Computer experts from the University of Cambridge claim not only to have
breached the Great Firewall of China, but have found a way to use the firewall
to launch denial-of-service attacks against specific Internet Protocol addresses
in the country.
The firewall, which uses routers supplied by Cisco, works in part by
inspecting Web traffic for certain
keywords that the Chinese government wishes to censor, including political
ideologies and groups it finds unacceptable.
The Cambridge research group tested
the firewall by firing data packets containing the word "Falun" at it, a
reference to the Falun Gong religious group, which is banned in China.
The researchers found that it was possible
to circumvent the Chinese intrusion detection systems by ignoring the forged
transmission control protocol resets injected by the Chinese routers, which
would normally force the endpoints to abandon the connection.
"The machines in China allow data packets in and out, but send a burst of
resets to shut connections if they spot particular keywords," explained Richard
Clayton of the University of Cambridge computer laboratory. "If you drop all the
reset packets at both ends of the connection, which is relatively trivial to do,
the Web page is transferred just fine."
Clayton added that this means the Chinese firewall can be used to launch
denial-of-service attacks against specific IP addresses within China, including
those of the Chinese government itself.
The IDS uses a stateless server, which examines each data packet both going
in and out of the firewall individually, unrelated to any previous request. By
forging the source address of a packet containing a "sensitive" keyword, people
could trigger the firewall to block access between source and destination
addresses for up to an hour at a time.
If an attacker had identified the machines used by regional government
offices, they could block access to Windows Update, or prevent Chinese embassies
abroad from accessing specific Chinese Web content.
"Due to the design of the firewall, a single packet addressed from a high
party official could block their Web access," said Clayton.
Even though this technique would block communication between only two
particular points on the Internet, the researchers calculated that a lone
attacker using a single dial-up connection could still generate a "reasonably
effective" denial-of-service attack. If an attacker generated 100 triggering
packets per second, and each packet caused 20 minutes of disruption, 120,000
pairs of endpoints could be prevented from communicating at any one time.
Clayton, speaking at the Sixth Workshop on Privacy Enhancing Technologies in
Cambridge last week, said that the researchers had reported their findings to
the Chinese Computer Emergency Response Team.