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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 to 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';">

Threats

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 load 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:

blocked-uri
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.
disposition
Either "enforce" or "reporting" depending on whether the Content-Security-Policy-Report-Only header or the Content-Security-Policy header is used.
document-uri
The URI of the document in which the violation occurred.
effective-directive
The directive whose enforcement caused the violation.
original-policy
The original policy as specified by the Content-Security-Policy HTTP header.
referrer
The referrer of the document in which the violation occurred.
script-sample
The first 40 characters of the inline script, event handler, or style that caused the violation.
status-code
The HTTP status code of the resource on which the global object was instantiated.
violated-directive
The name of the policy section that was violated.

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>
<html>
  <head>
    <title>Sign Up</title>
    <link rel="stylesheet" href="css/style.css">
  </head>
  <body>
    ... Content ...
  </body>
</html>
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

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Content-Security-PolicyChrome Full support 25
Notes
Full support 25
Notes
Notes Implemented as X-Webkit-CSP header in Chrome 14.
Edge Full support 14Firefox Full support 23
Notes
Full support 23
Notes
Notes Implemented as X-Content-Security-Policy header in Firefox 4.
IE Full support 10
Notes
Full support 10
Notes
Notes Implemented as X-Content-Security-Policy header, only supporting 'sandbox' directive.
Opera Full support 15Safari Full support 7
Notes
Full support 7
Notes
Notes Implemented as X-Webkit-CSP header in Safari 6.
WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 23Opera Android ? Safari iOS Full support 7.1
Notes
Full support 7.1
Notes
Notes Implemented as X-Webkit-CSP header in iOS 5.1.
Samsung Internet Android Full support Yes
base-uriChrome Full support 40Edge No support NoFirefox Full support 35IE No support NoOpera Full support 27Safari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile No support NoFirefox Android Full support 35Opera Android ? Safari iOS Full support 9.3Samsung Internet Android Full support Yes
block-all-mixed-contentChrome Full support YesEdge ? Firefox Full support 48IE No support NoOpera Full support YesSafari ? WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 48Opera Android ? Safari iOS ? Samsung Internet Android Full support Yes
child-src
Deprecated
Chrome Full support 40Edge Full support 15Firefox Full support 45IE No support NoOpera Full support 27Safari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile No support NoFirefox Android Full support 45Opera Android ? Safari iOS Full support 9.3Samsung Internet Android Full support Yes
connect-srcChrome Full support 25Edge Full support 14Firefox Full support 23
Notes
Full support 23
Notes
Notes Prior to Firefox 50, ping attributes of <a> elements weren't covered by connect-src.
IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
default-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
disown-opener
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoEdge Mobile No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
font-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
form-actionChrome Full support 40Edge Full support 15Firefox Full support 36IE No support NoOpera Full support 27Safari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile No support NoFirefox Android Full support 36Opera Android ? Safari iOS Full support 9.3Samsung Internet Android Full support Yes
frame-ancestorsChrome Full support 40Edge Full support 15Firefox Full support 33
Notes
Full support 33
Notes
Notes Before Firefox 58, frame-ancestors is ignored in Content-Security-Policy-Report-Only.
IE No support NoOpera Full support 26Safari Full support 10WebView Android ? Chrome Android Full support YesEdge Mobile No support NoFirefox Android Full support 33
Notes
Full support 33
Notes
Notes Before Firefox for Android 58, frame-ancestors is ignored in Content-Security-Policy-Report-Only.
Opera Android ? Safari iOS Full support 9.3Samsung Internet Android Full support Yes
frame-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
img-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
manifest-srcChrome Full support YesEdge No support NoFirefox Full support 41IE No support NoOpera Full support YesSafari No support NoWebView Android Full support YesChrome Android Full support YesEdge Mobile No support NoFirefox Android Full support 41Opera Android ? Safari iOS No support NoSamsung Internet Android Full support Yes
media-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
navigate-to
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoEdge Mobile No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
object-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
plugin-typesChrome Full support 40Edge Full support 15Firefox No support No
Notes
No support No
Notes
Notes See bug 1045899.
IE No support NoOpera Full support 27Safari Full support 10WebView Android Full support YesChrome Android Full support YesEdge Mobile No support NoFirefox Android No support NoOpera Android ? Safari iOS Full support 9.3Samsung Internet Android Full support Yes
referrer
DeprecatedNon-standard
Chrome No support 33 — 56Edge No support NoFirefox No support 37 — 62IE No support NoOpera No support ? — 43Safari No support NoWebView Android No support 37 — 56Chrome Android No support 33 — 56Edge Mobile No support NoFirefox Android No support 37 — 62Opera Android No support ? — 43Safari iOS No support NoSamsung Internet Android Full support Yes
report-sample
Experimental
Chrome Full support 59Edge ? Firefox ? IE ? Opera Full support 46Safari ? WebView Android Full support 59Chrome Android Full support 59Edge Mobile ? Firefox Android ? Opera Android Full support 46Safari iOS ? Samsung Internet Android Full support 7.0
report-toChrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoEdge Mobile No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
report-uri
Deprecated
Chrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
require-sri-for
Experimental
Chrome Full support 54Edge No support NoFirefox Full support 49
Disabled
Full support 49
Disabled
Disabled 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.
IE No support NoOpera Full support 41Safari No support NoWebView Android Full support 54Chrome Android Full support 54Edge Mobile No support NoFirefox Android Full support 49
Disabled
Full support 49
Disabled
Disabled 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.
Opera Android Full support 41Safari iOS No support NoSamsung Internet Android Full support 6.0
sandboxChrome Full support 25Edge Full support 14Firefox Full support 50IE Full support 10Opera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 50Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
script-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
strict-dynamicChrome Full support 52Edge No support NoFirefox Full support 52IE No support NoOpera Full support 39Safari No support NoWebView Android Full support 52Chrome Android Full support 52Edge Mobile No support NoFirefox Android No support NoOpera Android Full support 39Safari iOS No support NoSamsung Internet Android Full support 6.0
style-srcChrome Full support 25Edge Full support 14Firefox Full support 23IE No support NoOpera Full support 15Safari Full support 7WebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support 23Opera Android ? Safari iOS Full support 7.1Samsung Internet Android Full support Yes
upgrade-insecure-requestsChrome Full support 43Edge No support No
Notes
No support No
Notes
Notes Under consideration for future release.
Firefox Full support 42IE No support NoOpera Full support 30Safari Full support 10.1WebView Android Full support 43Chrome Android Full support 43Edge Mobile No support NoFirefox Android Full support 42Opera Android Full support 30Safari iOS Full support 10.3Samsung Internet Android Full support 4.0
worker-srcChrome Full support 59
Notes
Full support 59
Notes
Notes Chrome 59 and higher skips the deprecated child-src directive.
Edge No support NoFirefox Full support 58IE No support NoOpera Full support 48Safari No support NoWebView Android Full support 59
Notes
Full support 59
Notes
Notes Chrome 59 and higher skips the deprecated child-src directive.
Chrome Android Full support 59
Notes
Full support 59
Notes
Notes Chrome 59 and higher skips the deprecated child-src directive.
Edge Mobile No support NoFirefox Android Full support 58Opera Android Full support 48Safari iOS No support NoSamsung Internet Android Full support 7.0

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
Non-standard. Expect poor cross-browser support.
Non-standard. Expect poor cross-browser support.
Deprecated. Not for use in new websites.
Deprecated. Not for use in new websites.
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.

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