Content Security Policy (CSP)

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 malware distribution.

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 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 may 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:

  content="default-src 'self'; img-src https://*; child-src 'none';" />

Note: Some features, such as sending CSP violation reports, are only available when using the HTTP headers.


Mitigating cross-site scripting

A primary goal of CSP is to mitigate and report XSS attacks. XSS attacks exploit the browser's trust in 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 allowed domains, ignoring all other scripts (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 attribute 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 what 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. There are specific directives for a wide variety of types of items, so that each type can have its own policy, including fonts, frames, images, audio and video media, scripts, and workers.

For a complete list of policy directives, see the reference page for the Content-Security-Policy header.

Examples: Common use cases

This section provides examples of some common security policy scenarios.

Example 1

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

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

Example 2

A website 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' *

Example 3

A website 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; script-src

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 and (and not from subdomains of those sites).
  • Executable script is only allowed from

Example 4

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

Content-Security-Policy: default-src

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

Example 5

A website administrator of a web mail site wants to allow HTML in email, as well as images loaded from anywhere, but JavaScript or other potentially dangerous content can only come from the same origin as the mail server.

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

Note that this example doesn't specify a script-src, so the default-src directive will be used for JavaScript sources as a fallback.

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

A server can inform clients where to send reports using the Reporting-Endpoints HTTP response header. This header defines one or more endpoint URLs as a comma-separated list. If you want to define an endpoint named csp-endpoint which accepts reports at, the server's response header would look like this:

Reporting-Endpoints: csp-endpoint=""

You must then use the Content-Security-Policy header's report-to directive to refer to an endpoint where reports of that policy should be sent. For example, to send violation reports to you can send response headers that look like the following:

Reporting-Endpoints: csp-endpoint=""
Content-Security-Policy: default-src 'self'; report-to csp-endpoint

You need to set up a server to receive reports at the given URL ( in this case) that accepts a JSON object sent with an application/csp-report Content-Type. The server handling these requests can then store or process the incoming reports in a way that best suits your needs.

If you want to have multiple endpoints that handle different types of reports, the Reporting-Endpoints header can contain a comma-separated list of endpoints:

Reporting-Endpoints: csp-endpoint="",

Warning: The report-uri directive is deprecated and it's recommended to send CSP reports using report-to instead. See the report-uri documentation for details on how to specify both directives for backwards compatibility.

Violation report syntax

The report JSON object is sent with an application/csp-report Content-Type and contains the following data:


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.


Either "enforce" or "report" depending on whether the Content-Security-Policy-Report-Only header or the Content-Security-Policy header is used.


The URI of the document in which the violation occurred.


The directive whose enforcement caused the violation. Some browsers may provide different values, such as Chrome providing style-src-elem/style-src-attr, even when the actually enforced directive was style-src.


The original policy as specified by the Content-Security-Policy HTTP header.

referrer Deprecated Non-standard

The referrer of the document in which the violation occurred.


The first 40 characters of the inline script, event handler, or style that caused the violation. Only applicable to script-src* and style-src* violations, when they contain the 'report-sample'


The HTTP status code of the resource on which the global object was instantiated.

violated-directive Deprecated

The directive whose enforcement caused the violation. The violated-directive is a historic name for the effective-directive field and contains the same value.

Sample violation report

Let's consider a page located at It uses the following policy, disallowing everything but stylesheets from

Content-Security-Policy: default-src 'none'; style-src; report-to /_/csp-reports

The HTML of signup.html looks like this:

<!doctype html>
<html lang="en-US">
    <meta charset="UTF-8" />
    <title>Sign Up</title>
    <link rel="stylesheet" href="css/style.css" />
    Here be content.

Can you spot the mistake? Stylesheets are allowed to be loaded only from, yet the website tries to load one from its own origin ( A browser capable of enforcing CSP would send the following violation report as a POST request to, when the document is visited:

  "csp-report": {
    "blocked-uri": "",
    "disposition": "report",
    "document-uri": "",
    "effective-directive": "style-src-elem",
    "original-policy": "default-src 'none'; style-src; report-to /_/csp-reports",
    "referrer": "",
    "status-code": 200,
    "violated-directive": "style-src-elem"

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, if the signup.html attempted to load CSS from, the browser would not include the full path, but only the origin ( The CSP specification gives an explanation of this odd behavior. In summary, this is done to prevent leaking sensitive information about cross-origin resources.

Browser compatibility

BCD tables only load in the browser

Compatibility notes

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 the Content Security Policy not allowing the content.

See also