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Content-Security-Policy-Report-Only

Content-Security-Policy-Report-Only

The HTTP Content-Security-Policy-Report-Only response header allows web developers to experiment with policies by monitoring (but not enforcing) their effects. These violation reports consist of JSON documents sent via an HTTP POST request to the specified URI.

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

Header type Response header
Forbidden header name no
This header is not supported inside a <meta> element.

Syntax

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

Directives

The directives of the Content-Security-Policy header can also be applied to Content-Security-Policy-Report-Only.

The CSP report-uri directive should be used with this header, otherwise this header will be an expensive no-op machine.

Examples

This header reports violations that would have occurred. You can use this to iteratively work on your content security policy. You observe how your site behaves, watching for violation reports, or malware redirects, then choose the desired policy enforced by the Content-Security-Policy header.

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

If you still want to receive reporting, but also want to enforce a policy, use the Content-Security-Policy header with the report-uri directive.

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

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 "report" depending on whether the Content-Security-Policy header or the Content-Security-Policy-Report-Only 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-Report-Only 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-Report-Only: default-src 'none'; style-src cdn.example.com; report-uri /_/csp-reports

The HTML of signup.html looks like this:

<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="UTF-8" />
    <title>Sign Up</title>
    <link rel="stylesheet" href="css/style.css" />
  </head>
  <body>
    Page content
  </body>
</html>

Can you spot the violation? 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",
    "disposition": "report"
  }
}

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). This is done to prevent leaking sensitive information about cross-origin resources.

Specifications

Specification
Content Security Policy Level 3
# cspro-header

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
Content-Security-Policy-Report-Only
25
14
23
10
15
7
4.4
Yes
23
No
7
Yes

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only