CORS requests can omit the preflight request

Cross-Origin Resource Sharing CORS requests are required to send a preflight request if custom headers are included, to check that the host wishes to allow the full request to be made. An example of where this may be needed is for sites that use a custom header with a static value as part of thei...

CSRF risk with plugins and 307 redirects — Mozilla

Independent security researcher Kuza55 and Microsoft security researcher Tom Gallagher reported that when plugin-initiated requests receive a 307 redirect response, the plugin is not notified and the request is forwarded to the new location. This is true even for cross-site redirects, so any cust...