Quick answer: SSTI (Server Side Template Injection) is when an application passes user input into a template engine insecurely, letting the attacker run template expressions on the server. Template engines are text patterns with variables embedded to produce pages. The problem arises when user input is embedded into the template itself rather than as data; then the attacker can write expressions the engine will execute. In most template engines this goes as far as file reading and usually remote code execution. The root fix: never make user input part of the template; always pass it to the template as data.

SSTI is confused with XSS but is far more destructive: XSS runs code in the victim's browser, SSTI on the server itself. Email templates, report generators, customizable messages and theme engines are where this flaw is common. This article explains how SSTI arises, why it is so dangerous and how to prevent it.

How SSTI arises

A template engine combines two things: a fixed pattern and data to embed into it. Used correctly, user input is embedded only as data. Used wrongly, the developer places user input directly inside the template text. For example, if a welcome message adds the user's name to the template as pattern rather than data, the attacker writes a template expression instead of a name and the engine executes it.

The attacker typically follows these steps:

  1. Sends a simple math expression to test whether the engine interprets the input.
  2. If it does, tries to reach the engine's special objects and functions.
  3. From there moves to reading files, leaking environment variables and often running system commands.

SSTI versus XSS and injection

Flaw Where it runs Typical impact
XSS The victim's browser Session theft, UI deception
SSTI The server template engine File reading, code execution
SQL injection The database Data leak

SSTI, because it runs on the server, is usually the most destructive, and a successful exploit can lead to full server control like command injection.

Where it appears

  • Email and notification templates. If a user field is embedded into the template.
  • Report and document generators. Systems producing output from user content.
  • Customizable messages and themes. Places letting users edit the pattern.
  • Low code and automation tools. Fields that evaluate expressions.

Proper defense

1. Pass user input to the template as data

This is the strongest and most definitive defense. Input must not be concatenated into the template text; it must always go through the engine's data channel. So input is never interpreted as an expression.

2. Do not let users write templates

Users must not be given free template editing ability. If unavoidable, use a restricted logic less engine that runs no expressions and allows only safe variable placement.

3. Sandbox and least privilege

Template evaluation should run in a strictly bounded environment, and the engine's access to dangerous objects should be restricted. The application account should have only the privileges it needs.

4. Input validation

Values entering the template must be validated strictly against the expected format. This should be handled as part of secure code review.

SSTI scanning with KAOS

We scan for SSTI flaws at scale with KAOS, DSET's AI powered autonomous security scanning engine. With its multi agent architecture KAOS discovers endpoints that reflect user input, tries with harmless test payloads whether the engine interprets the input as an expression, and when it can show the engine actually evaluates the expression it verifies this in a controlled way; it reports only proven findings. So it distinguishes an XSS from an SSTI and shows the real impact without false positive noise. KAOS works alongside the human expert and documents every finding with a working proof.

Frequently asked questions

Are SSTI and XSS the same? No. XSS runs in the victim's browser, SSTI in the server's template engine. SSTI is usually far more destructive because it can lead to server control.

Is every template engine exposed to SSTI? Exposure depends on how the engine is used. It is safe if user input is passed as data; the flaw arises if it is made part of the template.

Does SSTI always lead to code execution? Not always, but in many engines it does. At minimum there is a risk of file reading and information disclosure; it must be taken seriously.

How is it tested? A harmless expression is sent where user input is reflected; if the engine evaluates it, there is an SSTI.

Sources

To detect SSTI and template injection flaws in your application with a working proof from KAOS and our expert team, contact DSET. From our Ankara Hacettepe Teknokent laboratory we provide penetration testing and secure code review.