Hackers stole personal data including Social Security numbers, addresses and account numbers of home mortgage holders at KeyBank, the bank reports, in the breach of a third-party vendor that serves multiple corporate clients.

The hackers obtained the information on July 5 after breaking into computers at the insurance services provider OverbySeawell Company, according to a letter that Cleveland-based KeyBank sent to affected residential mortgage customers.

KeyBank, which operates in 15 states and has close to $200 billion in assets, would not say how many of its customers were affected or answer any other questions about the breach. In a statement, it said it was notified of the data theft on Aug. 4 and KeyBank systems and operations were unaffected.

OverbySeawell did not respond to phone messages and emails sent to executives seeking comment. In the statement sent Friday to The Associated Press, KeyBank said Kennesaw, Georgia-based OverbySeawell “suffered a cybersecurity incident that compromised data of its corporate clients.” It did not elaborate.

According to its website, OverbySeawell’s customers include banks, credit unions, mortgage servicers, finance companies and property investors. Its products include a tracking system for real-time insurance monitoring that can be integrated with other financial industry software platforms.

It is a subsidiary of the Breckenridge Group, also of Kennesaw.

In an Aug. 26 letter shared with the AP by an affected mortgage-holder, KeyBank said the information acquired in the OverbySeawell breach related to their mortgage includes their name, address, mortgage account number and the first eight digits of their nine-digit Social Security number.

That’s plenty of information for identity thieves to commit serious fraud.

“We take this matter very seriously and have notified all affected individuals,” KeyBank said in the letter.

KeyBank said OverbySeawell had notified law enforcement and was investigating the breach with the help of third-party cybersecurity experts. It encouraged the mortgage holder to sign up for free fraud monitoring.