SSTI (Server Side Template Injection)

👉 Overview

👀 What ?

Server Side Template Injection (SSTI) is a vulnerability that occurs when user input is embedded in a template in an unsafe manner. It allows an attacker to inject code into a server-side template, leading to Remote Code Execution (RCE).

🧐 Why ?

Understanding SSTI is crucial because it's a common vulnerability that can lead to serious consequences if exploited, including loss of sensitive data or even total control of the server. Its exploitation is often undervalued because the consequences are not as immediate and obvious as those of other vulnerabilities.

⛏️ How ?

To exploit an SSTI vulnerability, an attacker needs to identify a user input field that is directly used in server-side templates. Once identified, they can inject malicious template code that will be executed server-side. This can be mitigated by properly validating and sanitizing user inputs, enforcing strict segregation of code and data, and using secure default configurations.

⏳ When ?

SSTI vulnerabilities have been recognized and exploited since the early 2000s, but they've gained more attention in recent years due to the rise in popularity of web development frameworks that use server-side templates.

⚙️ Technical Explanations

Template engines are widely used in web development to create dynamic web pages. They work by combining a template with a data model to generate a final document. If an attacker can control part or all of the template, they can execute arbitrary code on the server. This can be done by manipulating template expressions. The severity of this vulnerability depends on the features of the template engine, such as whether it supports arbitrary command execution, file read/write, or sandbox escape techniques.