Trading in seven stocks listed on the Hong Kong stock exchange was suspended on Wednesday after a hacking attack.
The attack was aimed at a website run by the exchange used to tell traders about company announcements.
The site was shut and trading in seven firms due to make announcements via the website was suspended for half a day.
Shares in HSBC, Cathay Pacific, China Power International and the Hong Kong exchange itself were among those suspended.
"Our current assessment (is) that this is a result of a malicious attack by outside hacking," said Charles Li, head of Hong Kong Exchanges & Clearing (HKEC), in a statement. HKEC runs the Hang Seng exchange.
Mr Li said the company was looking into the motive for the attack and what hackers sought to gain from it. The incident has been referred to the police as well as the Securities and Futures Commission.
The attack on the site made it temporarily unavailable. It is not yet clear whether the attack overwhelmed the site with data, making it unreachable, or whether hackers gained unauthorised access to it.
HKEC was investigating the attack and said if the site remained unstable on Thursday, announcements would be made via the Hang Seng's bulletin board. Additionally, the suspension of the seven shares would be lifted.
Price sensitive information due to be announced included HSBC announcing the sale of its US credit card arm and Cathy Pacific unveiling half year results. The suspended stocks are among the biggest on the Hang Seng index.
None of the other systems operated by Hong Kong Exchanges was hit in the attack and its securities and derivatives markets ran as normal.
The Hong Kong exchange is one of many stock markets that have been hit by hackers. The Zimbabwe stock exchange was attacked in early August and in February, the US Nasdaq revealed that cyber criminals had planted malicious code on its "Directors Desk" web application.