In the wake of several active attacks using one or more fraudulent digital certificates issued by Dutch company DigiNotar, Microsoft has released updates for Windows that adds all DigiNotar SSL certificates to the untrusted publishers list in the certificate store, thus blocking access to web resources that use the certificates. In addition, Google and Mozilla have updated the Chrome and Firefox browsers to block DigiNotar certificates.
Another Certification Authority, GlobalSign, has stopped issuing new certificates following indications that they have been targeted by hackers as well. Read more here: