Blind SSTI

In the ever-evolving landscape of web application security, one vulnerability that continues to haunt developers is Blind Server-Side Template Injection (SSTI). This stealthy exploit can have devastating consequences, allowing attackers to execute arbitrary code on the server, leading to data breaches, unauthorized access, and more. In this blog post, we'll delve into what Blind SSTI is, examine real-life examples, and provide practical mitigation guidelines with code samples.

Understanding Blind SSTI

What is Blind SSTI?Server-Side Template Injection occurs when an attacker injects malicious code into a server-side template. Unlike traditional SSTI, Blind SSTI doesn't provide direct feedback, making it more challenging to detect and exploit. It often occurs when user input is improperly validated or sanitized in template engines.

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Real-Life Examples

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Example 1: Jinja2 in Python

# Vulnerable code

from jinja2 import Template

def render_template(user_input):

   template = Template("Hello, {{ user_input }}!")

   result = template.render(user_input=user_input)

   return result

In this example, the render_template function uses Jinja2 to render a template. If user input is not properly sanitized, an attacker could inject code like {{ 7*7 }}, resulting in the execution of arbitrary code.

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Example 2: Thymeleaf in Java

// Vulnerable code

import org.thymeleaf.TemplateEngine;

import org.thymeleaf.context.Context;
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public class TemplateRenderer {

   private final TemplateEngine templateEngine = new TemplateEngine()
‍public String renderTemplate(String user_input) {
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   Context context = new Context();
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   context.setVariable("user_input", user_input);
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   return templateEngine.process("greet_template", context);
 }


}

In this Java example using Thymeleaf, an attacker might inject malicious code like ${7*7} into the user_input, leading to code execution.

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Mitigation Guidelines

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1. Input Validation and Sanitization

Ensure that all user inputs are properly validated and sanitized before being used in templates. Use a strict allowlist approach to only allow specific characters and patterns.

‍Example: Python with Jinja2

from jinja2 import Template, escape

def render_template(user_input):

   sanitized_input = escape(user_input)

   template = Template("Hello, {{ user_input }}!")

   result = template.render(user_input=sanitized_input)

   return result

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2. Context-Aware Escaping

Understand the context in which the user input is used and apply appropriate escaping mechanisms. Different template engines may require different approaches.

Example: Java with Thymeleaf

import org.thymeleaf.TemplateEngine;

import org.thymeleaf.context.Context;

import org.unbescape.html.HtmlEscape;

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public class TemplateRenderer {

   private final TemplateEngine templateEngine = new TemplateEngine();

   
    public String renderTemplate(String user_input) {
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   Context context = new Context();
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   context.setVariable("user_input", HtmlEscape.escapeHtml5(user_input));
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   return templateEngine.process("greet_template", context);
  }


}

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3. Use Template Engine Safeguards

‍Configure template engines to restrict their capabilities, such as limiting available functions or disabling certain features altogether.

Example: Jinja2

from jinja2 import Environment, select_autoescape

env = Environment(

   autoescape=select_autoescape(['html', 'xml']),    

enable_async=True  # Example configuration to enable asynchronous template execution

)
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Conclusion

Blind SSTI is a serious threat that demands proactive measures from developers. By implementing robust input validation, context-aware escaping, and utilizing template engine safeguards, we can significantly reduce the risk of SSTI vulnerabilities. Stay vigilant, update dependencies regularly, and keep evolving your security practices to stay one step ahead of potential attackers.

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