CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine

What is Improper Neutralization of Special Elements Used in a Template Engine?

• Overview: Improper Neutralization of Special Elements Used in a Template Engine (CWE-1336) occurs when a template engine processes user input without correctly neutralizing special elements, allowing these inputs to be interpreted as code or expressions by the template engine.

• Exploitation Methods:

• Attackers can manipulate input to inject malicious expressions or commands into templates.
• Common attack patterns include injecting expressions like "{{7*7}}" to evaluate as code, or embedding scripts for XSS-style attacks.

• Security Impact:

• Direct consequences include unauthorized code execution, data leakage, or manipulation of application behavior.
• Potential cascading effects include privilege escalation, data corruption, or full system compromise.
• Business impact includes loss of customer trust, legal repercussions, and financial losses from data breaches.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing input before processing it with a template engine.
• Security best practices involve using context-aware encoding, avoiding direct insertion of untrusted data, and employing input validation libraries.
• Recommended tools and frameworks include using secure template engines that provide built-in escaping mechanisms and regularly updating them to the latest versions.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Java, PHP, Python, JavaScript, Interpreted

Affected Technologies: AI/ML, Client Server

Vulnerable Code Example

import org.thymeleaf.TemplateEngine;
import org.thymeleaf.context.Context;

public class TemplateEngineService {
    private TemplateEngine templateEngine = new TemplateEngine();

    public String renderTemplate(String templateName, String userInput) {
        // Vulnerable: User input is directly used in the template without any sanitization
        Context context = new Context();
        context.setVariable("userInput", userInput);
        return templateEngine.process(templateName, context);
    }
}

Explanation:

• In the above code, user input is directly passed into the template engine without any form of sanitization or encoding. This makes the application susceptible to template injection attacks. An attacker could inject malicious code or expressions into the template, potentially leading to unauthorized data access or code execution.

How to fix Improper Neutralization of Special Elements Used in a Template Engine?

To fix this vulnerability, user input should be properly sanitized and validated before it is used in the template. This involves encoding special characters that could be interpreted as part of the template syntax. Use a library for encoding or sanitization that is designed for the template engine you are using. Additionally, consider using a whitelist approach to validate input against allowed patterns.

Fixed Code Example

import org.thymeleaf.TemplateEngine;
import org.thymeleaf.context.Context;
import org.owasp.encoder.Encode;

public class TemplateEngineService {
    private TemplateEngine templateEngine = new TemplateEngine();

    public String renderTemplate(String templateName, String userInput) {
        // Fixed: Encode user input to prevent template injection
        String sanitizedInput = Encode.forHtml(userInput);  // Encode input to neutralize special characters
        Context context = new Context();
        context.setVariable("userInput", sanitizedInput);
        return templateEngine.process(templateName, context);
    }
}

Key Fixes:

• Import a Security Library: Line 11 introduces the OWASP Java Encoder library, which is specifically designed for encoding user input to prevent injection attacks.
• Sanitize Input: Line 15 encodes the user input using Encode.forHtml(). This method ensures that any special characters in the input are converted into a safe format that cannot be interpreted as part of the template syntax.
• Use Sanitized Input: Line 16 sets the sanitized input in the context, ensuring that the template engine processes only safe data.

By implementing these changes, the risk of template injection vulnerabilities is significantly reduced, as special elements in user input are properly neutralized. This approach follows best practices for secure coding in Java applications.