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 custom headers that were added as part of the initial request would be forwarded intact across origins. This poses a CSRF risk for web applications that rely on custom headers only being present in requests from their own origin.