Content-Security-Policy

The HTTP Content-Security-Policy response header allows web site administrators to control resources the user agent is allowed to load for a given page. With a few exceptions, policies mostly involve specifying server origins and script endpoints. This helps guard against cross-site scripting attacks (XSS).

For more information, see also this article on Content Security Policy (CSP).

Syntax

Content-Security-Policy: <policy-directive>; <policy-directive>

Directives

Fetch directives

Fetch directives control locations from which certain resource types may be loaded.

child-src

Defines the valid sources for web workers and nested browsing contexts loaded using elements such as <frame> and <iframe>.

Instead of child-src, authors who wish to regulate nested browsing contexts and workers should use the frame-src and worker-src directives, respectively.

connect-src

Restricts the URLs which can be loaded using script interfaces

default-src

Serves as a fallback for the other fetch directives.

font-src

Specifies valid sources for fonts loaded using @font-face.

frame-src

Specifies valid sources for nested browsing contexts loading using elements such as <frame> and <iframe>.

img-src

Specifies valid sources of images and favicons.

manifest-src

Specifies valid sources of application manifest files.

media-src

Specifies valid sources for loading media using the <audio> , <video> and <track> elements.

object-src

Specifies valid sources for the <object>, <embed>, and <applet> elements.

prefetch-src

Specifies valid sources to be prefetched or prerendered.

script-src

Specifies valid sources for JavaScript.

style-src

Specifies valid sources for stylesheets.

webrtc-src

Specifies valid sources for WebRTC connections.

worker-src

Specifies valid sources for Worker, SharedWorker, or ServiceWorker scripts.

Document directives

Document directives govern the properties of a document or worker environment to which a policy applies.

base-uri

Restricts the URLs which can be used in a document's <base> element.

plugin-types

Restricts the set of plugins that can be embedded into a document by limiting the types of resources which can be loaded.

sandbox

Enables a sandbox for the requested resource similar to the <iframe> sandbox attribute.

disown-opener

Ensures a resource will disown its opener when navigated to.

Navigation directives

Navigation directives govern to which location a user can navigate to or submit a form to, for example.

form-action

Restricts the URLs which can be used as the target of a form submissions from a given context.

frame-ancestors

Specifies valid parents that may embed a page using <frame>, <iframe>, <object>, <embed>, or <applet>.

navigate-to

Restricts the URLs to which a document can navigate by any means (a, form, window.location, window.open, etc.)

Reporting directives

Reporting directives control the reporting process of CSP violations. See also the Content-Security-Policy-Report-Only header.

report-uri

Instructs the user agent to report attempts to violate the Content Security Policy. These violation reports consist of JSON documents sent via an HTTP POST request to the specified URI.

Though the report-to directive is intended to replace the deprecated report-uri directive, report-to isn’t supported in most browsers yet. So for compatibility with current browsers while also adding forward compatibility when browsers get report-to support, you can specify both report-uri and report-to:

Content-Security-Policy: ...; report-uri https://endpoint.example.com; report-to groupname

In browsers that support report-to, the report-uri directive will be ignored.

report-to

Fires a SecurityPolicyViolationEvent.

Other directives

block-all-mixed-content

Prevents loading any assets using HTTP when the page is loaded using HTTPS.

referrer

Used to specify information in the referer (sic) header for links away from a page. Use the Referrer-Policy header instead.

require-sri-for

Requires the use of SRI for scripts or styles on the page.

upgrade-insecure-requests

Instructs user agents to treat all of a site's insecure URLs (those served over HTTP) as though they have been replaced with secure URLs (those served over HTTPS). This directive is intended for web sites with large numbers of insecure legacy URLs that need to be rewritten.

CSP in workers

Workers are in general not governed by the content security policy of the document (or parent worker) that created them. To specify a content security policy for the worker, set a Content-Security-Policy response header for the request which requested the worker script itself.

The exception to this is if the worker script's origin is a globally unique identifier (for example, if its URL has a scheme of data or blob). In this case, the worker does inherit the content security policy of the document or worker that created it.

Multiple content security policies

CSP allows multiple policies being specified for a resource, including via the Content-Security-Policy header, the Content-Security-Policy-Report-Only header and a <meta> element.

You can use the Content-Security-Policy header more than once like in the example below. Pay special attention to the connect-src directive here. Even though the second policy would allow the connection, the first policy contains connect-src 'none'. Adding additional policies can only further restrict the capabilities of the protected resource, which means that there will be no connection allowed and, as the strictest policy, connect-src 'none' is enforced.

Content-Security-Policy: default-src 'self' http://example.com;
                         connect-src 'none';
Content-Security-Policy: connect-src http://example.com/;
                         script-src http://example.com/

Examples

Example: Disable unsafe inline/eval, only allow loading of resources (images, fonts, scripts, etc.) over https:

// header
Content-Security-Policy: default-src https:

// meta tag
<meta http-equiv="Content-Security-Policy" content="default-src https:">

Example: Pre-existing site that uses too much inline code to fix but wants to ensure resources are loaded only over https and disable plugins:

Content-Security-Policy: default-src https: 'unsafe-eval' 'unsafe-inline'; object-src 'none'

Example: Don't implement the above policy yet; instead just report violations that would have occurred:

Content-Security-Policy-Report-Only: default-src https:; report-uri /csp-violation-report-endpoint/

See Mozilla Web Security Guidelines for more examples.

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

• Content-Security-Policy-Report-Only
• Content Security in WebExtensions

• Display security and privacy policies In Firefox Developer Tools