Potential problem: 15 seconds' access to a device signed into Yahoo allows me to generate fraudulent credentials for a year.

1. Alice is signed into Yahoo on her laptop as `alice@yahoo.com`.
2. Eve borrows Alice's laptop for 15 seconds.
3. Eve opens a Persona dialog, types in `eve@yahoo.com`, and triggers an account linking email.
4. Eve returns Alice's laptop and goes home.
5. At home, Eve clicks the confirmation link on her own laptop, establishing this link: `["eve@yahoo.com", "alice@yahoo.com"]`.

Because the link is bidirectional, Eve can now type in `alice@yahoo.com` and get fraudulently authenticated. This ability would persist for a year, and I don't believe Alice would have any recourse.