Online discount broker Scottrade said Friday believes it was the victim of a data breach from late 2013 to early 2014 that targeted client names and addresses.
In a message on its website, the company said it is notifying an estimated 4.6 million clients whose information was stored on a compromised database.
"Although Social Security numbers, email addresses and other sensitive data were contained in the system accessed, it appears that contact information was the focus of the incident," the company said.
Scottrade said it confirmed a breach after being notified that federal law enforcement officials were investigating cybersecurity crimes involving the theft of information from financial services companies.
The company's stock-trading platform didn't appear to be affected in the cyber intrusion, Scottrade said, nor were client funds affected.
"Client passwords remained fully encrypted at all times and we have not seen any indication of fraudulent activity as a result of this incident," Scottrade said. The company is offering identity protection services to customers who were potentially affected.
A Scottrade spokesperson pointed NBC News to the online statement and did not comment further.
The revelation comes a day after credit reporting agency Experian announced that a data breach exposed private personal data of 15 million people who applied for T-Mobile wireless services.
October is also National Cyber Security Awareness Month, a collaborative effort by government and industry to inform Americans about ways to stay safe and secure online.