This one comes straight from Microsoft. Apparently there were nine fraudulent digital certificates being issued by a certification authority called "Comodo" but were quickly pulled. The initial release of the certificates pose a threat to internet browsers.
Microsoft is aware of nine fraudulent digital certificates issued by Comodo, a certification authority present in the Trusted Root Certification Authorities Store on all supported versions of Microsoft Windows.
Comodo advised Microsoft on March 16, 2011 that nine certificates had been signed on behalf of a third party without sufficiently validating its identity. These certificates may be used to spoof content, perform phishing attacks, or perform man-in-the-middle attacks against all Web browser users including users of Internet Explorer.
These certificates affect the following Web properties: login.live.com, mail.google.com, www.google.com, login.yahoo.com (3 certificates), login.skype.com, addons.mozilla.org, "Global Trustee"
Comodo has revoked these certificates, and they are listed in Comodo’s current Certificate Revocation List (CRL). In addition, browsers which have enabled the Online Certificate Status Protocol (OCSP) will interactively validate these certificates and block them from being used.
An update is available for all supported versions of Windows to help address this issue. For more information about this update, see Microsoft Knowledge Base Article 2524375.
Typically, no action is required of customers to install this update, because the majority of customers have automatic updating enabled and this update will be downloaded and installed automatically. For more information, including how to manually install this update, see the Suggested Actions section of this advisory.
Read more about it on Microsoft's website.