"For the past month," the hackers wrote, "anyone who has viewed Yahoo's page (and) used their search engine, now has a logic bomb/worm implanted deep within their computer."
The note demanded the release of noted hacker Kevin Mitnick, or the virus would "wreak havoc" on the Internet on Christmas Day.
The search company analyzed its networks, but found no malicious code - despite the message's claim that a virus was placed on the site. The page was reportedly removed after being on Yahoo!'s site for only 15 minutes. Noted hacker Kevin Poulsen dismissed the act as a hoax.
Company officials explained why they believe no such virus exists. "The intrusion was detected by our electronic monitoring system," said Diane Hunt, director of corporate communications for Yahoo! "If they attacked any other part of the system, we would have detected that as well."
The false home page only showed itself to users of the text-only browser known as Lynx, said a Yahoo! official, who went on to say that the hacker posting would not have been visible to users of Netscape's Communicator or Microsoft's Internet Explorer.
The group of hackers allegedly came from three groups, referred to in the posting as H4GiS, PANTS, and the MLF. The letter promised a nationwide failure of power grids "on exactly 01:00 hours February 14, 1997 (sic)" as a demonstration of their ability.
The subject of their demands is famed hacker Kevin Mitnick. Last year, Mitnick was indicted on charges related to an Internet crime spree. He was caught by network administrator Tsutomu Shimomura, and brought to fame in a book by New York Times reporter John Markoff.
The posting ended with a page-long diatribe, criticizing Shimomura and the Electronic Frontier Foundation.
Yahoo! has contacted law enforcement officials and will pursue the pranksters. "We are taking the appropriate steps," said Hunt, "even though no damage was done."