Morgan Stanley says an employee in its wealth management business stole data from about 900 clients and posted it on the internet.
The investment bank says the employee shared client names and account numbers, but not passwords or Social Security numbers.
Morgan Stanley says it detected the exposure and that the information was taken down.
The company says it's fired the employee and is working with regulators and law enforcement as they investigate the incident.
The breach affects about 10 per cent of Morgan Stanley's wealth-management clients.
The New York company says it hasn't discovered losses suffered by any client and is strengthening security. It did not immediately respond to an inquiry about the employee's identity.
Shares of Morgan Stanley fell $US1.14, or 3.00 per cent, to $US37.57 in morning trading.
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