server-side template injection (SSTI)

Charlie Yianni

min read

What is Server-Side Template Injection (SSTI)?

Server-Side Template Injection (SSTI) is a web application vulnerability that occurs when user input is unsafely embedded into templates. If the template engine processes this input as code, attackers can execute commands on the server or extract sensitive data.

Why SSTI Is Dangerous

SSTI vulnerabilities can lead to severe consequences, including:

Remote code execution on the server
Exposure of sensitive information (e.g., credentials, internal files)
Full compromise of the affected application or server

These attacks are especially critical because they often bypass client-side protections and target the backend directly.

How SSTI Works

Many web applications use template engines (like Twig, Jinja2, or FreeMarker) to generate dynamic content such as emails, invoices, or HTML pages. SSTI occurs when these templates include user input without proper sanitization or escaping. For example:

Hi {{ FirstName }}

If improperly handled, attackers could modify FirstName to inject malicious code into the template.

How to Prevent Server-Side Template Injection

Avoid using user input in templates where possible
Validate and sanitize input to allow only expected characters and formats
Use secure template engines with limited functionality (e.g., Mustache)
Keep template engines up to date with the latest security patches
Sandbox template execution to contain potential attacks and limit damage

How Intruder Helps

Intruder detects server-side template injection vulnerabilities during web application scans, helping teams:

Identify unsafe template usage quickly
Understand the risk level and impact of discovered vulnerabilities
Prioritize fixes with clear remediation guidance

Want to secure your web applications against SSTI? Start your free trial and detect critical vulnerabilities automatically.