The group known as the Iranian Cyber Army, which attacked Twitter in December, has defaced another high-profile website, according to security firm Sophos.
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This time the group has targeted China's biggest search engine, Baidu. The site's home page briefly displayed a message declaring the site had been hacked by the Iranian Cyber Army.
"It is not presently clear whether Baidu's site itself was compromised or, as in the case with the Twitter attack, its DNS records," said Graham Cluley senior technology consultant at Sophos.
If the website's DNS records were breached then the hackers would have been able to redirect visitor to web server under their control, he said in a blog post.
The Baidu website appeared to be back to normal in two hours, indicating that the motive for the attack was political rather than financial, said Cluley.
But it would have been easy for the hackers to create a cloned version of the main Baidu web page, he said, complete with an invisible link to a software exploit or piece of malware.
"Attacks like this are a reminder to everyone that you always need to have security scanning every webpage you visit, even if it is an established legitimate website," said Cluley.