Content Security Policy (CSP) is an added layer of security that helps to detect and mitigate certain types of attacks, including Cross Site Scripting (XSS) and data injection attacks. These attacks are used for everything from data theft to site defacement or distribution of malware.

CSP is designed to be fully backward compatible (except CSP version 2 where there are some explicitly-mentioned inconsistencies in backward compatibility; more details here section 1.1). Browsers that don't support it still work with servers that implement it, and vice-versa: browsers that don't support CSP simply ignore it, functioning as usual, defaulting to the standard same-origin policy for web content. If the site doesn't offer the CSP header, browsers likewise use the standard same-origin policy.

To enable CSP, you need to configure your web server to return the Content-Security-Policy HTTP header (sometimes you will see mentions of the X-Content-Security-Policy header, but that's an older version and you don't need to specify it anymore).

Alternatively, the <meta> element can be used to configure a policy, for example: <meta http-equiv="Content-Security-Policy" content="default-src 'self'; img-src https://*; child-src 'none';">


Mitigating cross site scripting

A primary goal of CSP is to mitigate and report XSS attacks. XSS attacks exploit the browser's trust of the content received from the server. Malicious scripts are executed by the victim's browser because the browser trusts the source of the content, even when it's not coming from where it seems to be coming from.

CSP makes it possible for server administrators to reduce or eliminate the vectors by which XSS can occur by specifying the domains that the browser should consider to be valid sources of executable scripts. A CSP compatible browser will then only execute scripts loaded in source files received from those whitelisted domains, ignoring all other script (including inline scripts and event-handling HTML attributes).

As an ultimate form of protection, sites that want to never allow scripts to be executed can opt to globally disallow script execution.

Mitigating packet sniffing attacks

In addition to restricting the domains from which content can be loaded, the server can specify which protocols are allowed to be used; for example (and ideally, from a security standpoint), a server can specify that all content must be loaded using HTTPS. A complete data transmission security strategy includes not only enforcing HTTPS for data transfer, but also marking all cookies with the secure flag and providing automatic redirects from HTTP pages to their HTTPS counterparts. Sites may also use the Strict-Transport-Security HTTP header to ensure that browsers connect to them only over an encrypted channel.

Using CSP

Configuring Content Security Policy involves adding the Content-Security-Policy HTTP header to a web page and giving it values to control resources the user agent is allowed to load for that page. For example, a page that uploads and displays images could allow images from anywhere, but restrict a form action to a specific endpoint. A properly designed Content Security Policy helps protect a page against a cross site scripting attack. This article explains how to construct such headers properly, and provides examples.

Specifying your policy

You can use the Content-Security-Policy HTTP header to specify your policy, like this:

Content-Security-Policy: policy

The policy is a string containing the policy directives describing your Content Security Policy.

Writing a policy

A policy is described using a series of policy directives, each of which describes the policy for a certain resource type or policy area. Your policy should include a default-src policy directive, which is a fallback for other resource types when they don't have policies of their own (for a complete list, see the description of the default-src directive). A policy needs to include a default-src or script-src directive to prevent inline scripts from running, as well as blocking the use of eval(). A policy needs to include a default-src or style-src directive to restrict inline styles from being applied from a <style> element or a style attribute.

Examples: Common use cases

This section provides examples of some common security policy scenarios.

Example 1

A web site administrator wants all content to come from the site's own origin (this excludes subdomains.)

Content-Security-Policy: default-src 'self'

Example 2

A web site administrator wants to allow content from a trusted domain and all its subdomains (it doesn't have to be the same domain that the CSP is set on.)

Content-Security-Policy: default-src 'self' *.trusted.com

Example 3

A web site administrator wants to allow users of a web application to include images from any origin in their own content, but to restrict audio or video media to trusted providers, and all scripts only to a specific server that hosts trusted code.

