Content Security Policy (CSP)

CSP gives administrators control over the website by limiting the resources that users are allowed to load within the website. Whitelist can be included in website content. If implemented properly, this header will prevent Cross-Site Scripting (XSS), ClickJacking and HTML exploits... Although it does not completely eliminate these attack possibilities, it certainly minimizes the damage. . Most major browsers support CSP.

Introduced in November 2012, CSP adds a layer of security against many website security vulnerabilities. CSP will become the most important tool for client-side security in the near future, as it can replace security headers such as: X-Frame-Options, X-XSS-Protection, to name a few. not enabled by default. All browsers do not support CSP, so verification is required before implementation.

CSP Directive Reference

The Content-Security-Policy header value is made up of one or more directives (defined below), multiple directives are separated with a semicolon ;

This documentation is provided based on the Content Security Policy Level 2 W3C Recommendation, and the CSP Level 3 W3C Working Draft

default-src

The default-src directive defines the default policy for fetching resources such as JavaScript, Images, CSS, Fonts, AJAX requests, Frames, HTML5 Media. Not all directives fallback to default-src. See the Source List Reference for possible values

default-src 'self' cdn.example.com;

script-src

Defines valid sources of JavaScript.

script-src 'self' js.example.com;

style-src

Defines valid sources of stylesheets or CSS

style-src 'self' css.example.com;

img-src

Defines valid sources of images.

img-src 'self' img.example.com;

connect-src

Applies to XMLHttpRequest (AJAX), WebSocket, fetch(), <a ping> or EventSource. If not allowed the browser emulates a 400 HTTP status code.

connect-src 'self';

font-src

Defines valid sources of font resources (loaded via @font-face).

font-src font.example.com;

object-src

Defines valid sources of plugins, eg <object>, <embed> or <applet>.

object-src 'self';

media-src

Defines valid sources of audio and video, eg HTML5 <audio>, <video> elements.

media-src media.example.com;

frame-src

Defines valid sources for loading frames. In CSP Level 2 frame-src was deprecated in favor of the child-src directive. CSP Level 3, has undeprecated frame-src and it will continue to defer to child-src if not present.

frame-src 'self';

sandbox

Enables a sandbox for the requested resource similar to the iframe sandbox attribute. The sandbox applies a same origin policy, prevents popups, plugins and script execution is blocked. You can keep the sandbox value empty to keep all restrictions in place, or add values: allow-forms allow-same-origin allow-scripts allow-popups, allow-modals, allow-orientation-lock, allow-pointer-lock, allow-presentation, allow-popups-to-escape-sandbox, and allow-top-navigation

sandbox allow-forms allow-scripts;

report-uri

Instructs the browser to POST a reports of policy failures to this URI. You can also use Content-Security-Policy-Report-Only as the HTTP header name to instruct the browser to only send reports (does not block anything). This directive is deprecated in CSP Level 3 in favor of the report-to directive.

report-uri /some-report-uri;

child-src

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

child-src 'self'

form-action

Defines valid sources that can be used as an HTML <form> action.

form-action 'self';

frame-ancestors

Defines valid sources for embedding the resource using <frame> <iframe> <object> <embed> <applet>. Setting this directive to 'none' should be roughly equivalent to X-Frame-Options: DENY

frame-ancestors 'none';

plugin-types

Defines valid MIME types for plugins invoked via <object> and <embed>. To load an <applet> you must specify application/x-java-applet.

plugin-types application/pdf;

base-uri

Defines a set of allowed URLs which can be used in the src attribute of a HTML base tag.

base-uri 'self';

report-to

Defines a reporting group name defined by a Report-To HTTP response header. See the Reporting API for more info.

report-to groupName;

worker-src

Restricts the URLs which may be loaded as a Worker, SharedWorker or ServiceWorker.

worker-src 'none';

manifest-src

Restricts the URLs that application manifests can be loaded.

manifest-src 'none';

prefetch-src

Defines valid sources for request prefetch and prerendering, for example via the link tag with rel="prefetch" or rel="prerender":

prefetch-src 'none'

navigate-to

Restricts the URLs that the document may navigate to by any means. For example when a link is clicked, a form is submitted, or window.location is invoked. If form-action is present then this directive is ignored for form submissions. Removed from the CSP 3 Spec.

navigate-to example.com

upgrade-insecure-requests

Automatically Converts URLs from http to https for links, images, javascript, css, etc.

upgrade-insecure-requests

block-all-mixed-content

Blocks requests to non secure http urls.

block-all-mixed-content

Not technically part of the CSP spec, may be removed in the future.

