attack-surface.md

Attack Surface Analysis for sveltejs/svelte

Attack Surface: 1. {@html ...} Misuse (Svelte-Specific XSS)

• Description: Rendering unsanitized user input directly as HTML using Svelte's {@html ...} tag. This is the most direct and dangerous Svelte-specific vulnerability. It's not a general XSS vulnerability; it's specific to how Svelte handles raw HTML.
• How Svelte Contributes: Svelte provides the {@html ...} tag for rendering raw HTML, which intentionally bypasses Svelte's built-in escaping mechanisms. This is a core Svelte feature that, if misused, creates a direct XSS vulnerability.
• Example:

  <script>
    let userInput = "<img src=x onerror=alert('XSS')>";
  </script>
  
  {@html userInput}

• Impact: Execution of arbitrary JavaScript in the context of the user's browser. This can lead to session hijacking, data theft, defacement, and other severe consequences.
• Risk Severity: Critical
• Mitigation Strategies:
  • Avoid {@html ...} whenever possible. Use Svelte's standard templating for dynamic content. This is the primary mitigation.
  • If {@html ...} is absolutely unavoidable, always sanitize the input using a robust HTML sanitization library like DOMPurify. This is not optional. Example:

    import DOMPurify from 'dompurify';
    
    let sanitizedInput = DOMPurify.sanitize(userInput);

  • Implement a strong Content Security Policy (CSP) to limit the impact of any successful XSS. A CSP can prevent the execution of inline scripts and restrict the sources from which scripts can be loaded. This is a defense-in-depth measure.
  • Educate developers on the dangers of {@html} and the absolute necessity of sanitization. This is a crucial preventative measure.

Attack Surface: 2. Unsafe bind: Directives (Svelte-Facilitated XSS/Injection)

• Description: Using bind: directives with potentially dangerous HTML attributes (e.g., innerHTML, href, src on <a> or <iframe> tags) and unsanitized user input. While not exclusive to Svelte, Svelte's bind: makes this easier to do incorrectly.
• How Svelte Contributes: Svelte's bind: directives provide convenient two-way data binding. This convenience can lead developers to directly bind user input to sensitive attributes without proper sanitization, creating an injection vulnerability. Svelte facilitates this vulnerability through its design.
• Example:

  <script>
    let userLink = "javascript:alert('XSS')";
  </script>
  
  <a bind:href={userLink}>Click me</a>

• Impact: Similar to {@html ...} misuse, this can lead to XSS or other injection attacks. The attack surface is generally smaller than {@html}, but still significant.
• Risk Severity: High
• Mitigation Strategies:
  • Avoid binding directly to dangerous attributes. Prefer safer alternatives or intermediate variables.
  • Sanitize the bound value before it's applied. Use a URL sanitization library for href attributes, and DOMPurify or a similar library for innerHTML and outerHTML. The sanitization must happen before the value is used in the binding.
  • Prefer Svelte's built-in components (e.g., <input>, <textarea>) for user input. These components often have some built-in protections (though they are not a substitute for proper sanitization).
  • Use input validation to restrict the types of values allowed in bound variables. This can help prevent unexpected or malicious input from being used.

Attack Surface: 3. Unintentional Data Exposure via Stores (Svelte-Specific Data Leak)

• Description: Sensitive data stored in Svelte stores being exposed to unauthorized components or directly to the client's JavaScript console. This is a risk specific to how Svelte manages shared state.
• How Svelte Contributes: Svelte's stores are a core feature for managing shared state. The framework itself doesn't inherently protect the data within stores; it's the developer's responsibility to manage access and scope appropriately. The ease of creating and using stores can lead to unintentional exposure if not handled carefully.
• Example: A store containing user authentication tokens being accessible to all components, even those that don't need access. Or, a store being directly logged to the console for debugging purposes in a production environment.
• Impact: Exposure of sensitive data, potentially leading to unauthorized access, data breaches, or privacy violations. The severity depends directly on the sensitivity of the exposed data.
• Risk Severity: High (depending on the sensitivity of the data. Could be Critical if highly sensitive data is exposed.)
• Mitigation Strategies:
  • Carefully scope stores. Use derived stores or custom stores with restricted access methods (getters/setters) to control which components can read or modify the store's data.
  • Avoid storing sensitive data directly in stores if possible. If unavoidable, encrypt or otherwise protect the data before it's placed in the store.
  • Sanitize user input before it is used to update a store. This prevents injection attacks that could modify the store's contents.
  • Use read-only stores where appropriate to prevent unintended modification of the store's data.
  • Avoid exposing the entire store object directly to the template; subscribe to specific properties instead. This limits the potential for accidental exposure.
  • Remove any debugging code (e.g., console.log(store)) before deploying to production. This is a critical step to prevent accidental data leaks.