Chrome Extension Data Access

TL;DR

Generally, Chrome extensions cannot directly access another extension’s private data (like local storage or background scripts) without explicit permission. However, there are ways they can indirectly interact and potentially share information, usually through the use of messaging APIs or shared storage areas like synced storage. Security is a key concern; Chrome actively prevents direct unauthorized access.

Understanding Extension Isolation

Chrome extensions operate in isolated environments for security reasons. Each extension has its own:

• Manifest file: Defines permissions and capabilities.
• Local storage: For storing data specific to that extension.
• Background scripts: Run independently of web pages.

This isolation prevents malicious extensions from interfering with each other or accessing sensitive user data.

How Extensions Interact (and Share Data)

1. Messaging API: This is the primary way extensions communicate.
   • An extension can send messages to another specific extension if it knows its ID.
   • The receiving extension must be listening for these messages and explicitly handle them.
   • Example (sending a message):

   chrome.runtime.sendMessage("extension-id", {message: "Hello from Extension A"}, function(response) {
     console.log("Response from Extension B:", response);
   });

2. Shared Storage (Synced Storage): Extensions can both access data stored in Chrome’s synced storage.
   • This is intended for cross-device synchronization, but extensions can use it to share information.
   • Example (setting a value in synced storage):

   chrome.storage.sync.set({'myKey': 'myValue'}, function() {
     console.log('Value saved');
   });

3. Content Scripts: Content scripts can interact with web pages and indirectly communicate with extensions.
   • They don’t directly access other extension data, but they can modify the page to trigger messages.
4. Native Messaging (Advanced): Allows communication between extensions and native applications.
   • This is a more complex method and requires setting up a native host application.

Permissions are Key

An extension must declare the necessary permissions in its manifest file to interact with other extensions or shared storage.

• "permissions": ["storage"]: Allows access to local and synced storage.
• "host_permissions": [...]: Required for content scripts to access specific websites.

Users are prompted to grant these permissions during installation.

Security Considerations

• Avoid storing sensitive data in local storage if possible. Use synced storage with caution, as it's accessible across devices.
• Carefully validate any messages received from other extensions. Don't trust the content without checking its origin and format.
• Regularly review your extension's permissions. Only request the permissions you absolutely need.

Checking Extension Permissions

1. Go to chrome://extensions in your Chrome browser.
2. Enable "Developer mode" (top right corner).
3. Click "Inspect view" on the extension you want to examine.
4. Open the "Manifest" tab to see the declared permissions.