People use computers in an Internet cafe in Beijing in January. (How Hwee Young/European Pressphoto Agency)

CHINA’S DETERMINATION to censor information inside the country is exemplified by the world’s largest digital cordon, known as the Great Firewall. People who try to gain access to Facebook in China, or search for terms considered a threat by the Communist Party, get screens saying their request or search has been “reset” or get nothing at all. All this is powered by a massive computer system. The Great Firewall reflects the fear of China’s rulers that information could undermine their monopoly on power. It also has isolated hundreds of millions of Chinese people from the world beyond.

Although limited, there are holes in the firewall. Technology makes it possible to penetrate the cordon and reach overseas Web sites. One nonprofit group outside of China, Greatfire.org, has attempted to publicize methods for circumvention of censorship and make some blocked Web sites accessible in China.

On March 17, the servers used by Greatfire.org were deluged with an enormous volume of Internet traffic. The cascade was a crude but effective technique known as a “dedicated denial of service” attack. The goal is to overwhelm a server so that it breaks down. The Greatfire.org servers, leased in the cloud in the United States, were hit with 2.6 billion requests per hour, or 2,500 times normal.

Now, a group of Internet researchers have pinpointed why: China struck back. The researchers at the Citizen Lab of the Munk School of Global Affairs at the University of Toronto have issued an unsettling report that suggests China activated an offensive cyberweapon against Greatfire.org. The researchers are calling this the “Great Cannon.”

In effect, China reached beyond its borders and carried out a destructive and powerful assault on an Internet site that it did not like. North Korea is believed last year to have struck across borders, using different techniques, against Sony Pictures Entertainment.

China has long attempted to intimidate academics, news media and others abroad. China also has carried out an aggressive and intrusive campaign of cyberespionage against military, government and commercial targets in the United States and elsewhere. But there is something new and disturbing in the appearance of the Great Cannon.

According to the Citizen Lab, China’s new weapon is co-located with the Great Firewall and is probably a government creation. But it uses a distinct offensive method that does not shoot directly at a specific target. Rather, it is implanted as a middleman on a busy commercial server and then unleashes its malevolent traffic on command. In the case of this assault, China apparently launched the Great Cannon from sites belonging to the Web search giant Baidu, one of China’s most successful Internet companies. In effect, a portion of innocent traffic that came to Baidu sites was re-directed to flood the Greatfire.org Web site in an attempt to poke its eyes out.

If China’s government would do this to Greatfire.org, a Web site that sought to thwart its censorship, then what will it do next? And to whom?