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attack-surface.md

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Attack Surface Analysis for pallets/jinja

Attack Surface: Server-Side Template Injection (SSTI)

  • Description: Attackers inject malicious Jinja2 syntax into the template, leading to code execution on the server. This is the most severe vulnerability associated with templating engines.
  • How Jinja2 Contributes: Jinja2's powerful features, including access to Python built-ins and potentially imported modules, provide the attacker with a wide range of capabilities if injection is successful. The template engine itself is the direct attack vector.
  • Example:
    • User input field: {{ config.__class__.__init__.__globals__['os'].popen('id').read() }}
    • This attempts to access the os module and execute the id command, revealing server user information.
  • Impact:
    • Remote Code Execution (RCE)
    • Information Disclosure (sensitive data, configuration, source code)
    • Denial of Service (DoS)
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strict Input Validation: Validate all user input against a whitelist of allowed characters/patterns. Reject anything that doesn't conform.
    • Context-Aware Escaping (Autoescaping): Ensure autoescaping is enabled (default in many frameworks). Understand its limitations and contexts (HTML, JS, etc.).
    • Sandboxing (SandboxedEnvironment): Use Jinja2's SandboxedEnvironment to restrict available functions and attributes within the template. This significantly limits the attacker's power.
    • Least Privilege: Run the application with minimal necessary privileges.
    • Avoid |safe: Minimize the use of the |safe filter. Only use it when absolutely necessary and you are certain the input is safe.
    • Avoid Exposing Sensitive Objects: Do not directly expose objects like config in templates.
    • Regular Security Audits & Penetration Testing: Specifically test for template injection.

Attack Surface: Cross-Site Scripting (XSS) - Through Direct Jinja2 Misuse

  • Description: While Jinja2's autoescaping aims to prevent XSS, direct misuse, such as disabling autoescaping or using the wrong escaping context within the template itself, can lead to vulnerabilities. This distinguishes it from general XSS, focusing on the Jinja2-specific aspect.
  • How Jinja2 Contributes: The vulnerability arises from directly misusing Jinja2's escaping features within the template. Jinja2's output mechanism, when improperly configured by the developer within the template, is the direct cause.
  • Example:
    • {{ user_input | safe }} - Directly disabling autoescaping for user input.
    • Using {{ ... }} in a JavaScript context without proper JavaScript escaping (even if HTML escaping is on). This is a direct misuse of Jinja2's output in the wrong context.
  • Impact:
    • Execution of arbitrary JavaScript in the victim's browser.
    • Session hijacking.
    • Phishing attacks.
    • Website defacement.
  • Risk Severity: High
  • Mitigation Strategies:
    • Rely on Autoescaping: Use Jinja2's autoescaping as the primary defense. Understand the different escaping contexts (HTML, JS, CSS, URL).
    • Avoid |safe (Crucially): Minimize its use. Only use it when absolutely necessary and you are certain the input is safe.
    • Context-Aware Output: Ensure that the output of Jinja2 expressions is used in the correct context (e.g., HTML, JavaScript, CSS) and that the appropriate escaping is applied automatically by Jinja2.
    • Content Security Policy (CSP): Implement a strong CSP as a defense-in-depth measure (although this is a broader mitigation, not solely Jinja2-specific).