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

Over the last few chapters, we’ve explored vulnerabilities like Remote Code Execution, OS Injection, XML Injection, and Blind SQL Injection — each one exposing just how dangerous simple inputs can become when left unchecked.

Today, I want to take you into the world of a lesser-known but quietly dangerous vulnerability — Server-Side Includes (SSI) Injection. It may not make headlines as often as XSS or SQL injection, but under the right conditions, it can be just as impactful.

A while ago, while exploring a test environment for a small web platform, I noticed something unusual: the website was using .shtml files to display user comments on a guestbook page. That was my first clue.

Out of curiosity, I submitted a comment containing this line:

<!--#echo var="DATE_LOCAL" -->

To my surprise, the next time I visited the guestbook page, it displayed the server’s local date and time — right where my comment was. That moment was a red flag. The server wasn’t just storing my input — it was executing it.