Content Security Policy
Extensions developed with WebExtension APIs 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 an extension, and how an extension can change the default CSP.
<!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, extensions 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 extensions by default. See default content security policy.
- the extension's author can change the default policy using the
content_security_policymanifest.json key, but there are restrictions on the policies that are allowed. See
The default content security policy for extensions is:
"script-src 'self'; object-src 'self';"
These policies are applied to any extension that has not explicitly set its own content security policy using the
content_security_policy manifest.json key. It has the following consequences:
This doesn't load the requested resource: it fails silently, and any object that you expect to be present from the resource is not found. There are two main solutions to this:
- download the resource, package it in your extension, and refer to this version of the resource
- allow the remote origin you need using the
content_security_policykey or, in Manifest V3, the
window.setTimeout("alert('Hello World!');", 500);
let f = new Function("console.log('foo');");
<script> tags and inline event handlers, meaning that the following are not permitted:
<div onclick="console.log('click')">Click me!</div>
Extensions wishing to use WebAssembly require
'wasm-unsafe-eval' to be specified in the
From Firefox 102 and Chrome 103,
'wasm-unsafe-eval' can be included in the
content_security_policy manifest.json key to enable the use of WebAssembly in extensions.
Manifest V2 extensions in Firefox can use WebAssembly without
'wasm-unsafe-eval' in their CSP for backward compatibility. However, this behavior isn't guaranteed, see bug 1770909. Extensions using WebAssembly are therefore encouraged to declare
'wasm-unsafe-eval' in their CSP.
For Chrome, extensions cannot use WebAssembly in version 101 or earlier. In 102, extensions can use WebAssembly (the same behavior as Firefox 101 and earlier). From version 103, extensions can use WebAssembly if they include
'wasm-unsafe-eval' in the
content_security_policy in the manifest key.