Cross-origin network requests

Regular web pages can use the fetch() or XMLHttpRequest APIs to send and receive data from remote servers, but they're limited by the same origin policy. Content scripts initiate requests on behalf of the web origin that the content script has been injected into and therefore content scripts are also subject to the same origin policy. Extension origins aren't so limited. A script executing in an extension service worker or foreground tab can talk to remote servers outside of its origin, as long as the extension requests cross-origin permissions.

Extension origin

Each running extension exists within its own separate security origin. Without requesting additional privileges, the extension can call fetch() to get resources within its installation. For example, if an extension contains a JSON configuration file called config.json, in a config_resources/ folder, the extension can retrieve the file's contents like this:

const response = await fetch('/config_resources/config.json');
const jsonData = await response.json();

If the extension attempts to use a security origin other than itself, say https://www.google.com, the browser disallows it unless the extension has requested the appropriate cross-origin permissions.

Request cross-origin permissions

To request access to remote servers outside an extension's origin, add hosts, match patterns, or both to the host_permissions section of the manifest file.

{
  "name": "My extension",
  ...
  "host_permissions": [
    "https://www.google.com/"
  ],
  ...
}

Cross-origin permission values can be fully qualified host names, like these:

• "https://www.google.com/"
• "https://www.gmail.com/"

Or they can be match patterns, like these:

• "https://*.google.com/"
• "https://*/"

A match pattern of "https://*/" allows HTTPS access to all reachable domains. Note that here, match patterns are similar to content script match patterns, but any path information following the host is ignored.

Also note that access is granted both by host and by scheme. If an extension wants both secure and non-secure HTTP access to a given host or set of hosts, it must declare the permissions separately:

"host_permissions": [
  "http://www.google.com/",
  "https://www.google.com/"
]

Fetch() vs. XMLHttpRequest()

fetch() was created specifically for service workers and follows a broader web trend away from synchronous operations. The XMLHttpRequest() API is supported in extensions outside of the service worker, and calling it triggers the extension service worker's fetch handler. New work should favor fetch() wherever possible.

Security considerations

Avoid cross-site scripting vulnerabilities

When using resources retrieved via fetch(), your offscreen document, side panel or popup should be careful not to fall victim to cross-site scripting. Specifically, avoid using dangerous APIs such as innerHTML. For example:

const response = await fetch("https://api.example.com/data.json");
const jsonData = await response.json();
// WARNING! Might be injecting a malicious script!
document.getElementById("resp").innerHTML = jsonData;
    ...

Instead, prefer safer APIs that do not run scripts:

const response = await fetch("https://api.example.com/data.json");
const jsonData = await response.json();
// JSON.parse does not evaluate the attacker's scripts.
let resp = JSON.parse(jsonData);

const response = await fetch("https://api.example.com/data.json");
const jsonData = response.json();
// textContent does not let the attacker inject HTML elements.
document.getElementById("resp").textContent = jsonData;

Limit content script access to cross-origin requests

When performing cross-origin requests on behalf of a content script, be careful to guard against malicious web pages that might try to impersonate a content script. In particular, do not allow content scripts to request an arbitrary URL.

Consider an example where an extension performs a cross-origin request to let a content script discover the price of an item. One not-so-secure approach would be to have the content script specify the exact resource to be fetched by the background page.

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    if (request.contentScriptQuery == 'fetchUrl') {
      // WARNING: SECURITY PROBLEM - a malicious web page may abuse
      // the message handler to get access to arbitrary cross-origin
      // resources.
      fetch(request.url)
        .then(response => response.text())
        .then(text => sendResponse(text))
        .catch(error => ...)
      return true;  // Will respond asynchronously.
    }
  }
);

chrome.runtime.sendMessage(
  {
    contentScriptQuery: 'fetchUrl',
    url: `https://another-site.com/price-query?itemId=${encodeURIComponent(request.itemId)}`
  },
  response => parsePrice(response.text())
);

In the approach above, the content script can ask the extension to fetch any URL that the extension has access to. A malicious web page may be able to forge such messages and trick the extension into giving access to cross-origin resources.

Instead, design message handlers that limit the resources that can be fetched. Below, only the itemId is provided by the content script, and not the full URL.

chrome.runtime.onMessage.addListener(
  function(request, sender, sendResponse) {
    if (request.contentScriptQuery == 'queryPrice') {
      const url = `https://another-site.com/price-query?itemId=${encodeURIComponent(request.itemId)}`
      fetch(url)
        .then(response => response.text())
        .then(text => parsePrice(text))
        .then(price => sendResponse(price))
        .catch(error => ...)
      return true;  // Will respond asynchronously.
    }
  }
);

chrome.runtime.sendMessage(
  {contentScriptQuery: 'queryPrice', itemId: 12345},
  price => ...
);

Prefer HTTPS over HTTP

Additionally, be especially careful of resources retrieved via HTTP. If your extension is used on a hostile network, an network attacker (aka a "man-in-the-middle") could modify the response and, potentially, attack your extension. Instead, prefer HTTPS whenever possible.

Adjust the content security policy

If you modify the default Content Security Policy for your extension by adding a content_security_policy attribute to your manifest, you'll need to ensure that any hosts to which you'd like to connect are allowed. While the default policy doesn't restrict connections to hosts, be careful when explicitly adding either the connect-src or default-src directives.