Hello everyone, hope you’re doing well.
Welcome back to my Medium series, The Injection Chronicles.

In the past few chapters, we’ve walked through the world of Remote Code Execution, OS Injection, XML Injection, and Blind SQL Injection — each one exposing how dangerous simple-looking features can become when misused.

Today, I want to introduce you to a subtle yet powerful vulnerability that lives inside many modern web apps: Server-Side Template Injection, or SSTI.

It’s one of those bugs that can appear harmless at first glance — but with the right payload, it can let an attacker read sensitive data, access environment variables, and even gain full control of the server.

What Is Server-Side Template Injection?

Most web applications use template engines to dynamically render pages. For example, when a user visits their profile page, a template might be used to render something like:

<h1>Welcome, {{ username }}!</h1>

The server fills in {{ username }} with the actual value — like "Alex" — before sending it to the browser.