Escaping content

What is an XSS attack?

Cross-site scripting (XSS) is a type of security vulnerability that allows attackers to run arbitrary JavaScript in the browsers of other, legitimate users of your site. This attack can be used to extract critical authentication information like session cookies from your users' browsers or to manipulate your site in unexpected ways.

XSS works by injecting code into the HTML output of your site in a place where you didn't expect it when writing the template code. Here's an example of a vulnerable snippet:

<footer>
  <p class="copyright">
    <?= $site->copyright() ?>
  </p>
</footer>

Looks harmless, right? But what if the copyright field of your site.txt contains this:

Title: ...

----

Copyright: <script>alert('Something malicious')</script>

The HTML output you will get from the snippet is this:

<footer>
  <p class="copyright">
    <script>alert('Something malicious')</script>
  </p>
</footer>

Yikes! Every time the page is loaded, every user's browser will now execute the malicious JavaScript – whatever code it contains. Of course it doesn't have to be a simple call to the alert() function, it can do everything JavaScript is capable of. And that can be very harmful.

This is just one example of an XSS attack. XSS can also occur with attribute values, strings printed inside legitimate <script> blocks etc. Often the attacker needs to be a bit more clever than in our example, but without protection it's always possible to misuse your template or snippet for an XSS attack.

How to protect against XSS attacks

There are three common ways to prevent XSS attacks:

1. Data validation
   This involves making 100 % sure that the content you output in your templates and snippets has the exact format you expect and doesn't contain unexpected characters that could be misued. While validating your content fields can be very useful to make sure your content fits your site and works correctly, it is hard to get right as it's often difficult to make the validator strict enough to block any harmful content. It's also difficult to even anticipate every possible attack.
2. Data sanitization
   Sanitizing data means removing any unwanted characters from it. Basically it is a stricter form of validation and can be useful in use-cases like comment forms, but it shares its disadvantages with data validation – it's hard to implement it securely.
3. Escaping
   Escaping means replacing every unsafe character with an escape sequence that tells the browser it should interpret the character as text and not as a special character. When printing content in the HTML body, for example, a harmful character could be escaped with an HTML entity: The < character would become <. It's important to use the right escaper for each output context, i.e. the place where your content is being output (HTML body, HTML attribute, inside a <script> tag, as part of a URL...). Otherwise attackers will be able to circumvent your escaper.

Depending on your use-case, one or more of these strategies might be a good fit. You can combine multiple of them and sometimes you need to – e.g. your validator might not know in which places your content is being output, so you need a context-sensitive escaper on top of your validator. The validator then only checks if the content follows the right structure.

Escaping data in templates and snippets

The esc() helper and escape()/esc() field methods

Often the most effective and also easiest solution to prevent XSS on your Kirby site is to use context-sensitive escaping.

For that, we provide the esc() helper and the escape()/esc() field methods.

Let's take our example snippet from above and add escaping to it:

<footer>
  <p class="copyright">
    <?= $site->copyright()->escape() ?>
  </p>
</footer>

It's that simple! Now let's see what the output looks like for the malicious content:

<footer>
  <p class="copyright">
    &lt;script&gt;alert('Something malicious')&lt;/script&gt;
  </p>
</footer>

Because all special characters in HTML have been escaped, the browser will no longer execute the malicious script but will print the field contents as-is (so the malicious HTML code itself will be displayed without doing any harm).

Context-sensitive escaping

You might wonder why Kirby doesn't escape everything automatically if it's so simple. As you've read above, escaping always needs to be context-sensitive. You cannot use the same escaping method for HTML text, attributes or URLs. This is why you need to tell Kirby what type of escaping you need. Here are a few examples of different contexts:

<img alt="<?= $image->alt()->escape('attr') ?>" src="<?= $image->url() ?>" />

<section style="--columns: <?= $section->columns()->escape('css')">
...
</section>

<script>
let yourVariable = "<?= $page->jsVariable()->escape('js') ?>";

// ...
</script>

<iframe src="https://map.example.com/?lat=<?= $map->lat()->escape('url') ?>&lon=<?= $map->lon()->escape('url') ?>"></iframe>

As you can see, you can pass the context type to the helper or field method and Kirby will automatically escape the content correctly.

Escaping KirbyText

As KirbyText is an extension of Markdown, it also supports raw HTML and allows the insertion of malicious JavaScript links:

## Nice headline

This is a paragraph with **some formatting**.

<script>alert('And here comes something malicious.')</script>

This [link is also malicious](javascript:alert(1)).

If you can't trust your KirbyText content but do want to allow formatting, you can enable the Markdown safe mode:

<?= $page->text()->kirbytext(['markdown' => ['safe' => true]]) ?>

This will create the following output:

<h2>Nice headline</h2>
<p>This is a paragraph with <strong>some formatting</strong>.</p>
<p>&lt;script&gt;alert('And here comes something malicious.')&lt;/script&gt;</p>
<p>This <a href="javascript%3Aalert(1)">link is also malicious</a>.</p>

You can globally enable Markdown safe mode with the markdown.safe option.