How to Escape Output in Templates

How to Escape Output in Templates

When generating HTML from a template, there is always a risk that a template variable may output unintended HTML or dangerous client-side code. The result is that dynamic content could break the HTML of the resulting page or allow a malicious user to perform a Cross Site Scripting (XSS) attack. Consider this classic example:

  • Twig
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    Hello {{ name }}
    
  • PHP
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    Hello <?php echo $name ?>
    

Imagine the user enters the following code for their name:

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<script>alert('hello!')</script>

Without any output escaping, the resulting template will cause a JavaScript alert box to pop up:

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Hello <script>alert('hello!')</script>

And while this seems harmless, if a user can get this far, that same user should also be able to write JavaScript that performs malicious actions inside the secure area of an unknowing, legitimate user.

The answer to the problem is output escaping. With output escaping on, the same template will render harmlessly, and literally print the script tag to the screen:

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Hello &lt;script&gt;alert(&#39;hello!&#39;)&lt;/script&gt;

The Twig and PHP templating systems approach the problem in different ways. If you're using Twig, output escaping is on by default and you're protected. In PHP, output escaping is not automatic, meaning you'll need to manually escape where necessary.

Output Escaping in Twig

If you're using Twig templates, then output escaping is on by default. This means that you're protected out-of-the-box from the unintentional consequences of user-submitted code. By default, the output escaping assumes that content is being escaped for HTML output.

In some cases, you'll need to disable output escaping when you're rendering a variable that is trusted and contains markup that should not be escaped. Suppose that administrative users are able to write articles that contain HTML code. By default, Twig will escape the article body.

To render it normally, add the raw filter:

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{{ article.body|raw }}

You can also disable output escaping inside a {% block %} area or for an entire template. For more information, see Output Escaping in the Twig documentation.

Output Escaping in PHP

Output escaping is not automatic when using PHP templates. This means that unless you explicitly choose to escape a variable, you're not protected. To use output escaping, use the special escape() view method:

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Hello <?php echo $view->escape($name) ?>

By default, the escape() method assumes that the variable is being rendered within an HTML context (and thus the variable is escaped to be safe for HTML). The second argument lets you change the context. For example, to output something in a JavaScript string, use the js context:

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var myMsg = 'Hello <?php echo $view->escape($name, 'js') ?>';

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.