Yahoo president Jeff Mallett said a "distributed denial of service attack" overwhelmed its Web hosting company's routers beginning at around 10:20 a.m. PST, and apparently ending shortly after 1 p.m. PST.
"It appears to be a coordinated outside attack on Yahoo routers here in California in our main data centers," Mallett said in an interview.
A denial of service attack is increasingly becoming a common networking prank. By deluging a Web site's equipment with too many requests for information, an attacker can effectively clog the system, slowing performance or altogether crashing the site. (See related story.)
These pranks have generally targeted much smaller sites to date. But Yahoo's outage today shows that even Internet giants with ostensibly enormous networking resources are not immune from such attacks.
GlobalCenter, the Web hosting unit of international communications carrier Global Crossing, hosts the majority of Yahoo's Web sites, with the exception of recently acquired properties such as GeoCities and Broadcast.com.
"As quickly and as early as possible we worked alongside with (Yahoo) to get them up and running again," said a GlobalCenter spokeswoman.
GlobalCenter's network was not affected and no other Web sites hosted by the company were impacted, the spokeswoman added.
According to one Internet performance measurement firm, Keynote Systems, the problem primarily affected Yahoo users in the United States.
Keynote director of public services Dan Todd said Yahoo was inaccessible within the United States, but was 59 percent accessible internationally.
Today's outage illustrates vulnerabilities in the technology supporting Internet businesses. Yahoo attracts millions of visitors and serves as the gateway to many other Web sites.
When it released its earnings report for the last three months of 1999, Yahoo noted that it is accessed by 120 million unique vistors and that it served an average of 465 million pages each day during December.
It was unclear how much Yahoo's competitors benefited from the temporary outage, if at all.
A spokesman for rival portal AltaVista, for example, said the company's network saw above average traffic this morning, but added the numbers were not all that dramatic.
"We're not sure really what we can attribute (the uptick) to," said spokesman David Emanuel. "It's not completely outside the normal parameters for ups and downs."
Representatives from Lycos today said the site experienced an "unusual traffic spike" while Yahoo was down, according to Ron Sege, executive vice president of Lycos.
"We've seen a significant increase in traffic this afternoon," Sege said.
Mallet said most of Yahoo's services were affected by the attack. However, certain sites, such as Yahoo Mail, Yahoo Shopping, and Yahoo Search and Directory were up and running at certain points.
Mallet said that the company plans to investigate the origin of the attacks.
"It did not come from one single point," Mallett said. "It appears to have come in from multiple servers simultaneously."
News.com's Stefanie Olsen and Corey Grice contributed to this report.