All of a sudden, when the client is trying to sign onto Yahoo to get email, they see a "This Connection is Untrusted" warning. They used Firefox on Mac, signed onto Yahoo, then clicked on the Mail link on the Yahoo home page.
The key phrases they see are:
login.yahoo.com uses an invalid security certificate.
The certificate is only valid for login.yahoo.net
(Error code: ssl_error_bad_cert_domain)
Is this an exploit?