attack-surface.md

Attack Surface Analysis for pallets/flask

Attack Surface: Server-Side Template Injection (SSTI) via Jinja2

• Description: Injecting malicious code into Jinja2 templates when user-controlled data is directly rendered without proper sanitization. This allows attackers to execute arbitrary code on the server.
• Flask Contribution: Flask uses Jinja2 as its default templating engine and provides functions like render_template_string which, if misused with unsanitized user input, directly enables SSTI vulnerabilities.
• Example:

  from flask import Flask, request, render_template_string
  
  app = Flask(__name__)
  
  @app.route('/')
  def index():
      user_input = request.args.get('name', 'World')
      template = '<h1>Hello {{ name }}</h1>' # Vulnerable if 'name' comes directly from user input
      return render_template_string(template, name=user_input)

  An attacker could access / with a crafted name parameter like /?name={{config.SECRET_KEY}} to potentially leak the secret key or execute arbitrary code.
• Impact: Remote Code Execution (RCE), complete server compromise, data breaches, denial of service, information disclosure.
• Risk Severity: Critical
• Mitigation Strategies:
  • Parameterize Templates: Avoid directly embedding user input into template strings. Pass data as variables to templates and let Jinja2 handle escaping. Use render_template instead of render_template_string with user input.
  • Input Sanitization: Sanitize and validate user inputs before using them in templates, even as variables, to prevent unexpected behavior.
  • Autoescaping: Ensure Jinja2's autoescaping is enabled and context-aware escaping is used where appropriate. Flask enables autoescaping by default for .html, .htm, .xml, and .xhtml extensions.
  • Principle of Least Privilege: Run the application with minimal necessary permissions to limit the impact of RCE.

Attack Surface: Session Management Vulnerabilities (Weak Secret Key & Session Fixation)

• Description: Weaknesses in Flask's session management, specifically related to a weak SECRET_KEY or improper session ID regeneration, allowing attackers to hijack user sessions.
• Flask Contribution: Flask's default session mechanism relies on a SECRET_KEY for signing cookies. A weak or exposed SECRET_KEY directly undermines session security. Flask also requires developers to explicitly regenerate session IDs to prevent session fixation.
• Example:
  • Weak Secret Key: Using a default or easily guessable SECRET_KEY like "dev" or "secret".
  • Session Fixation: Not regenerating session IDs after user login, allowing an attacker to pre-set a session ID for a victim.
• Impact: Session hijacking, account takeover, unauthorized access, privilege escalation, data manipulation.
• Risk Severity: High
• Mitigation Strategies:
  • Strong SECRET_KEY: Generate a strong, random, and long SECRET_KEY. Store it securely (environment variable, secrets management, not in code).
  • Key Rotation: Periodically rotate the SECRET_KEY.
  • Session Regeneration: Regenerate session IDs after successful authentication (e.g., using session.regenerate()).
  • Secure Session Cookies: Ensure session cookies are configured with httponly and secure flags to prevent client-side JavaScript access and transmission over insecure channels (HTTPS required for secure flag to be effective).

Attack Surface: Debug Mode Enabled in Production

• Description: Running a Flask application with debug=True in a production environment, exposing highly sensitive debugging tools and information.
• Flask Contribution: Flask's debug=True setting activates the Werkzeug debugger and reloader, which are intended only for development. In production, it exposes a dangerous interactive debugger and sensitive application details.
• Example: Deploying an application with app.run(debug=True) or FLASK_DEBUG=1 in a production server. Accessing an error page in this mode can reveal the debugger.
• Impact: Information disclosure (source code, configuration, environment variables), Remote Code Execution (via debugger console), denial of service, full server compromise.
• Risk Severity: Critical
• Mitigation Strategies:
  • Disable Debug Mode: Absolutely never run Flask applications in production with debug=True. Set debug=False or FLASK_DEBUG=0 in production environments.
  • Environment Configuration: Use environment variables or configuration files to strictly manage debug mode settings, ensuring it's disabled for all production deployments and enabled only in controlled development/testing environments.
  • Production Configuration Review: Double-check production configurations to guarantee debug mode is disabled before deployment and during maintenance.

Attack Surface: Blueprint Misconfiguration leading to Access Control Bypass

• Description: Incorrect configuration of Flask Blueprints, specifically URL prefixing or route registration, leading to unintended access to routes or functionalities that should be protected.
• Flask Contribution: Flask Blueprints are a core feature for modular application design. Misuse or misconfiguration of blueprint URL prefixes and route registrations can directly lead to unintended exposure of functionalities.
• Example:
  • Overlapping or incorrect URL prefixes in blueprints causing routes intended for administrators to be accessible to regular users.
  • Incorrectly assuming blueprint-level access control applies to all routes within, when specific routes might be inadvertently exposed due to routing conflicts.
• Impact: Unauthorized access to administrative functionalities, privilege escalation, data breaches, business logic bypass, circumvention of intended security measures.
• Risk Severity: High (if sensitive functionalities are exposed)
• Mitigation Strategies:
  • Careful Blueprint Planning & Review: Plan blueprint URL prefixes and route registrations meticulously to avoid overlaps and unintended exposures. Thoroughly review blueprint configurations.
  • Explicit Access Control: Implement and explicitly apply access control mechanisms (e.g., decorators, middleware) to each route within blueprints that requires protection. Do not rely solely on blueprint-level assumptions.
  • Route Testing & Auditing: Thoroughly test and audit route access, especially across different blueprints, to ensure access control is enforced as intended and no routes are unintentionally exposed.

Attack Surface: Static File Directory Traversal

• Description: Misconfigurations in serving static files through Flask, allowing attackers to use directory traversal techniques to access files outside the intended static directory, potentially including application code or sensitive data.
• Flask Contribution: Flask's send_from_directory function and static folder configuration are used to serve static files. Incorrect or insecure usage of send_from_directory without proper path sanitization can create directory traversal vulnerabilities.
• Example:

  from flask import Flask, send_from_directory
  
  app = Flask(__name__, static_folder='static')
  
  @app.route('/static_files/<path:filename>')
  def serve_static(filename):
      return send_from_directory(app.static_folder, filename) # Vulnerable if filename is not sanitized

  An attacker could access /static_files/../../app.py to attempt directory traversal and access application source code or configuration files.
• Impact: Information disclosure (source code, configuration files, backups, sensitive data), potential for further exploitation if sensitive files are accessed.
• Risk Severity: High (if sensitive files are exposed)
• Mitigation Strategies:
  • Restrict Static File Paths: Ensure static file paths are properly configured and restricted to the intended directory. Avoid serving the entire application directory as static.
  • Input Sanitization & Validation (if needed): If user input must be used to determine static file paths (which is generally discouraged), rigorously sanitize and validate the input to prevent directory traversal attempts. Use secure path manipulation functions.
  • Dedicated Web Server for Static Files: In production, strongly consider using a dedicated web server (like Nginx or Apache) to serve static files. This provides better security, performance, and built-in protection against directory traversal in many cases. Flask should primarily handle dynamic content.
  • Principle of Least Privilege (File System): Ensure the Flask application process has minimal file system permissions, limiting the impact even if directory traversal is successful.