A hacker broke into the Suffolk County National Bank's online banking system and got access to log-in information for more than 8,000 customers in November, the bank disclosed Monday.
The hacker apparently did not use the information to steal anyone's money, the bank said.
The hack took place during a six-day period ending Nov. 23, and the bank discovered the intrusion more than a month later, on Christmas Eve, the bank said.
The 8,378 accounts represent about 10 percent of the bank's online customer base.
The bank isolated the online-banking computer server where the intruder gained access to the log-in information, then "rebuilt the compromised server and took other measures to ensure the security of data on the server," Suffolk Bancorp, the bank's parent company, said in a prepared statement.
"To date, SCNB has found no evidence of any unauthorized access to online banking accounts, nor received any reports of unusual activity or reports of financial loss to its customers," the statement said.
The discovery of the intrusion came during an internal security review.
The bank sent letters Monday to retail and business customers whose log-in information was compromised, telling them it will pay for credit monitoring and fraud protection services for two years. The bank set aside $351,000 for expenses associated with the data intrusion and the subsequent investigation.
The Suffolk Bancorp statement said it "immediately launched an aggressive investigation of the incident with assistance from outside experts in forensics; notified the three major consumer credit reporting agencies Experian, Trans Union and Equifax; notified the federal Office of the Comptroller of the Currency, the state Consumer Protection Board; the state Office of Cyber Security and Critical Infrastructure Coordination; and law enforcement agencies."
Customers who need reassurance about their accounts are asked to call the manager of their local branch office. The bank operates 29 full-service offices throughout Suffolk County.