Exploiting Server-Side Template Injection (SSTI)

Overview

Server-Side Template Injection (SSTI) is a critical web vulnerability that occurs when user input is unsafely embedded into a server-side template engine (such as Jinja2 in Python, Twig in PHP, or FreeMarker in Java). This allows an attacker to inject template directives, enabling them to manipulate the template engine and often execute arbitrary code on the underlying server.

The Problem

Template engines are designed to dynamically generate HTML by combining static templates with dynamic data. A vulnerability arises when developers confuse template evaluation with standard string concatenation.

Vulnerable Code Example (Python/Flask/Jinja2):

# Bad: Unsafely concatenating user input directly into the template string
template_string = "Welcome to our site, " + request.args.get('name') + "!"
return render_template_string(template_string)

If the attacker sets their name as {{ 7 * 7 }}, the server evaluates the expression and returns "Welcome to our site, 49!". The attacker has successfully injected template syntax.

Solution and Configuration

The solution is to pass user input to the template engine as data context, not as part of the template structure itself.

Secure Implementation:

# Good: Passing input via context variables
template_string = "Welcome to our site, {{ name }}!"
return render_template_string(template_string, name=request.args.get('name'))

In this secure version, passing {{ 7 * 7 }} will literally output "Welcome to our site, {{ 7 * 7 }}!", neutralizing the exploit.

Technical Details

SSTI is highly dangerous because modern template engines are essentially full programming languages. If an attacker discovers an SSTI vulnerability, they will use "sandbox escape" techniques. In Python's Jinja2, for instance, everything is an object. An attacker can climb the object hierarchy using special attributes like __class__ and __mro__ to reach the base object class, find the subprocess.Popen class, and execute OS commands: {{ '.__class__.__mro__[1].__subclasses__()[407]('id', shell=True, stdout=-1).communicate()[0] }}. This payload instantly results in Remote Code Execution (RCE), giving the attacker full control over the Linux server.

Conclusion

SSTI is often mistaken for Cross-Site Scripting (XSS), but while XSS attacks the user's browser, SSTI targets the server itself. Developers must treat template engines with the same strict input validation and sanitization as SQL database queries, ensuring user data is never evaluated as code.