Only certain users were affected. "Users in North America, Asia Pacific and Latin America were not served these advertisements and were not affected," she said. "Additionally, users using Macs and mobile devices were not affected."
Presumably that means that Windows users on other continents -- including Europe, where the problem was first spotted -- were hit by the attacks.
"We will continue to monitor and block any advertisements being used for this activity," the Yahoo statement concludes. "We will be posting more information for our users shortly."
Update (Jan. 6): In a follow-up statement, Yahoo is now acknowledging that the problem started on December 31.