Shares in Sage opened more than four per cent lower and as the biggest faller on the FTSE 100 on monday morning after it revealed nearly 300 UK company records may have been compromised.
The accountancy software firm told affected customers on Friday that their information — including employee bank details and salary information — may have been stolen.
Firms are paying increasing attention to cyberthreats, after telephone and broadband provider TalkTalk's profits halved last year due to the fallout from a similar attack on its mobile sales site.
By early afternoon, the firm's stock was down 1.2 per cent to 731.1p.
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"We are investigating unauthorised access to customer information using an internal login," the company said in a statement.
"We cannot comment further whilst we work with the authorities to investigate but our customers remain our first priority and we are speaking directly with those affected," it added.
Media reports suggest that the company reported the breach to the City of London police over the weekend.
The breach was first reported on www.theantisocialengineer.com.
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Justin Harvey, chief security officer at Fidelis Cybersecurity said:
UK companies using Sage will likely feel extremely unsettled by the news that the software company has been breached. If the reports are true and an internal login has been used, this means that the employee was either involved, fell victim to a phishing attack, or alternatively, the hacker had managed to gain a foothold in Sage’s network and access the employee’s login details.
Ultimately, as a FTSE 100 company, Sage’s share price has already fallen and its reputation is likely to take a similar turn. This latest incident will serve as a reminder – particularly in the business to business community – how far reaching the impact of one compromised entry point can be.
With hundreds of companies and hundreds more employees at risk, security incidents such as this can be difficult to recover from.
Sage recently reported revenues grew 4.1 per cent year-on-year to £747m for the six months ended March 2016.