Server Side Template Injection (Blind)

Server Side Template Injection (SSTI) is a critical security vulnerability that allows attackers to execute arbitrary code on your web server. When it's detected in a blind form, it means that the vulnerability is not immediately visible through direct error messages or responses from the server. This guide will walk you through the steps to fix this vulnerability in your web application.

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Step 1: Understand Server Side Template Injection (SSTI)

Before you can fix the vulnerability, it's crucial to understand what SSTI is. In essence, SSTI occurs when user input is injected into server-side templates without proper validation or sanitization. Attackers can exploit this to execute malicious code on the server.

SSTI vulnerabilities are typically found in applications that use template engines like Jinja2 (Python), Twig (PHP), or Thymeleaf (Java). In blind SSTI, attackers exploit the vulnerability without immediate feedback, making it challenging to detect.

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Step 2: Confirm the Vulnerability

Start by confirming that your application indeed has a blind SSTI vulnerability. The external vulnerability scanner should provide details about where the vulnerability exists, such as URLs, parameters, or inputs that are vulnerable.

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Step 3: Isolate and Investigate

Determine which parts of your application are vulnerable. Look for input points where user data is injected into templates. Common sources include URL parameters, form fields, cookies, or any data retrieved from the client-side.

For example, in Python using Jinja2 templates, you might see something like this:

name = request.args.get('name')
template = Template("Hello, {{ name }}!")
output = template.render(name=name)

In this case, if 'name' is not properly validated, it could be exploited.

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Step 4: Sanitize and Validate User Input

To fix SSTI, you must validate and sanitize all user inputs that make their way into templates. Depending on your application's technology stack, here are some common steps:

Python (Jinja2):

from jinja2 import escape

name = request.args.get('name')
# Sanitize user input
name = escape(name)
template = Template("Hello, {{ name }}!")
output = template.render(name=name)

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PHP (Twig):

$loader = new \Twig\Loader\FilesystemLoader('/path/to/templates');
$twig = new \Twig\Environment($loader);

$name = htmlspecialchars($_GET['name'], ENT_QUOTES, 'UTF-8');
$template = $twig->load('template.html');
echo $template->render(['name' => $name]);

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Java (Thymeleaf):

import org.springframework.web.util.HtmlUtils;

String name = HtmlUtils.htmlEscape(request.getParameter("name"));
model.addAttribute("name", name);

These examples demonstrate how to escape user input before it's rendered in a template.

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Step 5: Implement Content Security Policies (CSP)

To further mitigate SSTI risks, implement Content Security Policies (CSP). CSP helps prevent the execution of inline scripts or unauthorized resources on your web pages. Here's an example of a CSP header:

Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-cdn.com;

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Step 6: Regular Security Testing

Once you've fixed the SSTI vulnerability, it's essential to conduct regular security testing. Continuously scan your application for new vulnerabilities and consider using a Web Application Firewall (WAF) for added protection.

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Step 7: Stay Informed

Keep up-to-date with the latest security best practices and vulnerabilities. Security is an ongoing process, and new threats emerge regularly. Subscribe to security mailing lists, forums, or follow relevant blogs to stay informed.

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Conclusion

Fixing Server Side Template Injection (Blind) vulnerabilities is critical to the security of your web application. By validating and sanitizing user input and implementing Content Security Policies, you can significantly reduce the risk of exploitation. Remember that security is an ongoing process, so stay vigilant and proactive in addressing new threats as they arise.

By following these steps, you can strengthen the security of your web application and protect it from potential SSTI attacks.

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