HTML Sanitizer

New in version 6.1: The HTML Sanitizer component was introduced in Symfony 6.1.

The HTML Sanitizer component aims at sanitizing/cleaning untrusted HTML code (e.g. created by a WYSIWYG editor in the browser) into HTML that can be trusted. It is based on the HTML Sanitizer W3C Standard Proposal.

The HTML sanitizer creates a new HTML structure from scratch, taking only the elements and attributes that are allowed by configuration. This means that the returned HTML is very predictable (it only contains allowed elements), but it does not work well with badly formatted input (e.g. invalid HTML). The sanitizer is targeted for two use cases:

Preventing security attacks based on XSS or other technologies relying on execution of malicious code on the visitors browsers;
Generating HTML that always respects a certain format (only certain tags, attributes, hosts, etc.) to be able to consistently style the resulting output with CSS. This also protects your application against attacks related to e.g. changing the CSS of the whole page.

Installation

You can install the HTML Sanitizer component with:

$ composer require symfony/html-sanitizer

Basic Usage

Use the Symfony\Component\HtmlSanitizer\HtmlSanitizer class to sanitize the HTML. In the Symfony framework, this class is available as the html_sanitizer service. This service will be autowired automatically when type-hinting for Symfony\Component\HtmlSanitizer\HtmlSanitizerInterface:

Note

The default configuration of the HTML sanitizer allows all “safe” elements and attributes, as defined by the W3C Standard Proposal. In practice, this means that the resulting code will not contain any scripts, styles or other elements that can cause the website to behave or look different. Later in this article, you’ll learn how to fully customize the HTML sanitizer.

Sanitizing HTML for a Specific Context

The default :method:`Symfony\\Component\\HtmlSanitizer\\HtmlSanitizer::sanitize` method cleans the HTML code for usage in the <body> element. Using the :method:`Symfony\\Component\\HtmlSanitizer\\HtmlSanitizer::sanitizeFor` method, you can instruct HTML sanitizer to customize this for the <head> or a more specific HTML tag:

// tags not allowed in <head> will be removed
$safeInput = $htmlSanitizer->sanitizeFor('head', $userInput);

// encodes the returned HTML using HTML entities
$safeInput = $htmlSanitizer->sanitizeFor('title', $userInput);
$safeInput = $htmlSanitizer->sanitizeFor('textarea', $userInput);

// uses the <body> context, removing tags only allowed in <head>
$safeInput = $htmlSanitizer->sanitizeFor('body', $userInput);
$safeInput = $htmlSanitizer->sanitizeFor('section', $userInput);

Sanitizing HTML from Form Input

The HTML sanitizer component directly integrates with Symfony Forms, to sanitize the form input before it is processed by your application.

You can enable the sanitizer in TextType forms, or any form extending this type (such as TextareaType), using the sanitize_html option:

// src/Form/BlogPostType.php
namespace App\Form;

// ...
class BlogPostType extends AbstractType
{
    // ...

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'sanitize_html' => true,
            // use the "sanitizer" option to use a custom sanitizer (see below)
            //'sanitizer' => 'app.post_sanitizer',
        ]);
    }
}

Sanitizing HTML in Twig Templates

Besides sanitizing user input, you can also sanitize HTML code before outputting it in a Twig template using the sanitize_html() filter:

{{ post.body|sanitize_html }}

{# you can also use a custom sanitizer (see below) #}
{{ post.body|sanitize_html('app.post_sanitizer') }}

Configuration

The behavior of the HTML sanitizer can be fully customized. This allows you to explicitly state which elements, attributes and even attribute values are allowed.

You can do this by defining a new HTML sanitizer in the configuration:

This configuration defines a new html_sanitizer.sanitizer.app.post_sanitizer service. This service will be autowired for services having an HtmlSanitizerInterface $appPostSanitizer parameter.

Allow Element Baselines

You can start the custom HTML sanitizer by using one of the two baselines:

Static elements: All elements and attributes on the baseline allow lists from the W3C Standard Proposal (this does not include scripts).
Safe elements: All elements and attributes from the “static elements” list, excluding elements and attributes that can also lead to CSS injection/click-jacking.

Allow Elements

This adds elements to the allow list. For each element, you can also specify the allowed attributes on that element. If not given, all allowed attributes from the W3C Standard Proposal are allowed.

Block and Drop Elements

You can also block (the element will be removed, but its children will be kept) or drop (the element and its children will be removed) elements.

This can also be used to remove elements from the allow list.

Allow Attributes

Using this option, you can specify which attributes will be preserved in the returned HTML. The attribute will be allowed on the given elements, or on all elements allowed before this setting.

Drop Attributes

This option allows you to disallow attributes that were allowed before.

Force Attribute Values

Using this option, you can force an attribute with a given value on an element. For instance, use the follow config to always set rel="noopener noreferrer" on each <a> element (even if the original one didn’t contain a rel attribute):

Force/Allow Link URLs

Besides allowing/blocking elements and attributes, you can also control the URLs of <a> elements:

Force/Allow Media URLs

Like link URLs, you can also control the URLs of other media in the HTML. The following attributes are checked by the HTML sanitizer: src, href, lowsrc, background and ping.

Custom Attribute Sanitizers

Controlling the link and media URLs is done by the Symfony\Component\HtmlSanitizer\Visitor\AttributeSanitizer\UrlAttributeSanitizer. You can also implement your own attribute sanitizer, to control the value of other attributes in the HTML. Create a class implementing Symfony\Component\HtmlSanitizer\Visitor\AttributeSanitizer\AttributeSanitizerInterface and register it as a service. After this, use with_attribute_sanitizers to enable it for an HTML sanitizer: