Category: Cybersecurity

Templating Engines – What a Hacker Needs to Know

A templating engine is a tool or framework used to dynamically generate HTML (or other text-based formats like XML, JSON, etc.) from data and templates. These templates contain static content (like the structure of a webpage) and placeholders for dynamic content (like user data or server-generated information). When the templating engine processes the template, it fills in the placeholders with actual data, creating a final document that can be rendered or transmitted.

Key Features of Templating Engines:

  1. Separation of Concerns: Templating engines separate the business logic (data processing) from the presentation logic (how the data is displayed). This makes it easier to manage and maintain code.
  2. Logic Support: Many templating engines allow simple logic (like loops and conditionals) within the template, allowing developers to display data conditionally or iterate over lists.
  3. Security Features: Templating engines often include built-in mechanisms to escape special characters, preventing injection attacks like Cross-Site Scripting (XSS).
  4. Reusability: Components and templates can be reused across different views or parts of the application, reducing redundancy and speeding up development.

Common Templating Engines in Various Tech Stacks:

  1. Node.js / JavaScript
    • Pug (formerly Jade): Known for its terse syntax, Pug is a popular templating engine for generating HTML in Node.js applications.
    • EJS (Embedded JavaScript): A simple templating engine that allows embedding JavaScript directly into HTML files.
    • Handlebars.js: Known for its logic-less design (with no logic inside the templates), making it a secure and clean option for templating.
  2. Ruby on Rails
    • ERB (Embedded Ruby): Ruby’s default templating engine that allows embedding Ruby code within HTML.
    • Haml (HTML Abstraction Markup Language): Known for its clean, indentation-based syntax, which simplifies writing HTML.
  3. Python
    • Jinja2: A very popular templating engine used in Python frameworks like Flask and Django (which uses its own variant called Django Templates). It’s powerful, extensible, and supports advanced templating logic.
  4. PHP
    • Blade: Laravel’s templating engine. Blade is known for its simplicity and the ability to use PHP code directly within templates.
    • Twig: A flexible templating engine used in frameworks like Symfony and Silex. It is fast, secure, and offers clear syntax.
  5. Java
    • JSP (JavaServer Pages): JSP is part of the Java EE specification and allows embedding Java code within HTML.
    • Thymeleaf: A modern Java templating engine widely used with Spring Boot. It supports both server-side and client-side rendering and is known for its natural template style, allowing templates to be valid HTML.
  6. .NET / ASP.NET
    • Razor: A templating engine used in ASP.NET MVC and .NET Core. It allows embedding C# code directly within HTML, offering a concise and powerful syntax for creating dynamic content.
  7. Go
    • Go HTML/template: The Go programming language provides a built-in templating engine for safely generating HTML output.
  8. Rust
    • Tera: A templating engine inspired by Jinja2 for Rust, providing safe, fast, and secure templating.

Use Cases for Templating Engines:

  • Web Applications: Templating engines are used to dynamically generate webpages that are served to the user based on the context (e.g., personalized user data).
  • Email Generation: They are often used to generate dynamic email templates that are sent to users (e.g., transactional emails, newsletters).
  • Document Generation: In some cases, templating engines are used to generate structured documents like PDFs, reports, or even JSON files.

Modern Alternatives:

In modern web development, templating engines are often supplemented or replaced by front-end frameworks like React, Vue, and Angular, which handle the rendering of templates on the client-side with more advanced state management. However, server-side templating engines are still widely used, especially in traditional MVC frameworks or when SEO and server-rendering performance are critical.

Each tech stack chooses a templating engine based on language, framework compatibility, and the specific needs of the application.

Templating Engine Security Considersations

When using templating engines, security is an important concern, as improper use can lead to vulnerabilities that expose applications to various attacks. Below are some common security considerations and best practices to keep in mind when using templating engines:

1. Cross-Site Scripting (XSS)

Cross-Site Scripting (XSS) occurs when malicious scripts are injected into web pages viewed by other users. This is one of the most common risks associated with templating engines, as they often render user-generated content. If the content is not properly sanitized, attackers can inject harmful scripts that execute in users’ browsers.

Mitigation:

  • Auto-escaping: Ensure the templating engine automatically escapes special characters (like <, >, &, "). Most modern templating engines have auto-escaping enabled by default (e.g., Jinja2, Twig, Razor, etc.).
  • Manually escaping: In cases where auto-escaping is disabled, manually escape output using the templating engine’s escape functions.
  • Sanitize inputs: Use a trusted sanitization library to filter or sanitize user inputs to ensure no dangerous HTML or JavaScript is included.

