A sign-in seal is a secret message or image that you select to help protect your account from phishing — a scam that tries to steal your password or personal information by spoofing a legitimate web site.
When you create a sign-in seal for your computer, you can be sure you’re on a legitimate Yahoo! site each time you use this computer to sign in to Yahoo!. Just look for the custom text or image you set up on this computer. If it’s not there, you might have landed on a “spoofed” site.
Your sign-in seal is saved on the computer you created it on. If you use more than one computer or browser, you may want to create a sign-in seal for each one.