The activeTab permission

The activeTab permission gives an extension temporary access to the currently active tab when the user invokes the extension - for example by clicking its browser action. Access to the tab lasts until the tab is navigated or closed.

This serves as an alternative for many uses of <all_urls>, but displays no warning message during installation:

Without activeTab With activeTab

Example

See the Page Redder sample extension: 

{
  "name": "Page Redder",
  "version": "2.0",
  "permissions": [
    "activeTab"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },
  "browser_action": {
    "default_title": "Make this page red"
  },
  "manifest_version": 2
}

// Called when the user clicks on the browser action.
chrome.browserAction.onClicked.addListener(function(tab) {
  // No tabs or host permissions needed!
  console.log('Turning ' + tab.url + ' red!');
  chrome.tabs.executeScript({
    code: 'document.body.style.backgroundColor="red"'
  });
});

Motivation

Consider a web clipping extension that has a browser action and context menu item. This extension may only really need to access tabs when its browser action is clicked, or when its context menu item is executed.

Without activeTab, this extension would need to request full, persistent access to every web site, just so that it could do its work if it happened to be called upon by the user. This is a lot of power to entrust to such a simple extension. And if the extension is ever compromised, the attacker gets access to everything the extension had.

In contrast, an extension with the activeTab permission only obtains access to a tab in response to an explicit user gesture. If the extension is compromised the attacker would need to wait for the user to invoke the extension before obtaining access. And that access only lasts until the tab is navigated or closed.

What activeTab allows

While the activeTab permission is enabled for a tab, an extension can:

Invoking activeTab

The following user gestures enable activeTab: