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Attack Surface Analysis for pallets/jinja

Attack Surface: Server-Side Template Injection (SSTI)

  • Description: Attackers inject malicious code into Jinja templates, leading to arbitrary code execution on the server.
  • Jinja Contribution: Jinja's core functionality of evaluating expressions ({{ ... }}) and executing statements ({% ... %}) within templates, when combined with unsanitized user input, directly enables SSTI.
  • Example:
    • Vulnerable Code: render_template_string('Hello {{ user_input }}', user_input=request.args.get('name'))
    • Attack Payload: {{ ''.__class__.__mro__[2].__subclasses__()[408]('whoami',shell=True,stdout=-1).communicate()[0].strip() }} (Example payload to execute system commands - specific payload may vary)
    • Explanation: This payload leverages Jinja's access to Python's object model to bypass intended template logic and execute arbitrary Python code, in this case, running the whoami command on the server.
  • Impact: Remote Code Execution (RCE), full server compromise, data breach, denial of service, and complete application takeover.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strict Input Sanitization: Never directly embed unsanitized user input into Jinja templates. Treat all user input as untrusted.
    • Parameterized Templates: Separate data from template logic. Pass data as variables to the template context instead of embedding user input directly within template strings.
    • Secure Templating Context (Jinja Environment): Restrict access to dangerous built-in functions and global variables within the Jinja environment. Create a sandboxed or minimal context, limiting available objects and functions.
    • Content Security Policy (CSP): While not a direct SSTI mitigation, CSP can limit the damage of successful exploitation by restricting actions an attacker can take after injecting code (e.g., prevent loading external scripts or executing inline JavaScript).
    • Web Application Firewall (WAF): Deploy a WAF to detect and block common SSTI payloads and malicious patterns in user input before it reaches the application.

Attack Surface: Cross-Site Scripting (XSS) via Inadequate Output Escaping

  • Description: Attackers inject malicious JavaScript code into web pages rendered by Jinja, which is then executed in users' browsers, leading to client-side attacks.
  • Jinja Contribution: While Jinja has autoescape enabled by default for HTML, developers can disable it globally, locally using {% autoescape false %}, or bypass it using the | safe filter. Incorrect or intentional disabling of autoescape, or misuse of | safe, directly contributes to XSS vulnerabilities.
  • Example:
    • Vulnerable Template: <div>{{ untrusted_user_content | safe }}</div>
    • Malicious Input: <script>alert('XSS Vulnerability!')</script>
    • Explanation: The | safe filter instructs Jinja to render untrusted_user_content without any HTML escaping. If this content contains malicious JavaScript, it will be executed in the user's browser.
  • Impact: Account hijacking, session theft, defacement of websites, redirection to malicious sites, theft of sensitive user information, and other client-side attacks.
  • Risk Severity: High
  • Mitigation Strategies:
    • Maintain Autoescape Enabled: Ensure Jinja's autoescape feature is enabled globally for HTML, XML, and other relevant output formats. Avoid disabling autoescape unless absolutely necessary and with extreme caution.
    • Avoid | safe Filter: Minimize and critically evaluate the use of the | safe filter. Only use it when you are absolutely certain the content is safe and has been rigorously sanitized before being passed to the template.
    • Context-Aware Escaping: While Jinja's autoescape is HTML-focused, be mindful of other output contexts (JavaScript, CSS, URLs). Manually escape for these contexts if needed, or use Jinja extensions that provide context-aware escaping.
    • Content Security Policy (CSP): Implement a strict CSP to significantly reduce the impact of XSS attacks by controlling the sources from which the browser can load resources and restricting inline script execution.
    • Input Validation and Sanitization (for allowed HTML): If you must allow users to input some HTML, use a robust and well-vetted HTML sanitization library (like Bleach in Python) to parse, clean, and remove potentially harmful HTML tags and attributes before rendering it with Jinja, even if using | safe.