A networking error has caused computers in Chile and the U.S. to come under the control of the Great Firewall of China, redirecting Facebook, Twitter, and YouTube users to Chinese servers.

Security experts are not sure exactly how this happened, but it appears that at least one ISP recently began fetching high-level DNS (domain name server) information from what's known as a root DNS server, based in China. That server, operated out of China by Swedish service provider Netnod, returned DNS information intended for Chinese users, effectively spreading China's network censorship overseas. China tightly controls access to a number of Web sites, using technology known colloquially as the Great Firewall of China.

The issue was reported Wednesday by Mauricio Ereche, a DNS admin with NIC Chile, who found that an unnamed local ISP reported that DNS queries for sites such as Facebook.com, Twitter.com and YouTube.com -- all of which have been blocked in China -- were being redirected to bogus addresses.

It is unclear how widespread the problem is. Ereche reported getting the bogus information from three network access points in Chile, and one in California, but on Thursday he said that the problem was no longer popping up. "The traces show us that we're not hitting the server in China," he wrote in a discussion group post.

This issue occurred because, for some reason, at least one outside ISP directed DNS requests to a root server based in China, networking experts say. This is something that service providers outside of China should not do because it allows China's censored network to "leak" outside of the country.

Researchers have long known that China has changed DNS routing information to redirect users of censored services to government-run servers instead of sites such as Facebook and Twitter. But this is the first public disclosure that those routes have leaked outside of China, according to Rodney Joffe, a senior technologist with DNS services company Neustar. "All of a sudden, the consequences are that people outside China may be subverted or redirected to servers inside China," he said.

By using a China-based root server, ISPs are essentially giving China a way to control all of their users' traffic over the network. That could mean big security problems for people whose network accepted the leaked routes, Joffe said.

The ISP that used the bad routes probably misconfigured its BGP (Border Gateway Protocol) system, used to route information on the Internet, according to Danny McPherson, chief security officer with Arbor Networks. "I don't think it was done intentionally, " he said. "This is an example of how easy it is for this information to be contaminated or corrupted or leaked out beyond the boundaries of what it was supposed to be."

In February 2008, BGP information from Pakistan -- which had just blocked YouTube -- was shared internationally, effectively knocking Google's video site offline for millions of users.

In an e-mail message, Netnod CEO Kurt Erik Lindqvist said his company is not hosting the bad routes on its server. They were most likely changed by machines somewhere on the Chinese network, McPherson said.

The incident shows that BGP remains a major weak link in the Internet, Joffe said. "It's really disconcerting form a security point of view and from a privacy point of view."

This is the first time that this type of behavior has been made public, but it has apparently been going on for some time. In a discussion group post on Wednesday, Nominet Researcher Roy Arends said that he has been studying this issue for a year.

Arends has compiled a list of 20 domain names that will trigger the kind of bad results, reported by Ereche. Arends is keeping the names of those domains secret, but he did publish some of his data in his discussion post.

"I wanted to keep this internal, however, the cat is out of the bag now," Arends wrote.