It is insecure to allow a webpage to request any resources from any other {{glossary("domain","domains")}} without limitation. CORS (Cross-Origin Resource Sharing) is a system that determines whether to block or fulfill these requests.
Learn more
General knowledge
- HTTP access control (CORS) on MDN
- {{Interwiki("wikipedia", "Cross-origin resource sharing")}} on Wikipedia
Technical reference
- Specification on W3C recommandation