Server-Side Template Injection (SSTI) vulnerabilities are often overlooked, but they can have severe consequences if exploited by an attacker. In this blog post, we will explore the nature of SSTI vulnerabilities, discuss ways to identify and exploit them and provide mitigation strategies to protect your web applications.

Understanding SSTI Vulnerabilities: Server-Side Template Injection occurs when an attacker can inject template code into a web application, causing the server to execute it. This can lead to information disclosure, remote code execution, or even complete server takeover. SSTI vulnerabilities typically arise from the improper handling of user input and the insecure use of template engines.

Identifying SSTI Vulnerabilities: To identify SSTI vulnerabilities, look for user input being passed to a template engine. Common signs include the use of double curly braces {{ }} or percent signs {% %} in the rendered HTML. You can test for SSTI by injecting simple expressions like {{7*7}} and observing if the server evaluates and returns the result.

Exploiting SSTI Vulnerabilities: Once you have identified an SSTI vulnerability, you can exploit it by injecting malicious template code. Depending on the template engine in use, the payloads will differ. Some common payloads include:

• Accessing server environment variables: {{env}} or {{request.environ}}
• Reading server files: {{'/etc/passwd'|file}}
• Remote code execution: {{''.__class__.__mro__[1].__subclasses__()[408]('/tmp/payload.py', 'r').read()()}}

Make sure to research the specific template engine in use to craft the appropriate payloads.

Post-Exploitation Activities: After exploiting an SSTI vulnerability, you can further compromise the target system by:

• Escalating privileges
• Gaining persistence
• Pivoting to other systems in the network
• Exfiltrating sensitive data

Mitigation Strategies: To protect your web applications from SSTI vulnerabilities, follow these best practices:

• Properly validate and sanitize user input
• Use a secure template engine that does not allow arbitrary code execution
• Employ a Content Security Policy (CSP) to prevent unauthorized code execution
• Regularly update your template engine to patch known vulnerabilities

Conclusion: Server-Side Template Injection vulnerabilities can pose a significant risk to your web applications. By understanding how to identify, exploit, and mitigate SSTI vulnerabilities, you can better protect your applications and ensure the security of your users’ data. Always remain vigilant and proactive in your approach to web application security.