Source List Reference

All of the directives that end with -src support similar values known as a source list. Multiple source list values can be space separated with the exception of 'none' which should be the only value.

Source Value	Example	Description
*	img-src *	Wildcard, allows any URL except data: blob: filesystem: schemes.
'none'	object-src 'none'	Prevents loading resources from any source.
'self'	script-src 'self'	Allows loading resources from the same origin (same scheme, host and port).
data:	img-src 'self' data:	Allows loading resources via the data scheme (eg Base64 encoded images).
domain.example.com	img-src domain.example.com	Allows loading resources from the specified domain name.
*.example.com	img-src *.example.com	Allows loading resources from any subdomain under example.com.
https://cdn.com	img-src https://cdn.com	Allows loading resources only over HTTPS matching the given domain.
https:	img-src https:	Allows loading resources only over HTTPS on any domain.
'unsafe-inline'	script-src 'unsafe-inline'	Allows use of inline source elements such as style attribute, onclick, or script tag bodies (depends on the context of the source it is applied to) and javascript: URIs
'unsafe-eval'	script-src 'unsafe-eval'	Allows unsafe dynamic code evaluation such as JavaScript eval()
'sha256-'	script-src 'sha256-xyz...'	Allows an inline script or CSS to execute if its hash matches the specified hash in the header. Currently supports SHA256, SHA384 or SHA512. CSP Level 2
'nonce-'	script-src 'nonce-rAnd0m'	Allows an inline script or CSS to execute if the script (eg: <script nonce="rAnd0m">) tag contains a nonce attribute matching the nonce specifed in the CSP header. The nonce should be a secure random string, and should not be reused. CSP Level 2
'strict-dynamic'	script-src 'strict-dynamic'	Enables an allowed script to load additional scripts via non-"parser-inserted" script elements (for example document.createElement('script'); is allowed). CSP Level 3
'unsafe-hashes'	script-src 'unsafe-hashes' 'sha256-abc...'	Allows you to enable scripts in event handlers (eg onclick). Does not apply to javascript: or inline <script> CSP Level 3

Content-Security-Policy Examples

Here a few common scenarios for content security policies:

Allow everything but only from the same origin

default-src 'self';

Only Allow Scripts from the same origin

script-src 'self';

Allow Google Analytics, Google AJAX CDN and Same Origin

script-src 'self' www.google-analytics.com ajax.googleapis.com;

Starter Policy

This policy allows images, scripts, AJAX, form actions, and CSS from the same origin, and does not allow any other resources to load (eg object, frame, media, etc). It is a good starting point for many sites.

default-src 'none'; script-src 'self'; connect-src 'self'; img-src 'self'; style-src 'self';base-uri 'self';form-action 'self'

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Content-Security-Policy Error Messages

In Chrome when a Content Security Policy Script Violation happens you get a message like this one in the Chrome Developer Tools:

Refused to load the script 'script-uri' because it violates the following Content Security Policy directive: "your CSP directive".

In Firefox you might see messages like this in the Web Developer Tools:

Content Security Policy: A violation occurred for a report-only CSP policy ("An attempt to execute inline scripts has been blocked"). The behavior was allowed, and a CSP report was sent.

In addition to a console message, a securitypolicyviolation event is fired on the window. See https://www.w3.org/TR/CSP2/#firing-securitypolicyviolationevent-events.

Server Side Configuration

Any server side programming environment should allow you to send back a custom HTTP response header. You can also use your web server to send back the header.

Apache Content-Security-Policy Header

Add the following to your httpd.conf in your VirtualHost or in an .htaccess file:

Header set Content-Security-Policy "default-src 'self';"

Nginx Content-Security-Policy Header

In your server {} block add:

add_header Content-Security-Policy "default-src 'self';";

You can also append always to the end to ensure that nginx sends the header regardless of response code.

IIS Content-Security-Policy Header

You can use the HTTP Response Headers GUI in IIS Manager or add the following to your web.config:

<system.webServer>
  <httpProtocol>
    <customHeaders>
      <add name="Content-Security-Policy" value="default-src 'self';" />
    </customHeaders>
  </httpProtocol>
</system.webServer>