Here's one question that's always bugged me - What's stopping a malicious user from sending an HTTP request from any API client like Postman, or even Curl from the CL? Something like a post with: {transferTo: myAccountId, amount: 1000000000}?

Obviously in any nontrivial web app it would fail because of authentication issues, but if a server doesn't do ANY sort of security checking, that should work, no? Does that mean that the onus is on the server developer of mybank.com? And if so, what would stop the malicious request from working on any server developed before the existence of CORS?