In this case, the USSD is not the bug. The fact that it can be triggered from HTML and cause a factory reset without user interaction is the bug. At least with older phones, after entry, it was necessary to hit dial before any effect was taken.
That's true, but sort of missing my point. Security bugs are very rarely "security bugs" in isolation. They're far more often unexpected interactions between subsystems. Here, the expectation of the browser is that it can fire a "phone number" Intent securely, because the dialer app will handle it. But the phone number intent also happens to hook to the USSD layer. It's not USSD's "fault", as the check needs to be in the browser according to the architecture. But USSD remains a booby trap because it's an unexpected legacy feature with surprising security behavior.
