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attack-surface.md

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Attack Surface Analysis for pallets/flask

Attack Surface: Weak Session Secret Key

  • Description: Flask uses a secret key to cryptographically sign session cookies, preventing tampering. A weak, default, or exposed key allows attackers to forge valid session cookies.
  • How Flask Contributes: Flask's built-in session management relies entirely on the security of the SECRET_KEY. The framework provides the mechanism; the developer is responsible for the key's strength and security. This is a direct Flask responsibility.
  • Example: An attacker discovers the SECRET_KEY is "changeme". They generate a signed cookie with user_id=1 (admin), gaining administrative access.
  • Impact: Complete account takeover, including administrative accounts. Data breach, system compromise.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Generate a Strong Key: Use os.urandom(24) (or better) for a cryptographically secure random key.
    • Secure Storage: Never hardcode the key. Use environment variables, a secrets management service (HashiCorp Vault, AWS Secrets Manager), or a config file outside the repository.
    • Key Rotation: Regularly rotate the secret key.
    • Never Use Defaults: Absolutely avoid example keys from documentation.

Attack Surface: Server-Side Template Injection (SSTI)

  • Description: User input is directly concatenated into a template string before rendering, allowing injection of Jinja2 syntax, executed on the server.
  • How Flask Contributes: Flask uses Jinja2 for templating. While Jinja2 is secure when used correctly, Flask's integration means developers must be aware of SSTI. The vulnerability is in how Jinja2 is used within Flask – a direct consequence of Flask's design.
  • Example: 
    # VULNERABLE!
@app.route("/unsafe")
def unsafe():
    name = request.args.get('name')
    template = "<h1>Hello " + name + "!</h1>"
    return render_template_string(template)
    /unsafe?name={{7*7}} results in "Hello 49!", proving code execution.
  • Impact: Remote code execution (RCE) on the server. Complete system compromise.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Use render_template: Always use render_template (or render_template_string with a pre-defined string) and pass user input as context variables, never directly into the template string. 
      # Safe
@app.route("/safe")
def safe():
    name = request.args.get('name')
    return render_template('hello.html', name=name)
      (hello.html: <h1>Hello {{ name }}!</h1>)
    • Avoid String Concatenation: Never build templates by concatenating user input with template code.

Attack Surface: Cross-Site Scripting (XSS) in Templates (Autoescaping Bypassed or Disabled)

  • Description: User data is rendered in a Jinja2 template without proper escaping, allowing injection of malicious JavaScript that executes in the victim's browser.
  • How Flask Contributes: Flask relies on Jinja2's autoescaping. The vulnerability arises when autoescaping is disabled, bypassed (| safe), or an outdated Jinja2 is used. This is directly tied to Flask's choice of templating engine and its default configuration.
  • Example: 
    <!-- VULNERABLE! -->
<p>Comment: {{ comment | safe }}</p>
    An attacker submits <script>alert('XSS')</script>.
  • Impact: Session hijacking, defacement, phishing, malware distribution, data theft (from the victim's browser).
  • Risk Severity: High
  • Mitigation Strategies:
    • Keep Autoescaping Enabled: This is the primary defense. Jinja2's autoescaping is on by default in Flask. Do not disable it.
    • Avoid | safe: Use | safe extremely sparingly, and only after thorough sanitization with a library like Bleach.
    • Content Security Policy (CSP): Implement a CSP for defense-in-depth.
    • Update Jinja2: Keep Jinja2 updated.

Attack Surface: Information Leakage in Error Messages (Debug Mode)

  • Description: Flask's debug mode provides detailed error messages, including stack traces and environment variables, potentially exposing sensitive information.
  • How Flask Contributes: Flask's debug mode is a built-in feature intended for development. It is the developer's responsibility to disable it in production, but the feature itself and its potential for information leakage are directly part of Flask.
  • Example: An unhandled exception in production with debug mode on reveals database credentials and file structure.
  • Impact: Exposure of sensitive information (credentials, configuration, code), aiding further attacks.
  • Risk Severity: High
  • Mitigation Strategies:
    • Disable Debug Mode in Production: app.debug = False or app.run(debug=False). Use environment variables (e.g., FLASK_ENV=production).
    • Custom Error Handlers: Use @app.errorhandler to show generic messages to users, logging details securely.
    • Log Management: Configure logging to capture details without exposing them to users.

Attack Surface: URL Traversal / Path Manipulation (using send_from_directory incorrectly)

  • Description: Attackers manipulate URL paths to access files outside the intended directory, even when using Flask's send_from_directory if it is misconfigured or if the base directory is not properly secured.
  • How Flask Contributes: While send_from_directory aims to be secure, it's a Flask-provided function, and its security depends on correct usage within the Flask application. The potential for misuse is directly related to Flask's API.
  • Example: If the base directory for send_from_directory is set too broadly (e.g., /), or if symbolic links are not handled correctly, an attacker might still be able to traverse outside the intended area.
  • Impact: Unauthorized access to sensitive files; information disclosure.
  • Risk Severity: High
  • Mitigation Strategies:
    • Careful Base Directory Selection: Choose a very specific and restricted base directory for send_from_directory. Avoid using root directories or directories with sensitive files.
    • Sanitize Filenames (Even with send_from_directory): While send_from_directory provides some protection, it's still good practice to sanitize the filename to remove any potentially dangerous characters.
    • Disable Symbolic Links (If Possible): If you don't need symbolic links, disable them within the served directory to prevent attackers from using them to bypass restrictions.
    • Regularly Audit: Review your file serving configuration to ensure it remains secure.