Understanding Server-Side Template Injection (SSTI) Risks, Exploitation, and Prevention

Server-Side Template Injection (SSTI) is a critical vulnerability that can compromise the security of web applications. It occurs when user input is embedded within templates processed by server-side templating engines without proper validation or sanitization. In this article, we’ll explore what SSTI is, how it can be exploited, the risks associated with it, and the strategies to prevent it.

What is Server-Side Template Injection (SSTI)?

Server-Side Template Injection (SSTI) is a vulnerability that arises when user-controlled data is injected into templates processed by server-side templating engines. These engines are responsible for generating dynamic content in web applications, and when they improperly handle user input, it can lead to remote code execution (RCE) and other severe security issues.

How is SSTI Exploited?

SSTI can be exploited by injecting specially crafted payloads into input fields that are later processed by server-side templating engines. Attackers typically manipulate template syntax to execute arbitrary code, access sensitive data, or perform other malicious actions. For example, in a Flask web application using the Jinja2 templating engine, an attacker might inject the following payload

pythonCopy code{{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }}

This payload leverages Jinja2’s object traversal syntax to execute the read() method on the /etc/passwd file, allowing the attacker to read sensitive system files.

Risks Associated with SSTI

SSTI poses significant risks to web applications and their users. Some of the key risks include

Remote Code Execution (RCE) → Exploiting SSTI can allow attackers to execute arbitrary code on the server, potentially compromising the entire application or underlying system.Information Disclosure → Attackers can use SSTI to access sensitive information, such as server-side configurations, environment variables, or internal system data.Data Tampering → SSTI attacks can modify template logic or inject malicious content…