Handlebars Templating: Security Best Practices

TLDR: This blog post provides a high-level overview of common security vulnerabilities in the Handlebars templating engine, emphasizing known issues and mitigation strategies. However, it’s not an exhaustive list, and developers should stay informed and conduct regular security audits and testing on their applications.

Handlebars is a popular templating engine that helps developers create dynamic HTML content. It allows you to separate your logic from your presentation, making your code more maintainable and easier to understand. However, as with any technology, Handlebars can introduce potential vulnerabilities if not used securely. In this blog post, we will discuss some common vulnerabilities in applications that use Handlebars for rendering HTML and provide recommendations for mitigating these risks.

Cross-Site Scripting (XSS)

Cross-Site Scripting (XSS) is one of the most common web application vulnerabilities. It occurs when an attacker injects malicious scripts into a web application, which are then executed by the victim’s browser. Handlebars automatically escapes content to prevent XSS, but there are still ways that attackers can exploit vulnerabilities in the templating engine.

Unescaped content: Handlebars provides an option to render unescaped content using triple braces `{{{ }}}` instead of double braces `{{ }}`. This can be dangerous if used with untrusted data, as it allows for the execution of malicious scripts.. — Solution: Always use double braces `{{ }}` to render content unless you are certain that the data is safe to display unescaped.Custom helpers: Developers often create custom Handlebars helpers to extend the functionality of the templating engine. If these helpers do not properly sanitize and escape user input, they can introduce XSS vulnerabilities.. — Solution: Always validate and escape user input in custom helpers before rendering it to the client.

2. Server-Side Template Injection (SSTI)

Server-Side Template Injection (SSTI) occurs when an attacker can inject template syntax into a server-side rendered template, resulting in the execution of arbitrary code. While Handlebars is designed for client-side rendering, it can be used on the server side as well.

Java-based server-side rendering: If an application is using Handlebars on the server side with a Java backend, it may be possible to exploit Java-based helpers and execute arbitrary code.. — Solution: Implement proper input validation and sanitization, limit the use of Java-based helpers, and use secure server-side rendering libraries, such as Handlebars.java.

3. Information Disclosure

Handlebars provides several built-in variables and helpers that can potentially expose sensitive information if not used securely.

Variable exposure: Some built-in variables that can expose sensitive data from the application’s context.. — Solution: Limit the use of built-in variables that can expose sensitive data, and ensure proper access controls are in place to restrict unauthorized access.Custom helpers: As mentioned earlier, custom helpers can also expose sensitive data if not implemented securely.. — Solution: Implement proper input validation and sanitization in custom helpers and limit the exposure of sensitive data.

Handlebars is a powerful templating engine that can greatly simplify the development of dynamic web applications. However, it is important to be aware of the potential vulnerabilities it can introduce and follow security best practices to mitigate these risks. By understanding and addressing the common vulnerabilities in applications that use Handlebars for rendering HTML, you can help create a more secure and robust web application.