Content Security Policy

WebExtensions have a Content Security Policy (CSP) applied to them by default. This restricts the sources from which they can load <script> and <object> resources, and disallows potentially unsafe practices such as the use of eval().

This article explains briefly what a CSP is, what the default policy is and what it means for a WebExtension, and how a WebExtension can change the default CSP.

Content Security Policy (CSP) is a mechanism to help prevent websites from inadvertantly executing malicious content. A website specifies a CSP using an HTTP header sent from the server. The CSP is mostly concerned with specifying legitimate sources of various types of content, such as scripts or embedded plugins. For example, a website can use it to specify that the browser should only execute JavaScript served from the website itself, and not from any other sources. A CSP can also instruct the browser to disallow potentially unsafe practices, such as the use of eval().

Like websites, WebExtensions can load content from different sources. For example, a browser action's popup is specified as an HTML document, and it can include JavaScript and CSS from different sources, just like a normal web page:

<!DOCTYPE html>

<html>
  <head>
    <meta charset="utf-8">
  </head>

  <body>

    <!--Some HTML content here-->

    <!--
      Include a third-party script.
      See also https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity.
    -->
    <script
      src="https://code.jquery.com/jquery-2.2.4.js"
      integrity="sha256-iT6Q9iMJYuQiMWNd9lDyBUStIq/8PuOW33aOqmvFpqI="
      crossorigin="anonymous">
    </script>
 
    <!-- Include my popup's own script-->
    <script src="popup.js"></script>
  </body>

</html>

Compared to a website, WebExtensions have access to additional privileged APIs, so if they are compromised by malicious code, the risks are greater. For this reason:

a fairly strict content security policy is applied to WebExtensions by default. See default content security policy.
the WebExtension's author can change the default policy using the content_security_policy manifest.json key, but there are restrictions on the policies that are allowed. See content_security_policy.

Default content security policy

The default content security policy for WebExtensions is:

"script-src 'self'; object-src 'self';"

This will be applied to any WebExtension that has not explicitly set its own content security policy using the content_security_policy manifest.json key. It has the following consequences:

Location of script and object resources

Under the default CSP you may only load <script> and <object> resources that are local to the add-on. For example, consider a line like this in a WebExtension's document:

 <script src="https://code.jquery.com/jquery-2.2.4.js"></script>

This will no longer load the requested resource: it will fail silently, and any object which you expected to be present from the resource will not be found. There are two main solutions to this:

download the resource, package it in your add-on, and refer to this version of the resource
use the content_security_policy key to allow the remote origin you need.

eval() and friends

Under the default CSP WebExtensions are not allowed to evaluate strings as JavaScript. This means that the following are not permitted:

eval("console.log('some output');");

window.setTimeout("alert('Hello World!');", 500);

var f = new Function("console.log('foo');");

Inline JavaScript

Under the default CSP inline JavaScript is not executed. This disallows both JavaScript placed directly in <script> tags and inline event handlers, meaning that the following are not permitted:

<script>console.log("foo");</script>

<div onclick="console.log('click')">Click me!</div>

If you are currently using code like <body onload="main()"> to run your script when the page has loaded, listen for DOMContentLoaded or load instead.

Content Security Policy

Default content security policy

Location of script and object resources

eval() and friends

Inline JavaScript

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