HIGH Authentication & Authorization

Server Side Template Injection in APIs

Q: How can I tell which template engine my API is using?

Look at error messages, response patterns, and application stack traces. Different engines have distinct syntax: Jinja2 uses double curly braces {{ }}, Handlebars uses triple braces {{{ }}}, and FreeMarker uses #[ ]#. You can also test with engine-specific payloads like {{ 7 * 7 }} for Jinja2 or ${7*7} for FreeMarker. middleBrick automatically detects the template engine by testing multiple syntax patterns and analyzing responses.

Q: Is SSTI only a problem for web applications, or do APIs face the same risk?

APIs are equally vulnerable, often more so because they process structured data that frequently gets embedded into templates. API-specific scenarios include error message generation, PDF creation from JSON data, email templating with API responses, and document generation services. The attack surface is broader in API contexts because data flows through more processing stages before reaching templates.

Q: Can SSTI lead to remote code execution, or is it just information disclosure?

SSTI can absolutely lead to remote code execution. Many template engines provide access to system functions, file operations, and even subprocess execution. For example, Jinja2 can access Python's built-in functions, allowing attackers to read files, execute commands, or establish reverse shells. The severity depends on the template engine configuration and the server's permissions, but RCE is a common outcome of successful SSTI exploitation.

What is Server Side Template Injection?

Server Side Template Injection (SSTI) occurs when an application uses user-supplied data in server-side template engines without proper sanitization. Template engines like Jinja2, Handlebars, Twig, or FreeMarker are designed to embed dynamic content into HTML, but when attackers can control template expressions, they can execute arbitrary code on the server.

The vulnerability arises when template variables are populated with untrusted input. For example, if a template contains {{ user_input }} and user_input comes from a query parameter, an attacker might inject {{ 7 * 7 }} to cause unintended template evaluation. More dangerously, many template engines support functions, object access, and even system calls through their syntax.

In APIs, SSTI often appears in error messages, email templates, PDF generation, or any dynamic content generation. Unlike traditional web applications where SSTI might leak HTML, API-based SSTI typically results in information disclosure, remote code execution, or server compromise through the template engine's capabilities.

How Server Side Template Injection Affects APIs

APIs are particularly vulnerable to SSTI because they often process JSON payloads, URL parameters, or headers that get embedded into templates without validation. Common API scenarios include:

Error message templates that include exception details or user-provided data
Email generation systems that use API data for personalized content
Document generation services that convert JSON to formatted output
Logging systems that create structured reports from API requests
Webhook processing that generates templated responses

An attacker exploiting SSTI in an API can achieve several objectives:

Information Disclosure: Extract sensitive data like database credentials, API keys, or internal system information through template expressions
Remote Code Execution: Many template engines allow calling system functions, reading files, or executing commands
Server Compromise: Escalate to full system control by leveraging template engine capabilities
Denial of Service: Cause infinite loops or resource exhaustion through malicious template expressions

For example, in a Jinja2 template, an attacker might inject {{ config.items() }} to dump the application configuration, or {{ ''.__class__.__mro__[2].__subclasses__()[40]('/etc/passwd').read() }} to read system files.

How to Detect Server Side Template Injection

Detecting SSTI requires both static analysis and dynamic testing. Here's what to look for:

Static Analysis:

Template files that directly interpolate user input without sanitization
Template engines used with user-controlled variables
Lack of content security policies or template sandboxing
Configuration files that enable dangerous template features

Dynamic Testing:

Submit template-specific syntax (like {{ 7 * 7 }} for Jinja2) and observe if it gets evaluated
Test common template injection payloads for the suspected engine
Monitor response times—template evaluation often causes delays
Look for error messages that reveal template engine details

How middleBrick Detects SSTI:

middleBrick's black-box scanning approach tests for SSTI by injecting template-specific payloads across all 12 security checks. The scanner automatically identifies template engine patterns and attempts controlled injections to detect evaluation without causing harm. For API endpoints, middleBrick tests:

Common template syntax across major engines (Jinja2, Handlebars, Twig, FreeMarker, etc.)
Mathematical operations that would only evaluate if templates are processed
Object property access patterns that reveal server-side processing
Time-based payloads that cause measurable delays if evaluated

The scanner reports findings with severity levels based on the potential impact and provides specific remediation guidance for the detected template engine.

Prevention & Remediation

Preventing SSTI requires a defense-in-depth approach:

1. Input Validation and Sanitization

Never directly embed user input into templates. Validate and sanitize all data before template processing:

 Real-World Impact
 SSTI vulnerabilities have caused significant breaches across industries. In 2015, a critical vulnerability in the popular Ruby on Rails framework allowed SSTI exploitation through the render method, affecting thousands of applications. More recently, CVE-2021-41262 in the Python package fastapi demonstrated how template injection in modern web frameworks can lead to remote code execution.
Financial services companies have suffered SSTI-related breaches where attackers extracted database credentials and customer data through template injection in reporting systems. E-commerce platforms have seen attackers manipulate pricing displays and order processing through SSTI in email templates and PDF generation.
The cost of SSTI exploitation extends beyond immediate data theft. Organizations face regulatory penalties, loss of customer trust, and expensive incident response. middleBrick's scoring system assigns critical severity to SSTI findings because of the high likelihood of remote code execution and the difficulty of detection without specialized testing.
By scanning your APIs with middleBrick before deployment, you can identify SSTI vulnerabilities early in the development lifecycle, when they're cheapest to fix. The scanner's 5-15 second runtime means you can integrate SSTI testing into your CI/CD pipeline without slowing down development velocity.

  Scan for server side template injection View all security checks 
  Frequently Asked Questions
How can I tell which template engine my API is using?
Look at error messages, response patterns, and application stack traces. Different engines have distinct syntax: Jinja2 uses double curly braces {{ }}, Handlebars uses triple braces {{{ }}}, and FreeMarker uses #[ ]#. You can also test with engine-specific payloads like {{ 7 * 7 }} for Jinja2 or ${7*7} for FreeMarker. middleBrick automatically detects the template engine by testing multiple syntax patterns and analyzing responses.
Is SSTI only a problem for web applications, or do APIs face the same risk?
APIs are equally vulnerable, often more so because they process structured data that frequently gets embedded into templates. API-specific scenarios include error message generation, PDF creation from JSON data, email templating with API responses, and document generation services. The attack surface is broader in API contexts because data flows through more processing stages before reaching templates.
Can SSTI lead to remote code execution, or is it just information disclosure?
SSTI can absolutely lead to remote code execution. Many template engines provide access to system functions, file operations, and even subprocess execution. For example, Jinja2 can access Python's built-in functions, allowing attackers to read files, execute commands, or establish reverse shells. The severity depends on the template engine configuration and the server's permissions, but RCE is a common outcome of successful SSTI exploitation.