Content-Security-Policy: default-src 'self'; img-src *; media-src media1.com media2.com; script-src userscripts.example.com

Here, by default, content is only permitted from the document's origin, with the following exceptions:

  • Images may loaded from anywhere (note the "*" wildcard).
  • Media is only allowed from media1.com and media2.com (and not from subdomains of those sites).
  • Executable script is only allowed from userscripts.example.com.

Example 4

A web site administrator for an online banking site wants to ensure that all its content is loaded using SSL, in order to prevent attackers from eavesdropping on requests.

Content-Security-Policy: default-src https://onlinebanking.jumbobank.com

The server only permits access to documents being loaded specifically over HTTPS through the single origin onlinebanking.jumbobank.com.

Example 5

A web site administrator of a web mail site wants to allow HTML in email, as well as images loaded from anywhere, but not JavaScript or other potentially dangerous content.

Content-Security-Policy: default-src 'self' *.mailsite.com; img-src *

Note that this example doesn't specify a script-src; with the example CSP, this site uses the setting specified by the default-src directive, which means that scripts can be loaded only from the originating server.

Testing your policy

To ease deployment, CSP can be deployed in report-only mode. The policy is not enforced, but any violations are reported to a provided URI. Additionally, a report-only header can be used to test a future revision to a policy without actually deploying it.

You can use the Content-Security-Policy-Report-Only HTTP header to specify your policy, like this:

Content-Security-Policy-Report-Only: policy 

If both a Content-Security-Policy-Report-Only header and a Content-Security-Policy header are present in the same response, both policies are honored. The policy specified in Content-Security-Policy headers is enforced while the Content-Security-Policy-Report-Only policy generates reports but is not enforced.

Enabling reporting

By default, violation reports aren't sent. To enable violation reporting, you need to specify the report-uri policy directive, providing at least one URI to which to deliver the reports:

Content-Security-Policy: default-src 'self'; report-uri http://reportcollector.example.com/collector.cgi

Then you need to set up your server to receive the reports; it can store or process them in whatever manner you feel is appropriate.

Violation report syntax

The report JSON object contains the following data:

The URI of the document in which the violation occurred.
The referrer of the document in which the violation occurred.
The URI of the resource that was blocked from loading by the Content Security Policy. If the blocked URI is from a different origin than the document-uri, then the blocked URI is truncated to contain just the scheme, host, and port.
The name of the policy section that was violated.
The original policy as specified by the Content-Security-Policy HTTP header.

Sample violation report

Let's consider a page located at http://example.com/signup.html. It uses the following policy, disallowing everything but stylesheets from cdn.example.com.
Content-Security-Policy: default-src 'none'; style-src cdn.example.com; report-uri /_/csp-reports
The HTML of signup.html looks like this:
<!DOCTYPE html>
    <title>Sign Up</title>
    <link rel="stylesheet" href="css/style.css">
    ... Content ...
Can you spot the mistake? Stylesheets are only allowed to be loaded from cdn.example.com, yet the website tries to load one from its own origin (http://example.com). A browser capable of enforcing CSP will send the following violation report as a POST request to http://example.com/_/csp-reports, when the document is visited:
  "csp-report": {
    "document-uri": "http://example.com/signup.html",
    "referrer": "",
    "blocked-uri": "http://example.com/css/style.css",
    "violated-directive": "style-src cdn.example.com",
    "original-policy": "default-src 'none'; style-src cdn.example.com; report-uri /_/csp-reports"

As you can see, the report includes the full path to the violating resource in blocked-uri. This is not always the case. For example, when the signup.html would attempt to load CSS from http://anothercdn.example.com/stylesheet.css, the browser would not include the full path but only the origin (http://anothercdn.example.com). The CSP specification gives an explanation of this odd behaviour. In summary, this is done to prevent leaking sensitive information about cross-origin resources.

