China's largest search engine, Baidu, was hacked on Tuesday morning by a group calling itself the 'Iranian Cyber Army'.
The same group attacked Twitter in mid-December using a similar Domain Name System hijack.
Baidu users were unable to access the site's offerings, and were greeted with a message from the hacking group with a black and red background and a flag, all connected with the Iranian party of Mir Hossein Mousavi, which challenged president Mahmoud Ahmadinejad's power and protested against his election in June.
Baidu has now responded to the hack, and said that the site is now running normally.
It is unclear why the group is attacking these particular web sites, as China and Iran have a relatively good relationship. China has resisted several calls from the US government to impose trading sanctions on Iran, although anti-government groups, such as Mousavi's, may object to this support.
There was similar confusion when the group attacked Twitter last month, because the micro-blogging site backed Mousavi supporters after the Iran election by delaying its maintenance schedule so that protesters in Tehran could be heard.
The same hacking group has also attacked another web site, Mowjcamp.org, which has been supportive of the protests.
Graham Cluley, senior technology consultant at security firm Sophos, suggested that the group may be choosing its targets to have the greatest impact.
"In China, Baidu outranks Google as the search engine of choice, receiving many millions of visits every day. That makes it an extremely attractive target for cyber criminals. Anyone who manages to breach its security has the potential to make a big impact," he said.
"Chinese surfers should be breathing a great sigh of relief that the hackers didn't exploit this opportunity to infect computers, and instead engaged in what appears to be political graffiti. Questions will be asked, however, as to how this high-profile hack was possible."
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