Exploiting XSS via CORS trust relationships

Even "correctly" configured CORS establishes a trust relationship between two origins. If a website trusts an origin that is vulnerable to cross-site scripting (XSS), then an attacker could exploit the XSS to inject some JavaScript that uses CORS to retrieve sensitive information from the site that trusts the vulnerable application.

Given the following request:

GET /api/requestApiKey HTTP/1.1
Host: vulnerable-website.com
Origin: https://subdomain.vulnerable-website.com
Cookie: sessionid=...

If the server responds with:

HTTP/1.1 200 OK
Access-Control-Allow-Origin: https://subdomain.vulnerable-website.com
Access-Control-Allow-Credentials: true

Then an attacker who finds an XSS vulnerability on subdomain.vulnerable-website.com could use that to retrieve the API key, using a URL like:

https://subdomain.vulnerable-website.com/?xss=<script>cors-stuff-here</script>