Hack to cost UK's TalkTalk up to $53 million

The company said, while presenting its half-year results Wednesday, that forensic analysis had found that 4 percent of its  customers have any personal data at risk.

Giving a breakdown, TalkTalk disclosed that  the total number of customers whose personal details were accessed were 156,959, and of these customers 15,656 bank account numbers and sort codes were accessed. 28,000 obscured credit and debit card numbers were also accessed by the hackers but cannot be used for financial transactions, as they were were 'orphaned', and cannot be identified by the stolen data, the company said.

The October hack is the third so far on the company. Previous attacks were in December 2014 and in February this year.

Police have so far arrested four persons in connection with the hack, under suspicion of offenses under the Computer Misuse Act. They were released on bail without charges while the police investigation continues. The company's board is also conducting an inquiry.

On Oct. 23, the company said it had been hit by a "significant and sustained cyberattack" on its website, and there was a possibility that customers' names, addresses, dates of birth, phone numbers, email addresses, TalkTalk account information, credit card details and/or bank details were compromised. This led to speculation that a significant proportion of the 4 million customers of the telecommunications operator were under threat.

By Oct. 30, the company disclosed that during the hack less than 21,000 unique bank account numbers and sort codes, under 28,000 obscured credit and debit card details of which the middle six digits had been removed, and less than 15,000 customer dates of birth had been accessed. Under 1.2 million customer email addresses, names and phone numbers were also part of the personal data accessed.

TalkTalk said it expects some near-term effects on customer churn from the cyberattack. While it was to early to assess the impact of the cyberattack on its business, it said early data on customer churn and retention after the hack are encouraging. The company has offered a free upgrade of services to customers, allowing them to choose from free TV content, a mobile SIM with monthly allowances of free text, data and calls, unlimited U.K. landline and mobile calls, and a broadband health checkup.

CEO Dido Harding told the BBC that the one-off costs on the hack would go towards covering for lost revenue as the company's online sales sites were down for the last three weeks, additional IT and technology costs, higher number of calls to its call center, and the cost of responding to the incident.