Any company worth its weight that stands to lose big time from a security vulnerability will have a page with info on how to report it to them, and searching "[company name] security" will usually get you right to it. Disclosure is what you do after you've contacted them and not heard back.

http://www.apple.com/support/security/

http://www.microsoft.com/security/msrc/report.aspx

https://www.dropbox.com/terms/#security

http://www.amazon.com/gp/help/customer/display.html/ref=hp_l...