Vite + Vue 3 built project Content-Security-Policy Error (CSP) 'script-src "self"' because of new Function constructor in built files

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05-10-2024

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Vite + Vue 3: Unlocking the 'script-src "self"' CSP Error with the new Function Constructor

The Frustrating "script-src 'self'" Problem

When building a Vite + Vue 3 application, you might encounter a Content-Security-Policy (CSP) error stating "script-src 'self'" when using the new Function constructor in your built files. This error can be frustrating, as your application may function correctly in development, but breaks in production due to this strict CSP directive.

The Scenario: A Code Example and Its CSP Implications

Let's imagine a simple scenario where you use new Function to dynamically execute some JavaScript code:

// Example.vue

<template>
  <button @click="executeCode">Run Code</button>
</template>

<script>
export default {
  methods: {
    executeCode() {
      const codeToExecute = "console.log('Hello from dynamic code!')";
      new Function(codeToExecute)(); 
    }
  }
};
</script>

This code snippet dynamically executes codeToExecute within the executeCode method. In development, this works flawlessly. However, when you build your application with Vite and deploy it to a production environment with a "script-src 'self'" CSP directive, you'll encounter the CSP error.

Understanding the Root Cause

Here's why the CSP error occurs:

• CSP 'script-src "self"' Restriction: The "script-src 'self'" CSP directive limits the execution of scripts to those originating from the same domain as the current webpage.
• Dynamic Code Execution: The new Function constructor allows for creating functions on the fly, dynamically. This means the generated function originates from your code, not directly from the HTML, bypassing the CSP restrictions.

Resolving the 'script-src "self"' Error: A Practical Solution

There are several approaches to address this problem, but the most reliable and recommended method involves pre-compiling the dynamically generated code. Here's how:

1. Transform new Function Usage: Instead of relying on new Function, pre-compile the dynamically generated code into a regular function using a library like babel-loader. This way, the function is created during the build process, not during runtime.

2. Example Implementation: Let's adjust the example above to use babel-loader:

   // Example.vue
   
   <template>
     <button @click="executeCode">Run Code</button>
   </template>
   
   <script>
   import { compile } from 'vue-template-compiler';
   
   export default {
     methods: {
       executeCode() {
         const codeToExecute = "console.log('Hello from dynamic code!')";
         const compiledFunction = compile(codeToExecute, {
           sourceRoot: 'example.vue'
         });
   
         const fn = new Function(compiledFunction.render);
         fn();
       }
     }
   };
   </script>
   

   This approach leverages Vue's template compiler to pre-compile the code, effectively resolving the CSP issue.

Additional Considerations

• Security: Although using new Function can be convenient, it's generally best to avoid it if possible, as it can increase the risk of security vulnerabilities.
• CSP Optimization: If you're using a Content Security Policy, consider carefully configuring it to balance security and flexibility. For example, you might allow specific scripts from trusted third-party domains using a "script-src" directive that includes additional origins.

Conclusion

The "script-src 'self'" CSP error with the new Function constructor in a Vite + Vue 3 project can be effectively addressed by pre-compiling your dynamic code. This ensures secure and compliant application behavior while preserving the functionality you need.

By understanding the CSP restrictions and implementing the appropriate solutions, you can develop robust and production-ready applications with Vite and Vue 3, all while maintaining the benefits of dynamic code execution.