Software company CircleCi has confirmed that some customers’ data were stolen in a data breach last month.

Although the employee’s access was secured with two-factor authentication, the company said that the intruder gained access through a laptop that was infected with malware, reports TechCrunch.

This vulnerable system allowed the theft of session tokens, which were used to keep the employee logged in to specific applications.

The company accepted responsibility for the breach, labeling it a “systems failure” and added that its antivirus software failed to find the malware on the employee’s laptop that was stealing tokens.

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According to CircleCi, the theft of the session token allowed the hackers to use the employee’s identity to access some of the company’s production systems, which include customer data.

“Because the targeted employee had privileges to generate production access tokens as part of the employee’s regular duties, the unauthorized third party was able to access and exfiltrate data from a subset of databases and stores, including customer environment variables, tokens and keys,” said Rob Zuber, chief technology officer, CircleCi.

“We encourage customers who have yet to take action to do so in order to prevent unauthorized access to third-party systems and stores,” Zuber added.

Moreover, he said that the hackers had access from December 16 through January 4.

Several customers have already informed the company of unauthorized access to their systems, Zuber added.

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