Access-Control-Allow-Origin: *

Controls origins (websites) that are allowed to load data from this web service over JavaScript-based APIs as part of Cross-Origin Resource Sharing (CORS) standard. By default, a web browser will refuse to load data over XmlHttpRequest from a website that is not in the same origin, which is a precaution against various types of data stealing attacks. The target server has to explicitly allow the origin domain using the Access-Control-Allow-Origin (ACAO) header, or it may allow all origins to access it using a wildcard *. The latter however creates a potential security issue if the website in question is transactional and processing sensitive data, so the wildcard should be only used on websites consciously offering public APIs.

Link: <https://fonts.gstatic.com>; rel=preconnect; crossorigin

Indicates location of sub-resources for the given page, in machine-readable format. It may come in a number of types, for example next page, previous page, index page, preloading resources etc.

Strict-Transport-Security: max-age=31536000

HTTP Strict Transport Security (HSTS) is an opt-in security enhancement that is specified by a web application through the use of a special response header.

Server: ESF

Announces web server software and optionally version details.

X-XSS-Protection: 0

Controls an Cross-Site Scripting (XSS) filters built into the majority of web browsers. The filter is usually turned on by default anyway, but requirement to set the header to 1 became part of canonical set of "secure" HTTP headers. Over time, vulnerabilities in the "sanitizing" mode filter were found, so 1; mode=block became the recommended value. Some companies decided that they don't really need a browser-side XSS filter to mess with their web services which are XSS-free anyway and they became consciously disabling the XSS filter by setting the header to 0.

X-Frame-Options: SAMEORIGIN

Instructs the browser if the current website can be embedded in HTML frame by another website. Since this allows the parent website to control the framed page, this creates a potential for data theft attacks ("clickjacking") and most sensitive websites won't allow them to be framed at all (deny) or just allow parts of them to be embedded in frames created by themselves only (samesite).

X-Content-Type-Options: nosniff

A non-standard but widely accepted header introduced originally by Microsoft to disable "content sniffing" or heuristic content type discovery in absence or mismatch of a proper HTTP Content-Type declaration, which led to a number of web attacks. In general, presence of the header with its only defined value of nosniff is considered as part of a properly secured HTTP response.