What is Server-Side Template Injection?

Server-Side Template Injection (SSTI) is a critical vulnerability in web applications. Attackers exploit this flaw by injecting harmful code into server-side templates, enabling unauthorized access, data breaches, or even complete server takeover.

Unlike client-side templating, which affects end-user experience, SSTI occurs within the server, making it distinct from cross-site scripting vulnerabilities. Despite their usefulness, SSTs in feature-rich web applications can inadvertently expose sensitive user inputs, emphasizing the need for proactive security measures to mitigate SSTI risks.

Working of SST (Server-Side Template)

The most common method for web frameworks to dynamically create HTML is by using templates. It comprises both the static parts of the HTML output, as well as the syntax that describes how it will place dynamic content.

These template systems are not safe from suspicious template authors. For example, a website should not permit its users to supply their own templates, since a template author can execute cross-site scripting attacks and they can also obtain properties of these template variables that may comprise sensitive data.

Then these template engines process template files, which supports in populating dynamic information into web pages. The template engine generates the HTML output reply when an HTTP request comes in.

Some of the most commonly used server-side template engines are Jinja2 or Jinja, Freemaker, Mako, Velocity, Smarty, Tornado, Genshi, Twig, Mustache, etc.

SSTs (Server-Side Templates) facilitate easy dynamic HTML generation, but they are susceptible to SSTI attacks. While SSTs enhance webpage customization and performance by processing user information directly on the server, their vulnerability underscores the importance of robust security measures in web development.