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 theContent-Security-Policy
header or theContent-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>
<head>
<title>Sign Up</title>
<link rel="stylesheet" href="css/style.css">
</head>
<body>
... 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
BCD tables only load in the browser
See also
Content-Security-Policy
- CSP
report-uri
directive