Resolving Content Security Policy Issues in Chrome Extension Manifest V3

Overcoming Content Security Policy Errors in Manifest V3 Extensions

Developing a Chrome extension can be an exciting project, but it often comes with unique challenges—especially with the recent updates in Manifest V3. One common hurdle developers face is configuring the Content Security Policy (CSP) correctly. This policy is essential for maintaining security, yet it can also introduce unexpected errors that prevent the extension from functioning as intended. 🚧

Imagine spending days perfecting an extension, only to have it rejected by the Chrome Web Store due to an invalid CSP configuration. This issue can be particularly frustrating when your extension needs to communicate with external APIs securely, like an API endpoint at `api.example.com`. Attempting to set up CSP to allow such external access might seem straightforward, but the recent Manifest V3 changes can complicate this setup significantly.

In this post, we’ll dive into a developer’s journey with CSP validation errors in Manifest V3. Through trial and error, you’ll see various attempts to correctly format the `content_security_policy` field. Each attempt reflects a step closer to the solution, along with useful insights drawn from common errors and official documentation.

Whether you’re building an AdBlocker, a productivity tool, or any other extension, this guide will clarify CSP requirements, help you troubleshoot validation errors, and ensure your extension is secure and compliant. Let’s get into the nitty-gritty of overcoming these CSP obstacles!

Understanding Key Components in Content Security Policy Configuration for Chrome Extensions

The example scripts provided aim to overcome a common challenge in configuring Content Security Policy (CSP) settings for Chrome extensions, especially in Manifest V3. The first configuration approach in the manifest file utilizes the host_permissions attribute. This command specifies the external domains that the extension can access directly, in this case, “https://api.example.com/*.” By adding this to the manifest, we inform Chrome that our extension plans to securely communicate with an external API, a necessity for features that depend on fetching external data. The second essential element, the content_security_policy, restricts resources the extension is allowed to load. Here, it defines which scripts are permitted in specific extension environments, such as sandboxed pages, while adhering to Chrome’s stringent security requirements.

The example script provided in the background service worker script, background.js, leverages a function that calls the external API. This function uses the JavaScript fetch command to handle asynchronous HTTP requests, which are essential for retrieving data from APIs. When an API request is needed, the function connects to the designated endpoint and returns the data. This functionality helps maintain a clean separation of concerns, where each function performs one action, making the code modular and reusable. To facilitate this process, the script uses chrome.runtime.onMessage.addListener to listen for specific commands—like “fetchData”—from other components of the extension, ensuring effective communication between various parts of the codebase.

The example also includes another crucial aspect: error handling. The script wraps the API call in a try-catch block, which is crucial in any network-dependent function. If the API request fails, the script logs an error message to inform the developer about potential issues, such as an invalid URL or a network problem. Handling errors in this manner also ensures that the extension remains robust and doesn’t fail entirely if one network request fails. It provides a smoother user experience, as errors are isolated and handled gracefully, instead of disrupting the entire extension’s functionality.

Lastly, to ensure code quality, a set of unit tests validate the integrity of these configurations. Using a testing framework, the unit test script applies the fetchMock library to simulate API responses, thus providing a controlled environment for testing. These tests verify that CSP rules are appropriately configured, confirming whether the extension can access external resources securely and as intended. Each of these tests serves to check the extension’s behavior under multiple scenarios, ensuring that it functions across Chrome versions and that the CSP rules are compatible with the Chrome Web Store’s security policies. By having this testing suite, developers can confidently upload their extension, knowing it complies with Chrome’s security standards and avoids the common “Invalid value for ‘content_security_policy’” error. 🛠️

{
  "manifest_version": 3,
  "name": "AdBlocker Upsia",
  "version": "1.0",
  "permissions": ["storage"],
  "host_permissions": ["https://api.example.com/*"],
  "content_security_policy": {
    "extension_pages": "script-src 'self'; object-src 'self';",
    "sandbox": "sandbox allow-scripts; script-src 'self' https://api.example.com;"
  }
}

// background.js
chrome.runtime.onInstalled.addListener(() => {
  console.log("Service Worker registered");
});

// Function to make API call securely
async function fetchDataFromAPI() {
  try {
    const response = await fetch('https://api.example.com/data', {
      method: 'GET',
      headers: { 'Content-Type': 'application/json' }
    });
    const data = await response.json();
    console.log("API data received:", data);
    return data;
  } catch (error) {
    console.error("API fetch error:", error);
  }
}

// Call API when a message is received
chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
  if (message.command === "fetchData") {
    fetchDataFromAPI().then(data => sendResponse({ data }));
    return true; // keeps the response channel open
  }
});

// test/cspTest.js
const { expect } = require('chai');
const { describe, it } = require('mocha');
const fetchMock = require('fetch-mock');

describe("CSP Configuration Tests", () => {
  it("should allow secure API call with valid CSP", async () => {
    fetchMock.get('https://api.example.com/data', { status: 200, body: { key: "value" } });

    const data = await fetchDataFromAPI();
    expect(data).to.have.property('key');
  });

  it("should throw error on invalid API call attempt", async () => {
    fetchMock.get('https://api.fake.com/data', 403);

    try {
      await fetchDataFromAPI();
    } catch (error) {
      expect(error).to.exist;
    }
  });
});

Configuring CSP for External API Integration in Chrome Extensions

When developing with Chrome Extension Manifest V3, integrating external APIs securely requires a clear understanding of the updated Content Security Policy (CSP) rules. Manifest V3 introduced stricter policies to enhance security, but these changes have made certain setups more challenging, particularly when connecting with external APIs like https://api.example.com. Extensions must follow these new guidelines, balancing both security and functionality. Without correct configuration, the extension may trigger errors during submission, such as "Invalid value for 'content_security_policy'," which indicates an issue with CSP syntax or permissions.

A key element here is the CSP’s role in restricting or permitting the resources that the extension can load. Extensions that use dynamic content, like calling an external API for data, need to specify allowed domains directly in the host_permissions field. This entry authorizes the extension to connect to designated URLs securely. Additionally, separating CSP directives—such as specifying a sandboxed environment for sensitive scripts—can improve the extension's compliance with Manifest V3’s updated policies. Implementing object-src and script-src policies also enables developers to define which types of content can load from external sources.

Another essential aspect involves background service workers. Manifest V3 replaces background pages with service workers, which allows the extension to maintain secure, ongoing communication with APIs without requiring persistent background access. By using a service worker, you can manage API calls asynchronously and handle responses effectively. This approach not only aligns with Manifest V3's security improvements but also optimizes the extension’s performance, as service workers consume fewer resources. Implementing these techniques allows developers to build secure and efficient extensions that comply with Chrome’s latest standards. 🌐