Server-Side Template Injection (SSTI): Finding and Exploiting

Server-Side Template Injection (SSTI): Finding and Exploiting It Safely

Most modern web apps use templates. A template is like a page layout with blanks that the server fills with real data, such as a username or a message. It keeps code clean and makes it easy to reuse the same layout again and again.

When templates are used in the wrong way, a small mistake can let an attacker run code on the server. That problem is called Server-Side Template Injection (SSTI). Instead of sending plain text, an attacker sends special template syntax that the server treats as code.

SSTI matters for security testers, bug bounty hunters, and developers because it can lead to real damage. An attacker may read private data, steal API keys, or even take full control of the server. This post explains what SSTI is, how to spot it, how attackers exploit it, and what you can do to stop it.

What Is Server-Side Template Injection (SSTI) and Why Is It So Dangerous?

SSTI happens when user input goes into a server-side template and gets treated as code instead of data.

Think of a simple template like:

Hello, {{username}}!

Here, {{username}} It is a placeholder. The server replaces it with the real username before sending the page to the browser. If the app handles this safely, the user only controls the value, not the behavior.

With SSTI, the app dangerously uses user input. Instead of only filling in a value, the input becomes part of the template logic itself. For example, an attacker might send:

{{7*7}}

If the server is using a template engine like Jinja2, it might evaluate this and return it 49 in the response. At that point, you are not just showing user data; you are executing code on the server.

From there, things get serious very fast. In many engines, template code can:

Read environment variables or config files
Access sensitive data, such as passwords or tokens
Call internal functions or objects
Run system commands and open network connections

That means one small template bug can lead to remote code execution (RCE) and full server compromise. For bug bounty hunters, SSTI often means high or critical severity reports. For defenders, it is a direct path to data theft, service outages, and lateral movement inside the network.

If you want a deeper reference later, the PortSwigger Web Security Academy has a helpful page on server-side template injection basics.

How Template Engines Work in Modern Web Apps

A template engine is like a document with blanks and small rules that the server uses to build HTML, emails, or PDFs. The engine takes a template file plus some data, then produces the final output.

Some popular template engines include:

Jinja2 in Python
Twig in PHP
FreeMarker in Java
Razor in ASP.NET

In a safe setup, the template has two parts:

Data: values that come from users or the database, such as names or comments
Code or logic: loops, conditions, functions, and special objects that the template engine uses to control behavior

Trouble starts when user input stops being pure data and slips into the code side. If a string from a form field is treated like template code, it can trigger SSTI.

How SSTI Happens: From User Input to Server Execution

The basic flow behind SSTI is simple:

A user sends some input, such as a name, message, or subject line.
The web app passes that input straight into a template.
The template engine treats part of that input as logic, not just plain text.
The server evaluates it and returns the result to the user.

So if you type {{7*7}} into a field and the response shows49, the template engine is doing math based on your input. That is a red flag.

Other signals are:

User input appears in error messages with template syntax around it
The response changes when you use different template patterns, such as ${7*7} or <%= 7*7 %>
You see stack traces that mention template engines, views, or expression evaluation

In short, if the server interprets your input as logic, you might have SSTI.

Real Risks of SSTI: Data Theft, RCE, and Full Takeover

Once SSTI is confirmed, the impact can be severe. Depending on the engine and configuration, an attacker may:

Read environment variables, like AWS_ACCESS_KEY or DB user names
Access local files, such as /etc/passwd or configuration files with secrets
Steal API keys and database passwords, then use them outside the app
Run system commands, which is remote code execution (RCE)
Move deeper into the network, scan internal services, or deploy backdoors

If you are learning to test for this, the OWASP Web Security Testing Guide has a detailed section on testing for server-side template injection.

How to Find Server-Side Template Injection: Practical Testing Steps

This section is for legal, authorized testing only. Always have permission before you poke at a system.

Where to Look for SSTI: Inputs, URLs, and Hidden Fields

SSTI usually hides in features that build content on the server side. Some common places are:

Search boxes and site-wide search
Contact forms or feedback forms
Profile settings such as display name, bio, or status
Email templates, such as welcome or password reset emails
Notification or message templates inside admin panels
CMS fields that render HTML from user content
Query string parameters that control views or reports

A good mindset is: if this feature takes user input and then builds HTML, emails, or documents on the server, it could use a template engine. That makes it a candidate for SSTI testing.

Using Simple Payloads Like {{7*7}} to Detect SSTI

You rarely need complex payloads to detect SSTI. Start with simple math or string tests and see how the app reacts.

