Time Warner Cable is warning about 320,000 customers that some of their data may have been stolen.
But the cable giant denied that its systems have been breached, instead blaming malware or third-party companies that store its customers' data.
The company said that email addresses and passwords may have been taken in the breach. Affected customers have been notified by email and direct mail.
"We have not yet determined how the information was obtained, but there are no indications that TWC's systems were breached," said spokesperson Eric Mangan.
"The emails and passwords were likely previously stolen either through malware downloaded during phishing attacks or indirectly through data breaches of other companies that stored TWC customer information, including email addresses," he said.
The FBI informed the cable and internet giant that some account details had been compromised, according to Reuters.
It wouldn't be the first time that users have had their accounts hacked through a separate breach. Many use the same email address and password as their internet provider to log into streaming services, other email accounts, social networks, and other websites -- which can all be vulnerable to hacking.
The company did not say if the breach was connected to a similar attack against Comcast, which led to 200,000 accounts being reset in November.