(CNN) China's cybercensors have long used a "Great Firewall" to block its citizens from reading critical articles from Western news websites or consuming other content it disapproves of.

But it's no longer enough for them, says a study published Friday . They've developed a new IT weapon and have attacked servers outside their borders, including in the United States.

The study's authors have named it the "Great Cannon," and it operates in plain sight.

Going on the attack so visibly and handily within another country's borders will probably draw international ire, the study's authors say, and Beijing may have counted on that.

"This is a powerful attack capability, and we are curious about the risk and benefit analysis that led the Chinese government to reveal it with this highly visible denial of service attack," said researcher John Scott-Railton.

Enter the Cannon

The reason Chinese censors are taking that risk: Free-speech cyberactivists have found ways to get around the Great Firewall and give Chinese readers greater access to the West's free press.

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Enter the "Great Cannon."

It blasts targeted Web servers with massive distributed denial of service attacks, and it uses the Web browsers of unsuspecting Web surfers to do it.

The Cannon wrecked two online services with DDoS attacks in March, say the researchers from the University of Toronto's Citizen Lab , the International Computer Science Institute , the University of California-Berkeley and Princeton University.

Many of the researchers focus on the abuse of information technology to undermine civil liberties and human rights. And they are afraid this new cyberweapon could easily be used for an array of powerful attacks beyond what they've already observed.

"A modest technical change could turn the Great Cannon into a malware delivery device for infecting the computer of a target individual anywhere in the world who visits a Chinese server," Scott-Railton said.

This might include all emails headed in and out of China, he said. "The device could replace genuine attachments with malicious files, for example."

Surprising target

One of the Great Cannon's targets that the researchers studied was an obvious one -- Greatfire.org, run by Chinese expats bent on fighting Beijing's censorship. They monitor Chinese citizens' access to international news sites such as German news service Deutsche Welle or The Tibet Post

But the other target may seem odd at first glance. GitHub is a popular Silicon Valley hosting service used by programmers who want to share code with each other.

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The two attacks were connected, however, the study says. GreatFire.org hosted two GitHub repositories that contained computer code allowing Chinese readers to get around the Great Firewall and read The New York Times in Chinese

Critical articles from the Times are a particular fly in the ointment for Beijing, and China has turned away at least three of the paper's reporters in short succession, according to a U.S. congressional commission on China

GitHub said it thought the attackers were trying to coerce it into taking content offline. GreatFire.org says it suspects the attack may have been in response to a Wall Street Journal article on its struggle to circumvent Chinese censors.

and media reports. Both services suspected China was behind their attacks and used the Great Firewall to carry it out, according to statements

The Great Firewall

By triggering attacks and analyzing them, the researchers concluded that Beijing has developed a tool distinctly different from the Great Firewall. They are confident it is also in China and say it is technically similar to the Great Firewall.

The firewall, in a manner of speaking, stands aside and watches all digital traffic going in and out of China, the researchers say.

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If it sees requests going out into the world for content it doesn't want citizens to see, the researchers say, it discreetly injects forged messages to the foreign server and the Chinese user's computer to make them stop communicating.

The user might see an HTTP 403 reply -- "Sorry, you're not authorized to see this page."

Not only does the Great Firewall monitor tons of traffic, but its systems have to do a lot of processing to discern what to block and what not, so it's work-intensive.

Great Cannon pinpoints targets

The Great Cannon takes on a much lighter load, because it doesn't care about all that traffic. Instead, it targets traffic between a handful of Web addresses. But it uses Web traffic unrelated to its targets to build its attack against them.

Users going to Baidu, one of China's most prolific Web services and most successful Internet companies, can become unknowing proxy warriors against the Great Cannon's targets, the study says.

In the overwhelming number of cases, when traffic came into China from the outside world, the Great Cannon let it through to Baidu's advertising servers.

But in a tiny fraction of the cases the researchers observed, it picked out computers it wanted to use in the attack, and sent bad code back to the user's browser. "The malicious script enlisted the requesting user as an unwitting participant in the DDoS attack against GreatFire.org and Github," the authors wrote.

Their browsers mercilessly fired requests at both sites and paralyzed them.

"At the time of writing they number 2.6 billion requests per hour," GreatFire.org wrote during an outage in March . "Websites are not equipped to handle that kind of volume so they usually 'break' and go offline."

GitHub said the March incident was the biggest DDoS attack in its history. Back then, the programmers noticed that there were unique aspects about the attack.

"These include every vector we've seen in previous attacks as well as some sophisticated new techniques that use the web browsers of unsuspecting, uninvolved people to flood github.com with high levels of traffic," they wrote.

Baidu denies any involvement in the attacks and says its internal security has remained intact, the researchers said. But government cybercensors' monitoring of traffic to and from Baidu's servers could hurt its reputation as a major player in international commerce.

Fully encrypting Web traffic should help to defend against the Great Cannon, Scott-Railton said.

Xi Jinping's tightening grip

Chinese President Xi Jinping is a Communist Party hardliner, and since he took office in November 2012, Chinese citizens have felt the grip tightening again on freedoms they thought they had gained, journalists and activists say.

Xi and the Politburo "are responding to new threats by falling back on repressive tactics" rather than "experimenting with more liberal policies," think tank Freedom House wrote in an analysis. And repression has particularly targeted grass-roots activists, online opinion leaders and ordinary citizens on the Internet.

Xi also has a reputation for eyeing Western values with suspicion and considering American IT companies, such as Intel and Google, partners of the U.S. government.

"Deployment of the GC (Great Cannon) may also reflect a desire to counter what the Chinese government perceives as U.S. hegemony in cyberspace," the researchers write.

The authors say the United States and Great Britain already have methods for intercepting unencrypted traffic and launching attacks.