Hackers took advantage of the fact that some website companies stored the users' information without encrypting, making it easy for hackers to break into, Kaspersky, an anti-virus company, told China Daily on Thursday through an e-mail.

Netizens should use different e-mail addresses and codes to register on different websites to avoid being hacked, suggested Kaspersky.

The mass leakage showed that many websites ignored having to protect the privacy of their users, said Chen Tao, a member of the Beijing Lawyers' Association.

"Existing laws and regulations fail to specify how Internet companies responsible for information leakage might be penalized," Chen said. "As a result, most of them get away by issuing an apology without being fined or punished by authorities."

The amendment to the Criminal Law rules that hackers could face a penalty of up to five years in jail.

"The Criminal Law should also impose the compulsory responsibilities of protecting information of their users on Internet companies," Chen said.

Stricter rules are needed to keep Internet companies from dereliction of duty, "especially when real-name registration was required by some websites such as the Sina weibo", Chen added.

To make things worse, some small Internet companies even sold the information of their members to public relations companies to make profits, Kang Lingyi, the former hacker, said.

"I always received text messages from unidentified people who said they wanted to pay for the information of our website's users, which I refused all the time," he said.

By Thursday, none of the Internet companies that were reported to have their users' information exposed received any punishment from authorities.

The Ministry of Industry and Information Technology issued a notice on Wednesday saying websites should remind their users as soon as possible to avoid using the same account and code in different websites, and that netizens should change their codes to avoid the possibility of being hacked.

"Internet security is a worldwide question for both the websites and the netizens," Kang said. "Under the current situation, the only way for netizens to protect themselves was to change their logging codes from time to time."