.3 allows inline scripts and styles by default, this policy will change in Magento 2.4. We recommend that you prepare your custom code/extension for the future by defining whitelists and avoiding use of inline styles and scripts.

To whitelist a script or style HTML tag, use Magento\Csp\Api\InlineUtilInterface by adding it as a dependency to your class that renders HTML. You can also use $csp variables inside a .phtml template.

Inside a .phtml template:

1
2
3
<div>This is an HTML page</div>
<?= /* @noEscape */ $csp->renderTag('script', ['type' => 'text/javascript'], "\n    let myVar = 1;\n    console.log('myVar = ' + myVar);\n") ?>
<?= /* @noEscape */ $csp->renderTag('style', [], "\n    .#my-element {\n        width: 100%;\n    }\n") ?>

Inside a class that renders HTML:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
class HtmlRenderer
{
    /**
     * @var \Magento\Csp\Api\InlineUtilInterface
     */
    private $cspUtil;

    //...

    public function renderSomeHtml(): string
    {
        $html = '<div>This is an HTML page</div>';
        $html .= $this->cspUtil->renderTag(
            'script',
            ['type' => 'text/javascript'],
            "\n    let myVar = 1;\n    console.log('myVar = ' + myVar);\n"
        );

        return $html;
    }
}

CSP does not currently allow whitelisting of JavaScript inside on<Event> HTML attributes or as styles inside a style attribute. Replace these constructions with separate style and script HTML tags whitelisted via Magento\Csp\Api\InlineUtilInterface.

You can also whitelist inline CSS and JS inside style and script tags in a csp_whitelist.xml file. To do this, get a sha256 hash of a tag’s content and encode it as BASE64, then add it to your module’s csp_whitelist.xml. Programmatically, it could look like this:

1
$whitelistHash = base64_encode(hash('sha256', $content, true));

Add the corresponding policy to a csp_whitelist.xml file:

1
2
3
4
5
6
7
8
9
10
<?xml version="1.0"?>
<csp_whitelist xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Csp:etc/csp_whitelist.xsd">
    <policies>
        <policy id="script-src">
            <values>
                <value id="my-scripts-hash" type="hash" algorithm="sha256">B4yPHKaXnvFWtRChIbabYmUBFZdVfKKXHbWtWidDVF8=</value>
            </values>
        </policy>
    </policies>
</csp_whitelist>

When unsafe-inline is allowed for script-src or style-src policies, whitelisted inline scripts/styles hashes will not appear in the Content-Security-Policy header.

Advanced CSP configuration

To configure other CSPs such as sandbox policy, which does not consist of whitelisted hosts and hashes, or for more advanced fetch policy configurations, like removing inline support from script-src, you would have to create a config.xml file inside your custom module and rewrite the default values. For reference, see Magento\Csp\etc\config.xml.

Report-Uri configuration

Regardless of restrict or report-only mode, CSP violations may be reported to an endpoint for collection. The URL to use for reporting by browsers can be configured in your custom module’s config.xml file:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <default>
        <csp>
            <mode>
                <storefront>
                    <report_uri>http://csp-reporting-service.com/my-project/endpoint</report_uri>
                </storefront>
                <admin>
                    <report_uri>http://csp-reporting-service.com/my-project/endpoint</report_uri>
                </admin>
            </mode>
        </csp>
    </default>
</config>

Page specific Content-Security-Policies

Magento can send unique policies for a specific page. To do so, implement Magento\Csp\Api\CspAwareActionInterface in a controller responsible for the page and define the modifyCsp method. It receives existing CSPs read from configs and allows you redefine them by returning a new list. See the example below:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Mypage extends \Magento\Framework\App\Action\Action implements \Magento\Csp\Api\CspAwareActionInterface
{
    /**
     * My custom page.
     *
     * @inheritDoc
     */
    public function execute()
    {
        return $this->resultPageFactory->create();
    }

    public function modifyCsp(array $appliedPolicies): array
    {
        $appliedPolicies[] = new \Magento\Csp\Model\Policy\FetchPolicy(
            'form-action',
            false,
            ['https://my-site.com'],
            ['https']
        );

        return $appliedPolicies;
    }
}

You do not need to define other policy options in contexts like unsafe-inline. The same policy options read from config will be merged later.