SSTI - Jinja2 Scanner
Tests for server-side template injection in Jinja2/Flask applications.
What is SSTI - Jinja2?
Jinja2 Server-Side Template Injection (SSTI) occurs when user input is unsafely embedded into Jinja2 templates, allowing attackers to inject template directives that execute on the server. Jinja2 is the default template engine for Flask and widely used in Python web applications. Attackers can escape the sandbox and achieve remote code execution.
Why is This Important?
Jinja2 SSTI is particularly dangerous because it often leads directly to remote code execution. Python's introspection capabilities allow attackers to traverse object hierarchies and access dangerous classes. Flask applications are especially vulnerable when rendering user input as templates.
How It Works
1. Input Discovery
Maps all user input points including forms, headers, cookies, and API parameters for injection testing.
2. Injection Testing
Executes sophisticated injection payloads designed to bypass filters and WAFs while detecting vulnerabilities.
3. Exploitation Validation
Confirms vulnerabilities through safe exploitation, providing proof-of-concept and impact assessment.
Key Capabilities
Advanced injection detection engine combining signature-based and AI-powered analysis for comprehensive coverage.
- Multi-vector injection testing across all input types
- WAF and filter bypass techniques built-in
- Database-specific payload optimization
- Out-of-band detection for blind vulnerabilities
- Automated proof-of-concept generation
Frequently Asked Questions
How do attackers achieve RCE through Jinja2 SSTI?
Attackers use Python's __mro__, __subclasses__, __globals__, and __builtins__ to traverse from string objects to dangerous classes like subprocess.Popen or os.system. A common payload is {{''.__class__.__mro__[1].__subclasses__()}} to enumerate available classes for exploitation.
What is the difference between safe and vulnerable template rendering?
Vulnerable: render_template_string(user_input). Safe: render_template('template.html', variable=user_input). The key difference is whether user input becomes part of the template syntax or is just data passed to a predefined template.
Can Jinja2's sandboxed environment prevent SSTI?
Jinja2's SandboxedEnvironment provides some protection but has been bypassed multiple times. It blocks access to certain attributes and methods, but researchers regularly discover new bypass techniques. It should not be relied upon as the sole defense.
How do I prevent Jinja2 SSTI?
Never pass user input to render_template_string(), always use predefined templates with variables, enable autoescaping, use Jinja2's SandboxedEnvironment as defense in depth, validate and sanitize user input, and consider using a logic-less template engine for user-generated content.
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