The Treasury Department, Secret Service, Federal Trade Commission and Transportation Department Web sites were all down at varying points over the holiday weekend and into this week, according to officials inside and outside the government. Some of the sites were still experiencing problems Tuesday evening. and private sites also may be linked, officials there said.
U.S. officials refused to publicly discuss any details of the cyber attack, and would only generally acknowledge that it occurred. It was not clear whether other government sites also were attacked.
Others familiar with the U.S. outage, which is called a denial of service attack, said that the fact that the government Web sites were still being affected three days after it began signaled an unusually lengthy and sophisticated attack. The officials spoke on condition of anonymity because they were not authorized to speak on the matter.
Web sites of major South Korean government agencies, banks and Internet sites also were paralyzed in a suspected cyber attack Tuesday. Ahn Jeong-eun, a spokeswoman at the Korea Information Security Agency, said the U.S. and South Korean attacks appeared to be linked.
The South Korean sites included the presidential Blue House, the Defense Ministry, the National Assembly, Shinhan Bank, Korea Exchange Bank and top Internet portal Naver. They went down or had access problems since late Tuesday, Ahn said.
The Homeland Security Department confirmed that officials had received reports of "malicious Web activity" and they were investigating the matter, but had no further comment. Two government officials acknowledged that the Treasury and Secret Service sites were brought down, and said the agencies were working with their Internet service provider to resolve the problem.
Ben Rushlo, director of Internet technologies at Keynote Systems, called it a "massive outage" and said problems with the Transportation Department site began Saturday and continued until Monday, while the FTC site was down Sunday and Monday.
Keynote Systems is a mobile and Web site monitoring company based in San Mateo, Calif. The company publishes data detailing outages on Web sites, including 40 government sites it watches.
According to Rushlo, the Transportation Web site was "100 percent down" for two days, so that no Internet users could get through to it. The FTC site, meanwhile, started to come back online late Sunday, but even on Tuesday Internet users still were unable to get to the site 70 percent of the time.
"This is very strange. You don't see this," he said. "Having something 100 percent down for a 24-hour-plus period is a pretty significant event."
He added that, "The fact that it lasted for so long and that it was so significant in its ability to bring the site down says something about the site's ability to fend off (an attack) or about the severity of the attack."
Denial of service attacks against Web sites are not uncommon, and are usually caused when sites are deluged with Internet traffic so as to effectively take them off-line. Mounting such an attack can be relatively easy using widely available hacking programs, and they can be made far more serious if hackers infect and use thousands of computers tied together into "botnets."
For instance, last summer, in the weeks leading up to the war between Russia and Georgia, Georgian government and corporate Web sites began to see "denial of service" attacks. The Kremlin denied involvement, but a group of independent Western computer experts traced domain names and Web site registration data to conclude that the Russian security and military intelligence agencies were involved.
Documenting cyber attacks against government sites is difficult, and depends heavily on how agencies characterize an incident and how successful or damaging it is.
Government officials routinely say their computers are probed millions of times a day, with many of those being scans that don't trigger any problems. In a June report, the congressional Government Accountability Office said federal agencies reported more than 16,000 threats or incidents last year, roughly three times the amount in 2007. Most of those involved unauthorized access to the system, violations of computer use policies or investigations into potentially harmful incidents.
The Homeland Security Department, meanwhile, says there were 5,499 known breaches of U.S. government computers in 2008, up from 3,928 the previous year, and just 2,172 in 2006.