Sony shares dived as investors tried to gauge the damage from cyber attacks on the Japanese entertainment and technology giant's online networks that have compromised millions of users.
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Chief Executive Howard Stringer apologised in a blog post late Thursday for the massive data breach in his first public remarks on the attack, but uncertainty over the cost of the assault prompted investors to sell.
The electronics giant firm was 2.80 per cent down by noon in Tokyo trade Friday, after earlier plunging 4.1 per cent as Japanese markets reopened following the Golden Week holiday.
While markets were closed, Sony had said cyber assaults on its online videogame network were broader than first thought, with intruders possibly making off with credit and debit card data.
Hackers breached the Sony Online Entertainment (SOE) network as well as the PlayStation Network and the Qriocity streaming music service, the consumer electronics giant said, affecting another 25 million accounts, bringing the total affected to more than 100 million.
Sony had previously said a security breach of its PlayStation Network had lead to 77 million users' data being stolen.
"I know this has been a frustrating time for all of you," Stringer said in a blog post to Sony customers.
"Let me assure you that the resources of this company have been focused on investigating the entire nature and impact of the cyber-attack we've all experienced and on fixing it."
He added that there was currently no evidence credit card or personal information had been "misused".
"Games are one of Sony's main pillars, so it's very unsettling to investors that they don't have enough information yet to estimate how much compensation costs, legal expenses and missed revenue will arise from the incidents," a portfolio manager at a Japanese securities firm told Dow Jones Newswires.
The breach is a blow for Sony as it focuses on pushing its content such as games and music through hardware platforms such as game consoles, smartphones and tablet computers amid competition from Apple's iTunes and App store.
Analysts say Sony, already reeling from the impact on production of Japan's March 11 earthquake, faces costs in beefing up security and compensating consumers. Its shares are 22 per cent below pre-March 11 earthquake levels.