Computer firm Microsoft is hurrying to fix a bug with the Internet Explorer browser after it emerged that hackers have already exploited the glitch with attacks on US companies.
Microsoft said the bug could allow hackers to take over a computer, install malicious programmes and create user accounts.
Cybersecurity firm FireEye said so far there had been "targeted attacks seemingly against U.S.-based firms, currently tied to defence and financial sectors".
News Corporation chairman Rupert Murdoch has suggested hackers are "still" trying to gain access to his companies' servers.
News Corp's Wall Street Journal and the independently-owned New York Times revealed in recent days that their networks were penetrated at the weekend, potentially by China-based hackers.
News International has confirmed that a Sun journalist has been arrested on suspicion of computer hacking.
A News International spokeswoman said: "We can confirm a journalist from the Sun was arrested this morning."