SSTI Explanation

Server-side Template Injection (SSTI) is a type of security vulnerability that occurs when an attacker is able to inject malicious code or templates into a web application's server-side template engine. Server-side template engines are used to generate dynamic web pages by combining static templates with data from the server, which can include user input.

The most common context where SSTI vulnerabilities are found is in web applications that use template engines, like Jinja2 (Python), Thymeleaf (Java), or Twig (PHP). These template engines allow developers to embed dynamic content, such as user-provided data, into HTML, XML, or other markup languages. However, if these engines are not properly secured, an attacker may exploit this feature to execute arbitrary code on the server.

Here's a simplified example in Python using Jinja2: 

from jinja2 import Environment, FileSystemLoader
# User input
user_input = "{{ 7 * 7 }}"
# Create a Jinja2 environment
env = Environment(loader=FileSystemLoader('/path/to/templates'))
# Render the template with user input
template = env.get_template('template.html')
output = template.render(user_input=user_input)
print(output)

In this example, if an attacker provides the user_input with malicious code like {{ 7 * 7 }}, the server-side template engine would evaluate this expression and execute it on the server, potentially causing harm or leaking sensitive information.

Preventing SSTI vulnerabilities involves properly sanitizing and validating user input and avoiding the direct execution of user-provided content within templates. Developers should also configure and harden their server-side template engines to prevent code execution and evaluate the risk of SSTI in their web applications. Regular security testing, code reviews, and staying up to date with security best practices are essential for mitigating SSTI vulnerabilities.