Common detection patterns include:

{{7*7}} for Jinja2-style engines
${7*7} for some Java-based engines
<%= 7*7 %> for some JSP or older template systems
#{7*7} for some frameworks that use expression language

The expected result is usually 49. If the app prints 49 Instead of the exact string you sent, you likely have a template evaluation. If it returns an error message that mentions the engine, that is also useful.

For more example payloads and ideas, you can study practical guides like this walkthrough on finding and exploiting SSTI.

Fingerprinting the Template Engine (Jinja2, Twig, FreeMarker, Razor, and More)

Once you see signs of SSTI, the next step is to figure out which engine you are dealing with. Different engines use different syntax and built-in functions, so knowing the engine helps you understand the risk.

Ways to fingerprint the engine include:

Trying different syntax styles, such as {{ }}, ${ }, <% %>, or @{ }
Watching how errors are displayed, for example, references to Jinja2, Twig_Template, freemarker.core, or RazorView
Looking at technology hints in headers, cookies, or page content, such as PHPSESSID, .aspx, or Java stack traces

Once you know the engine, you can look up its features and what kind of access template code it has by default.

Using Tools Like Burp Suite and Tplmap to Speed Up SSTI Hunting

Manual testing is important, but tools can make the process faster.

Many security testers use an intercepting proxy like Burp Suite to:

Capture requests from the browser
Modify inputs and replay them quickly
Add SSTI payloads in many parameters without clicking through the UI

On top of that, tools such as Tplmap can help automate SSTI detection and exploitation. They try different payloads, guess the engine type, and check if RCE is possible. Some articles focus on more complex SSTI payloads, such as this guide on advanced SSTI exploitation techniques.

Automation speeds up hunting, but you still need to understand how templates work and what is safe or unsafe.

How Attackers Exploit SSTI and How You Can Prevent It

Now that you know how SSTI appears, it helps to see how attackers think and how developers can cut them off.

From Simple Test to Full Exploit: Reading Files and Running Commands

An attacker will often start with a small test payload, such as {{7*7}}. After they confirm template execution, they try to:

Access internal objects or globals that the engine exposes.
Use those objects to read files from disk, such as config or key stores.
Reach dangerous functions that can run system commands or open network sockets.
Turn the bug into stable remote code execution or a shell on the server.

This is why SSTI is so serious. Template engines often sit close to the app logic. If misused, they act as a bridge from user input to full control of the system. For a deeper exploitation focus, resources such as server-side template injection exploitation with RCE examples can show what is possible in real cases.

Secure Coding Practices to Stop SSTI in Your Templates

Good coding habits reduce the risk of SSTI a lot. Key practices include:

Never pass raw user input into functions that evaluate or compile template code
Use variable binding and safe render methods instead of string concatenation to build templates
Avoid building templates on the fly with user data in the template syntax itself
Limit template features that can run code, such as eval, import, or direct function calls
Keep business logic in code, not inside complex template expressions

As a reference for developers, this guide on preventing server-side template injection outlines common pitfalls and safe patterns.

Input Validation, Output Encoding, and Safe Template Configuration

SSTI is easier when user input arrives in the template untouched. You can reduce that risk with:

Input validation: allow only expected characters and patterns, set length limits, and reject strange syntax in fields that should be simple
Output encoding: escape special template characters before inserting user data so it stays data, not code
Safe configuration: turn off dangerous template extensions, sandbox the environment, and restrict access to system objects

Keeping template engines and frameworks up to date also matters. Security patches often close risky features or tighten default behavior. Review third-party themes, plugins, and snippet libraries too, since they may add unsafe template logic.

You can also see broader prevention tips in guides like this SSTI prevention overview.

Testing and Monitoring Your App So SSTI Does Not Come Back

Prevention is not a one-time task. You want routine checks so a new feature does not bring SSTI back.

Practical steps:

Add SSTI checks to regular security testing and code review
Include simple payloads like {{7*7}} in automated test suites for key templates
Use scanners that include SSTI checks, but always review results by hand
Monitor logs for odd template errors or inputs that contain template syntax

A helpful mental checklist is: always treat user input as data, never as code. If you keep that rule in mind when designing templates, you are far less likely to introduce SSTI.

Conclusion

Server-Side Template Injection is a simple idea with a huge impact. When user input is treated as template logic instead of data, attackers can test with basic payloads, spot evaluation, and pivot to remote code execution and full server takeover.

You saw how to recognize SSTI, where to look for it, and how attackers move from harmless {{7*7}} to reading files and running commands. You also saw how careful template design, strict input handling, and safe engine configuration can stop these bugs before they ship.

Use this knowledge for defense and authorized testing only. As a next step, pick one template in your own app today and check if any user input could ever be treated as code. That small review may prevent a very large problem later.

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