2. Server-Side Template Injection (SSTI)

Server-Side Template Injection (SSTI) occurs when user input is inserted directly into the template, and the templating engine evaluates it as executable code. This can result in arbitrary code execution on the server.

Mitigation:

  • Avoid dynamic template evaluation: Never pass user input directly into templates for evaluation. This is particularly dangerous when user data is used to dynamically generate templates.
  • Whitelisting allowed input: If user input must be inserted into a template, use whitelisting to strictly define allowed characters or structures.
  • Disable template execution in user-supplied content: If users can control parts of the template, ensure that template evaluation is disabled or restricted in these areas.

3. Cross-Site Request Forgery (CSRF)

CSRF attacks occur when a malicious actor tricks a user’s browser into making an unwanted request to the server on behalf of the user (usually via an authenticated session).

Mitigation:

  • Include anti-CSRF tokens in templates: Ensure that CSRF tokens are embedded in forms and that the server validates these tokens with every form submission or sensitive request.
  • Use framework security features: Many frameworks (like Django, ASP.NET, or Spring) offer built-in protections for CSRF. Make sure these are enabled in templates that handle forms or sensitive actions.

4. Output Encoding

Output encoding ensures that data rendered in different contexts (HTML, JavaScript, CSS, etc.) is properly encoded for that context. If output is not correctly encoded, it can lead to XSS or other injection vulnerabilities.

Mitigation:

  • Use context-aware output encoding, depending on where the data is placed in the document (e.g., HTML, JavaScript, or attributes). For example:
    • Escape HTML content before inserting it into a page.
    • Properly escape any content inside <script> tags to prevent JavaScript injection.
    • Use a library or built-in functions of the templating engine to handle the proper encoding.

5. Template Inheritance Abuse

Some templating engines allow inheritance (one template extending another). If improperly configured, this can allow attackers to inject or modify templates by exploiting template inheritance features.

Mitigation:

  • Restrict template paths: Limit the locations from which templates can be loaded. Ensure templates are only loaded from safe, predefined directories.
  • Control template inheritance: Ensure that inheritance mechanisms are locked down and cannot be abused by user input or external factors.

6. Data Exposure

Templating engines may expose sensitive data in templates, either by mistake or misconfiguration. For example, debugging information, error messages, or sensitive variables could be unintentionally rendered in a web page.

Mitigation:

  • Disable debug mode in production: Ensure that debugging features and detailed error messages are disabled in production environments. Display only user-friendly error messages.
  • Sanitize data before rendering: Avoid passing sensitive data (e.g., passwords, API keys) directly to templates. Ensure sensitive data is not exposed via templates or error messages.
  • Minimal data passing: Pass only the data necessary for rendering the template. Avoid passing full objects or unnecessary details to prevent accidental exposure.

7. Template Caching Issues

Many templating engines use caching to improve performance, storing precompiled templates. This caching can introduce security concerns if not managed properly.

Mitigation:

  • Invalidate cached templates on change: Ensure that cached templates are properly invalidated and recompiled when the source templates are updated.
  • Secure cache location: Ensure that the cache is stored in a secure location and cannot be tampered with or accessed by unauthorized parties.

8. Template File Inclusion

Some templating engines support including other files within a template (e.g., include statements in Twig or Jinja2). If user input can control which files are included, attackers could potentially include arbitrary files, leading to data leakage or code execution.

Mitigation:

  • Restrict file inclusion: Use strict control over which files can be included in templates. Do not allow user input to determine file paths without validation or sanitization.
  • Sanitize input controlling includes: If user input affects file inclusion, sanitize it to prevent directory traversal attacks or inclusion of unintended files.

9. Denial of Service (DoS)

Complex template rendering or dynamic content generation may inadvertently lead to Denial of Service (DoS) attacks. For example, improperly configured template loops could consume server resources, leading to performance degradation or crashes.

Mitigation:

  • Limit template complexity: Avoid overly complex logic in templates that could result in performance bottlenecks. Restrict the use of recursive functions or unbounded loops.
  • Use resource limits: Set limits on the size of input data, the depth of recursion, or the number of iterations a template can handle.

By following these security considerations and using the templating engine’s built-in features, you can significantly reduce the risk of security vulnerabilities in your web applications.

In summary, templating engines make connecting the front-end UI with a backend datasource. Every convienience comes with potentials risks so choose wisely.