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari
Content-Security-Policy 251 14 232 103 15 74
base-uri 40 No 35 No 27 10
block-all-mixed-content Yes ? 48 No Yes ?
child-src 40 15 45 No 27 10
connect-src 25 14 236 No 15 7
default-src 25 14 23 No 15 7
disown-opener No No No No No No
font-src 25 14 23 No 15 7
form-action 40 15 36 No 27 10
frame-ancestors 40 15 337 No 26 10
frame-src 25 14 23 No 15 7
img-src 25 14 23 No 15 7
manifest-src Yes No 41 No Yes No
media-src 25 14 23 No 15 7
navigation-to No No No No No No
object-src 25 14 23 No 15 7
plugin-types 40 15 No9 No 27 10
referrer 33 — 56 No 3710 No Yes — 43 No
report-sample 59 ? ? ? 46 ?
report-to No No No No No No
report-uri 25 14 23 No 15 7
require-sri-for 54 No 4911 No 41 No
sandbox 25 14 50 10 15 7
script-src 25 14 23 No 15 7
strict-dynamic 52 No 52 No 39 No
style-src 25 14 23 No 15 7
upgrade-insecure-requests 43 No12 42 No 30 No
worker-src 59 No 58 No 48 No
Feature Android webview Chrome for Android Edge mobile Firefox for Android IE mobile Opera Android iOS Safari
Content-Security-Policy Yes Yes Yes 23 ? ? 7.15
base-uri Yes Yes No 35 No ? 9.3
block-all-mixed-content Yes Yes ? 48 No ? ?
child-src Yes Yes No 45 No ? 9.3
connect-src Yes Yes ? 23 No ? 7.1
default-src Yes Yes ? 23 No ? 7.1
disown-opener No No No No No No No
font-src Yes Yes ? 23 No ? 7.1
form-action Yes Yes No 36 No ? 9.3
frame-ancestors ? Yes No 338 No ? 9.3
frame-src Yes Yes ? 23 No ? 7.1
img-src Yes Yes ? 23 No ? 7.1
manifest-src Yes Yes No 41 No ? No
media-src Yes Yes ? 23 No ? 7.1
navigation-to No No No No No No No
object-src Yes Yes ? 23 No ? 7.1
plugin-types Yes Yes No No No ? 9.3
referrer 33 — 56 33 — 56 No 3710 No Yes — 43 No
report-sample 59 59 ? ? ? 46 ?
report-to No No No No No No No
report-uri Yes Yes ? 23 No ? 7.1
require-sri-for 54 54 No 4911 No 41 No
sandbox Yes Yes ? 50 10 ? 7.1
script-src Yes Yes ? 23 No ? 7.1
strict-dynamic 52 52 No No No 39 No
style-src Yes Yes ? 23 No ? 7.1
upgrade-insecure-requests 43 43 No 42 No 30 No
worker-src 59 59 No 58 No 48 No

1. Implemented as X-Webkit-CSP header in Chrome 14.

2. Implemented as X-Content-Security-Policy header in Firefox 4.

3. Implemented as X-Content-Security-Policy header, only supporting 'sandbox' directive.

4. Implemented as X-Webkit-CSP header in Safari 6.

5. Implemented as X-Webkit-CSP header in iOS 5.1.

6. Prior to Firefox 50, ping attributes of <a> elements weren't covered by connect-src.

7. Before Firefox 58, frame-ancestors is ignored in Content-Security-Policy-Report-Only.

8. Before Firefox for Android 58, frame-ancestors is ignored in Content-Security-Policy-Report-Only.

9. See Bugzilla bug 1045899.

10. Will be removed, see Bugzilla bug 1302449.

11. From version 49: this feature is behind the security.csp.experimentalEnabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

12. Under consideration for future release.

A specific incompatibility exists in some versions of the Safari web browser, whereby if a Content Security Policy header is set, but not a Same Origin header, the browser will block self-hosted content and off-site content, and incorrectly report that this is due to a the Content Security Policy not allowing the content.

See also

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