GitHub, a U.S.-based coding website, has suffered a four-day cyber attack, suspected to be linked to mainland Chinese authorities.

The site is the host for Greatfire.org, an organization which monitors Chinese online censorship activity and offers Chinese users some ways around online restrictions, and CN-NYTimes, the Chinese-language version of the New York Times.

From Thursday (March 26), GitHub underwent a ‘distributed denial of service’ attack in which huge numbers of page requests overwhelm a site’s servers, causing all or part of a site to become unavailable.

The attack is seen as part of an escalation by Chinese authorities, who already have tens of thousands of staff operating interventionist tools within China, to extend their control of the Internet to domains overseas.

The Wall Street Journal quoted an Internet security expert as saying that the attack had to be organized by “someone with the ability to tamper with all the Internet traffic coming in to China.”

The attack worked by redirecting requests from outside China for pages on Chinese search engine Baidu to the Greatfire and CN-NYTimes pages on GitHub. By only using traffic from outside China and taking it away from Baidu, the Chinese authorities can plausibly deny knowledge of the operation, and a Chinese site can claim to be the victim.

Baidu has issued a statement denying any involvement in the cyber-attack.

Communicating by Twitter and its own security pages, GitHub says that it has deployed defences against the DDOS assault, but that the attack has since evolved into one targeting other pages and assets.

The websites of certain overseas media including The New York Times, The Wall Street Journal and The Guardian are blocked in China. So too are many services provides by Google and Microsoft.

In January, Greatfire reported on a Chinese attack on Microsoft Outlook that it said involved the issuing of fake site security certificates by the China Internet Network Information Center.