The registration details of users on Tianya.cn, one of the mainland's largest internet forums, were leaked after the site was hacked - one of the biggest leaks of user information in Chinese cyberspace.
The information of more than 40 million forum users became available on Sunday, according to woo yun.org, a website that carries updates on online security loopholes.
Shi Xiaohong, vice-president of Qihoo 360 Technology, a mainland security software maker, said it was the biggest leak of user information in the history of the internet on the mainland, with a total of 110 million user codes hacked and posted online. It is not known how many of the passwords still worked.
Online security breaches have been in the news recently after six million user names and codes on csdn.net, a popular website for software programmers, were leaked online last Wednesday.
Internet users also reported password leaks on other major websites. IT Times reported that the information of eight million users on duo wan.com, a game website, was leaked. But 7k7k.com, another game website; Renren.com, one of the largest social networking websites; and Sina.com, one of the largest news portals on the mainland; denied other reports of leaks.
The incidents are worrying website operators and internet users.
'Tianya.cn used plain text passwords in the early days for historical reasons,' Tianya.cn said on its Weibo microblog page. 'The stolen data was the back-up data before 2009. We adopted an encryption algorithm to tackle the security issues in 2010.'