Handlebars HTML Escaping

HTML escaping is an important security measure that ensures the safe transmission of data between a client and server. When user input is not properly sanitized or escaped, it can lead to cross-site scripting (XSS) attacks, where malicious scripts are injected into a website, allowing attackers to steal sensitive information or execute code on a victim's machine.

Handlebars is a popular JavaScript templating engine that provides a simple and intuitive syntax for generating dynamic HTML content. However, it can also be susceptible to XSS vulnerabilities if not properly handled. Fortunately, Handlebars has built-in support for HTML escaping, which allows developers to safely include user input in their templates without risking security breaches.

In this article, we will explore how Handlebars handlebars handle html escaping and provide some code examples to illustrate its usage.

What is HTML Escaping?

HTML escaping is the process of encoding special characters in HTML content to prevent them from being interpreted as HTML tags or attributes. For example, the < character is used to start a new element in HTML, but if it appears in user input that is not properly escaped, an attacker could inject malicious code into your website and take control of your server.

HTML escaping can be achieved using various techniques, such as encoding special characters with HTML entities (e.g., < becomes <), converting them to their ASCII equivalents (e.g., < becomes 0x3C), or wrapping the input in a CDATA section (e.g., <![CDATA[ user input ]]>).

How Does Handlebars Handle HTML Escaping?

Handlebars has built-in support for HTML escaping, which allows developers to safely include user input in their templates without risking security breaches. By default, Handlebars will escape any output that is not a valid HTML entity or attribute value. This means that if you include user input in your template that contains special characters, they will be automatically encoded to prevent them from being interpreted as HTML tags or attributes.

For example, let's say we have a template that includes user input:

<div>{{userInput}}</div>

If the user input contains any special characters, such as < or >, Handlebars will automatically escape them using HTML entities:

<div>&lt;user input&gt;</div>

However, if we want to include user input that contains valid HTML tags or attributes, we can use the {{!}} syntax to disable escaping for that specific section of the template. This is useful when we need to pass raw HTML code to the server without any modification:

<div>{{#if userInputContainsHtml}}
    <div>{{userInput}}</div>
{{else}}
    {{!}}{{userInput}}{{/if}}
</div>

In this example, if the user input contains valid HTML code (e.g., <p>Hello, world!</p>), it will be passed through to the server without any modification. If it does not contain any HTML tags or attributes, Handlebars will automatically escape it using HTML entities.

Handlebars also provides a built-in helper function called {{safeHtml}} that allows us to safely include user input in our templates without escaping it. This can be useful when we need to pass raw HTML code to the server and trust the source of the input:

<div>{{#if safeToIncludeHtml}}
    <div>{{safeHtml userInput}}</div>
{{else}}
    {{!}}{{userInput}}{{/if}}
</div>

In this example, if the input has been verified as safe to include in our template (e.g., via a whitelist or other security measures), we can use the {{safeHtml}} helper function to pass it through to the server without any modification. If it is not safe, Handlebars will automatically escape it using HTML entities.

Conclusion

Handlebars provides built-in support for HTML escaping, which allows developers to safely include user input in their templates without risking security breaches. By default, Handlebars will escape any output that is not a valid HTML entity or attribute value, and we can use the {{!}} syntax or {{safeHtml}} helper function to disable escaping or pass raw HTML code to the server when necessary.

As with any security measure, it's important to carefully review your templates and ensure that they are properly sanitized and validated before passing them through to the server. By following best practices for secure coding and leveraging Handlebars' built-in features for HTML escaping, you can minimize the risk of XSS attacks and keep your website safe from attackers.