content_security_policy

Type String
Mandatory No
Manifest version 2 or higher
Example Manifest V2:
"content_security_policy": "default-src 'self'"
Manifest V3:
"content_security_policy": {
  "extension_pages": "default-src 'self'"
}

Extensions have a content security policy (CSP) applied to them by default. The default policy restricts the sources from which extensions can load code (such as <script> resources) and disallows potentially unsafe practices such as the use of eval(). See Default content security policy to learn more about the implications of this.

You can use the "content_security_policy" manifest key to loosen or tighten the default policy. This key is specified in the same way as the Content-Security-Policy HTTP header. See Using Content Security Policy for a general description of CSP syntax.

For example, you can use this key to:

  • Restrict permitted sources for other types of content, such as images and stylesheets, using the appropriate policy directive.
  • Allow the extension to take advantage of WebAssembly by including the 'wasm-unsafe-eval' source in the script-src directive.
  • Loosen the default script-src policies (Manifest V2 only):

There are restrictions on the policy you can specify with this manifest key:

  • The script-src directive must include at least the 'self' keyword and may only contain secure sources. The set of permitted secure sources differs between Manifest V2 and Manifest V3.
  • The policy may include default-src alone (without script-src) if its sources meet the requirement for the script-src directive.
  • The object-src keyword may be required, see object-src directive for details.
  • Directives that reference code – script-src, script-src-elem, worker-src, and default-src (if used as a fallback) – share the same secure source requirement. There are no restrictions on CSP directives that cover non-script content, such as img-src.

In Manifest V3, all CSP sources that refer to external or non-static content are forbidden. The only permitted values are 'none', 'self', and 'wasm-unsafe-eval'. In Manifest V2, a source for a script directive is considered secure if it meets these criteria:

  • Wildcard hosts are not permitted, such as "script-src 'self' *".
  • Remote sources must use https: schemes.
  • Remote sources must not use wildcards for any domains in the public suffix list (so "*.co.uk" and "*.blogspot.com" are not allowed, although "*.foo.blogspot.com" is permitted).
  • All sources must specify a host.
  • The only permitted schemes for sources are blob:, filesystem:, moz-extension:, https:, and wss:.
  • The only permitted keywords are: 'none', 'self', 'unsafe-eval', and 'wasm-unsafe-eval'.

object-src directive

The object-src directive may be required in some browsers that support obsolete plugins and should be set to a secure source such as 'none' if needed. This may be necessary for browsers up until 2022.

  • In Firefox, "object-src" it optional from Firefox 106. In earlier versions, if "object-src" isn't specified, "content_security_policy" is ignored and the default CSP used.
  • In Chrome, "object-src" is required. If it's missing or deemed insecure, the default ("object-src 'self'") is used and a warning message logged.
  • In Safari, there is no requirement for "object-src".

See W3C WebExtensions Community Group issue 204, Remove object-src from the CSP, for more information.

Manifest V2 syntax

In Manifest V2, there is one content security policy specified against the key like this:

"content_security_policy": "default-src 'self'"

Manifest V3 syntax

In Manifest V3, the content_security_policy key is an object that may have any of these properties, all optional:

Name Type Description
extension_pages String The content security policy used for extension pages. The script-src and worker-src directives may only have these values:
  • 'self'
  • 'none'
  • 'wasm-unsafe-eval'/code>
sandbox String The content security policy used for sandboxed extension pages.

Examples

Valid examples

Note: Valid examples demonstrate the correct use of keys in CSP. However, extensions with 'unsafe-eval', remote script, blob, or remote sources in their CSP are not allowed for Firefox extensions per the add-on policies and due to significant security issues.

Note: Some examples include the object-src directive, which provides backward compatibility for older browser versions. See object-src directive for more details.

Require that all types of content should be packaged with the extension:

  • Manifest V2
    "content_security_policy": "default-src 'self'"
    
  • Manifest V3
    "content_security_policy": {
      "extension_pages": "default-src 'self'"
    }
    

Allow remote scripts from "https://example.com":

  • Manifest V2
    "content_security_policy": "script-src 'self' https://example.com; object-src 'self'"
    
  • Manifest V3 does not allow remote URLs in script-src of extension_pages.

Allow remote scripts from any subdomain of "jquery.com":

  • Manifest V2
    "content_security_policy": "script-src 'self' https://*.jquery.com; object-src 'self'"
    
  • Manifest V3 does not allow remote URLs in script-src of extension_pages.

Allow eval() and friends:

  • Manifest V2
    "content_security_policy": "script-src 'self' 'unsafe-eval'; object-src 'self';"
    
  • Manifest V3 does not allow 'unsafe-eval' in script-src.

Allow the inline script: "<script>alert('Hello, world.');</script>":

  • Manifest V2
    "content_security_policy": "script-src 'self' 'sha256-qznLcsROx4GACP2dm0UCKCzCG+HiZ1guq6ZZDob/Tng='; object-src 'self'"
    
  • Manifest V3 does not allow CSP hashes in script-src of extension_pages.

Keep the rest of the policy, but also require that images should be packaged with the extension:

  • Manifest V2
    "content_security_policy": "script-src 'self'; object-src 'self'; img-src 'self'"
    
  • Manifest V3
    "content_security_policy": {
      "extension_pages": "script-src 'self'; img-src 'self'"
    }
    

Enable the use of WebAssembly:

  • Manifest V2 For backward compatibility, Manifest V2 extensions in Firefox can use WebAssembly without the use of 'wasm-unsafe-eval'. However, this behavior isn't guaranteed. See bug 1770909. Extensions using WebAssembly are therefore encouraged to declare 'wasm-unsafe-eval' in their CSP. See WebAssembly on the Content Security Policy page for more information.
    "content_security_policy": "script-src 'self' 'wasm-unsafe-eval'"
    
  • Manifest V3
    "content_security_policy": {
      "extension_pages": "script-src 'self' 'wasm-unsafe-eval'"
    }
    

Invalid examples

Policy that omits the "object-src" directive:

"content_security_policy": "script-src 'self' https://*.jquery.com;"

However, this is only invalid in browsers that support obsolete plugins. See object-src directive for more details..

Policy that omits the "self" keyword in the "script-src" directive:

"content_security_policy": "script-src https://*.jquery.com; object-src 'self'"

Scheme for a remote source is not https:

"content_security_policy": "script-src 'self' http://code.jquery.com; object-src 'self'"

Wildcard is used with a generic domain:

"content_security_policy": "script-src 'self' https://*.blogspot.com; object-src 'self'"

Source specifies a scheme but no host:

"content_security_policy": "script-src 'self' https:; object-src 'self'"

Directive includes the unsupported keyword 'unsafe-inline':

"content_security_policy": "script-src 'self' 'unsafe-inline'; object-src 'self'"

Browser compatibility

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