A week ago reports emerged that Yahoo's ad network was serving malware. Those initial reports said that only European sites were affected between December 31 and January 3. After further investigation, Yahoo now says the period of attack was from December 27, 2013 - January 3, 2014, and that a small number of users outside of Europe may have been affected.
Yahoo adds that the vector for the attack was a compromised account. They have shut down that account and are working with law enforcement to investigate the breach.
Yahoo advises that users run Windows Update to apply the latest fixes to Windows, update Java to the current version, download the latest version of Adobe products and run an updated antivirus program.