Access-Control-Allow-Origin: http://www.googletagmanager.com

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.

Server: Google Tag Manager

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.