name: stackblitz-security-basics description: 'Secure WebContainer deployments: CSP headers, sandbox isolation, input validation.

Use when working with WebContainers or StackBlitz SDK.

Trigger: "stackblitz security".

' allowed-tools: Read, Write, Grep version: 1.0.0 license: MIT author: Jeremy Longshore jeremy@intentsolutions.io tags:

• saas
• ide
• webcontainers
• stackblitz compatibility: Designed for Claude Code

StackBlitz Security Basics

Overview

Secure WebContainer deployments: CSP headers, sandbox isolation, input validation.

Instructions

Step 1: WebContainer Security Model

WebContainers run in the browser sandbox -- no access to host filesystem, network is limited to HTTP, and all code runs in the user's browser tab. Key security points:

// WebContainers are inherently sandboxed:
// - No file system access to host
// - No raw network sockets
// - Memory isolated to browser tab
// - Cross-origin isolation via COOP/COEP headers

Step 2: Validate User Input

// If users can provide code to run in WebContainer, validate:
function sanitizeFileTree(tree: FileSystemTree): FileSystemTree {
  const sanitized: FileSystemTree = {};
  for (const [name, entry] of Object.entries(tree)) {
    // Block path traversal
    if (name.includes('..') || name.startsWith('/')) continue;
    // Block sensitive files
    if (name === '.env' || name.endsWith('.key')) continue;
    sanitized[name] = entry;
  }
  return sanitized;
}

Step 3: Content Security Policy

Content-Security-Policy: default-src 'self'; script-src 'self' 'wasm-unsafe-eval'; frame-src https://*.webcontainer.io;

Security Checklist

• COOP/COEP headers set correctly
• User-provided code sandboxed in WebContainer
• No secrets passed to WebContainer file system
• CSP headers configured
• Input validation on file paths

Resources

• WebContainer Security

Next Steps

For production, see stackblitz-prod-checklist.