Every Comcast ISP user has a master account, which is accessible through their Zimbra webmail site. This account can be used to access your payment information, e-mail settings, user account creation and services you purchase from Comcast. Even if you do not use their mail service, you still will have a master account. It is strongly recommended that, if you are a Comcast user, you change your password as soon as possible.
Comcast performed out-of-schedule maintenance on their mail servers last night, hopefully to fix this exploit. No more information is available at this time on what maintenance was performed.