Dries Buytaert

Explanation

Specifies a security policy to help browsers detect and block cross-site scripting (XSS) and data injection attacks.
upgrade-insecure-requests instructs browsers to replace insecure URLs (HTTP) with secure URLs (HTTPS).

Allows web developers to debug content security policies. Violations are reported to the specified report-uri, but not enforced.

The Report-To header instructs the browser where to send "Network Error Logging" (NEL) reports to.
group defines the reporting group. Groups allow reports to be grouped logically; e.g. there could be a group for network errors and a second group for browser feature deprecation reports.
max_age defines the number of seconds the browser should remember these settings. Prevents the browser from having to parse the JSON on each request.
endpoints defines one or more URLs where the reports need to be send to. Multiple URLs can be specified for failover and load-balancing. Endpoints can be assigned a weight to distribute load, with each endpoint receiving a specified fraction of the reporting traffic. Endpoints can also be assigned a priority to set up fallback collectors.

The type of the message body, specified as a MIME type.

The date and time after which the page should be considered stale and all caches should be refreshed.
Notice Because there is a Cache-Control header with a max-age and/or s-maxage directive, the Expires header will be ignored. Consider removing Expires to save bandwidth and processing power.

The date and time at which the request was made. A browser uses it for age calculations rather than using its own internal date and time; e.g. when compairing against Max-Age or Expires.

private means the page can only be stored by the browser, but not by CDNs, Varnish or any other shared caches.
max-age specifies the maximum amount of seconds a page is considered valid. The higher max-age, the longer a page can be cached.
Warning Because max-age is set to 0 seconds, nothing will ever be cached in shared caches or browsers. Caching is effectively disabled!

The date and time at which the origin server believes the page was last modified.
Notice Because there is an Etag header, Last-Modified is likely to be ignored. The ETag hash is more accurate than the date/time in Last-Modified. Consider removing Last-Modified to save bandwidth and processing power.

A unique identifier that changes every time a page at a given URL changes. It acts as a fingerprint. A cache can compare Etag values to see if the page has changed and became stale. For example, a browsers will send the ETag value of a cached page in an If-None-Match header. The web server compares the ETag value sent by the browser with the ETag value of the current version of the page. If both values hasHeaderWithName, the web server sends back a 304 Not Modified status and no body. This particular Etag value starts with w/ which means that it is a weak identifier; while unlikely, multiple pages might have the same identifier. Weak identifiers are used because strong identifiers can be difficult and costly to generate.

Instructs the browser not to change the MIME type of the request. Browsers sometimes sniff or inspect the document to determine if the specified MIME type is correct. A browser changing the MIME type of a document can introduce security vulnerabilities. These vulnerabilities can occur when a website allows users to upload content and the user disguises a particular file type as something else. For example, a JavaScript file is uploaded as a JPEG file. When the browser changes the MIME type from image/jpeg to test/javascript, it gives attackers an opportunity to perform a cross-site scripting attack.

1; mode=block enables the browser's cross-site scripting (XSS) filtering. Browsers that support X-Xss-Protection will stop rendering the page when an attack is detected.

The size of the message body, in bytes.

Some of the software used to generate or serve this page.

Used to promote alternative services through which the same resource can be retrieved.
h3 stands for HTTP/3. The number after the dash indicates the draft; e.g. h3-27 would be draft 27 of the HTTP/3 protocol.
ma is the number of seconds the alternative service is considered fresh.

missing Add a Strict-Transport-Security header. The Strict-Transport-Security header or HSTS header is used to instruct browsers to only use HTTPS, instead of using HTTP. It helps enforce secure communication.

missing Add a Referrer-Policy header. When a visitor navigates from one page to another, browsers often pass along referrer information. The Referrer-Policy header controls how much referrer information a browser can share. This is important to configure when private information is embedded in the path or query string and passed onto an external destination.

missing Add a Permissions-Policy header. Restrict access to device features like the camera, microphone, location, accelerometer and much more.

missing Add a Cross-Origin-Embedder-Policy to specify how this page can be loaded by cross-origin resources.

missing Add a Cross-Origin-Opener-Policy header to opt-in into better browser isolation.

missing Add a Cross-Origin-Resource-Policy header to specify who can load this page.

missing Add a X-Permitted-Cross-Domain-Policies header to prevent Flash, Adobe Reader and other clients from sharing data across domains.