Discount brokerage firm Scottrade says that 4.6 million customer contact details may be at risk after the firm received a warning from the FBI that its systems had been hacked between late 2013 and early 2014.
The brokerage says that it only became aware of the intrusion after a tip-off from Federal law enforcement officials who had been investigating cybersecurity crimes involving the theft of information from Scottrade and "other financial services companies".
The firm says that the criminals targeted client names and street addresses, indicating that the data may have been lifted to perpetrate pump'n'dump stock scams on penny stocks.
Says Scottrade: "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."
News of the breach comes just five months after the firm hired a new chief information security officer to lead the company's cybersecurity strategy.
The Scottrade break-in caps an inauspicious start to the US Government's Cybersecurity Awareness Month campaign, which has been marred by a rash of successful attacks on FXCM, Experian, Trump hotels and the American Bankers Association.