"Gullible" is in the dictionary; "thin ears" is not.
It may be in Korean dictionaries, all right, where having thin ears means to easily change one's mind following hearsay and other people's opinions.
Now, you probably want Yahoo! Mail to turn a thick ear to people who pretend to be you trying access to your emails. Then, when you want to change the password guarding your account the ear should be thin, of course — and it is, if you know where to whisper and how:
›› Do you think somebody may have guessed your Yahoo! Mail password? Change it now, and then change and change again, to help keep your Yahoo! Mail account secure.

