Simplifying SSTI: Key Facts for Everyone

Today, I completed my first CTF (Capture The Flag) challenge. For those unfamiliar, CTFs are competitions that focus on cybersecurity topics. If you've heard of LeetCode, think of CTFs as similar but focused on cybersecurity (though it's not a perfect comparison).

I was both excited and nervous, eager to learn something new. So, I jumped right in. I started with the beginner-friendly problems and clicked on the first one listed. It was called SSTI1. I read the description and the topics it covered, then launched the instance.

A website link appeared, and I spent some time trying to debug it, but soon found myself stuck. With no clear way forward, I decided to check the hint. It pointed to Server-side Template Injection, a concept I never heard of. I did some research online, aiming to understand what it was all about, how it works and can be abused and ways to prevent it.

I am happy to share what I learnt and help others like me understand the same, even if you are someone completely new to programming and web security.

Here are the topics covered:

What is SSTI?
How an Attack Happens?
Why This is Dangerous?
Why Does This Happen?
Real-World Analogy
How to Protect Against SSTI?
Final Thoughts

What is SSTI?

Imagine you have a website with forms where users can enter text — like a name or a comment — and the website displays that text back to them on a page.

For example:

You enter your name as John.
The website then says, “Hello, John!” on the page.

Behind the scenes, the web server processes your input and generates that message using a template system. This is like filling in blanks in a template:

Template: “Hello, {{ user_name }}!”

Here, {{ user_name }} is a placeholder that the server replaces with your actual input, like "John."

How an Attack Happens?

Now, here’s where it gets dangerous.

If the server isn’t careful, it might not just accept your name but might accidentally treat your input as code.

For example:

Let’s say instead of typing “John,” a hacker types something tricky like:
```
  {{ 7 * 7 }}
```
If the server’s template system doesn’t handle input properly, it might actually run that code!
It would calculate 7 * 7 and display 49 on the page.

Why This is Dangerous?

A skilled hacker can take it further and type even more dangerous things, like:

{{ system('ls') }}

If the server blindly runs this input as part of the template, the command system('ls') might execute on the server and show a list of files.

In the worst-case scenario, the hacker could run harmful commands that:

Steal sensitive data from the server.
Delete files.
Access databases.
Control the server entirely!

Why Does This Happen?

This happens because some template engines (the systems used to fill in the placeholders) are too powerful and can evaluate not just simple placeholders but also real programming code.

If the server doesn’t protect itself properly, it may trust user input too much and run whatever code the hacker tries to inject.

Real-World Analogy

Imagine you’re filling out a form at the bank, and the teller says:

“Write your name, and we’ll greet you on the screen.”

You write:

John

The screen says, “Hello, John!”

Now imagine if you wrote:

Erase all accounts.

If the bank’s system blindly runs that as a command, it could erase all their customer accounts. That’s essentially what happens in an SSTI attack — only it’s code running on the server instead of text.

How to Protect Against SSTI?

Here are the key points to keep in mind while using server-side templating:

Don’t trust user input: Always treat anything a user types as potentially harmful.
Use escaping: Convert user input into harmless text instead of code (like turning <script> into <script>).
Use safer template engines: Some template engines automatically protect against SSTI (like Django’s template system).
Validate input: Only allow input that matches what you expect (e.g., names with letters only).
Disable Debug Mode: Never leave debugging enabled in production.

Scenario: Example of SSTI in Python (Flask)

Here’s the insecure code:

from flask import Flask, request, render_template_string

app = Flask(__name__)

@app.route('/')
def home():
    user_input = request.args.get('name', '')
    template = f"Hello, {user_input}!"
    return render_template_string(template)

if __name__ == '__main__':
    app.run(debug=True)

1. Use a Safer Template Engine

Instead of directly rendering user input in render_template_string(), use a template engine that escapes user input automatically.

Example with Jinja2 (Flask’s default):

pythonCopyEditfrom flask import Flask, request, render_template

app = Flask(__name__)

@app.route('/')
def home():
    user_input = request.args.get('name', '')
    return render_template('home.html', name=user_input)  # Use template file, not string

if __name__ == '__main__':
    app.run(debug=True)

In home.html:

htmlCopyEdit<!DOCTYPE html>
<html lang="en">
<head>
    <title>Welcome</title>
</head>
<body>
    <h1>Hello, {{ name }}</h1>  <!-- Safe: Jinja2 automatically escapes input -->
</body>
</html>

This method automatically escapes any potentially dangerous user input, like:

{{ 7*7 }}

It will display literally as {{ 7*7 }}, not 49.

2. Manually Escape Input

If you must use render_template_string() (not recommended), escape user input with flask.escape or markupsafe:

from flask import Flask, request, render_template_string, escape

app = Flask(__name__)

@app.route('/')
def home():
    user_input = request.args.get('name', '')
    escaped_input = escape(user_input)  # Escape dangerous characters
    return render_template_string(f"Hello, {escaped_input}!")  # Safely render input

if __name__ == '__main__':
    app.run(debug=False)

Now, if the user inputs {{7*7}}, it will be escaped and displayed as {{7*7}}, without executing.

3. Validate User Input

Only allow specific types of input (like letters or numbers):

import re
from flask import Flask, request, render_template_string

app = Flask(__name__)

@app.route('/')
def home():
    user_input = request.args.get('name', '')
    if not re.match(r'^[a-zA-Z]+$', user_input):  # Allow only letters
        user_input = "Invalid input"
    return render_template_string(f"Hello, {user_input}!")

if __name__ == '__main__':
    app.run(debug=False)

Now, if someone tries to input {{7*7}}, the input won’t match the regex, and the server will return “Invalid input.”

4. Disable Debug Mode in Production

Flask’s debug=True can expose sensitive debugging information during an SSTI attack.
Always disable debug mode in production:

app.run(debug=False)

And that’s what I learned today. I wish to improve upon my knowledge and see its use in real-world scenarios. Hope it was of help to you as well.

Final Thoughts

If you’re also new like me, don’t get discouraged if you don’t know everything at first – every challenge is an opportunity to learn. Yesterday, I didn’t know such a thing existed and today I’m writing an article about it. I hope to tackle more such challenges and carry forward in this journey. Wish me luck and I wish for you too!

What About You?

Have you ever solved an SSTI challenge or participated in a CTF? Let me know in the comments, and feel free to share your own tips and experiences!

SSTI Made Easy: Essential Information for All