Online trading company TD Ameritrade alerted more than 6 million customers Friday that a security breach occurred with its client information database.
The database contained such sensitive information as clients' names, Social Security numbers, dates of birth, addresses, phone numbers and trading activity.
Ameritrade, however, stressed that it has no evidence that Social Security numbers and client demographics, such as birth dates and trading activity information, were retrieved or used to commit identity theft. The company also notes that Ameritrade's user log-ins and passwords were not part of the database.
The discovery was made a couple of weeks ago, when the online broker learned that investment-related spam had infiltrated the brokers' system. The malicious code allowed a hacker to access some of the information stored in the database.
A TD Ameritrade spokeswoman declined to give further details of the security breach, noting that the investigation is still ongoing.
But one security expert said it could have happened one of two ways.
"There are only two different ways this could have happened. There was either a vulnerability with their Web site and it was hacked, or someone internally gained access with a Trojan horse," said Graham Cluley, senior technology consultant at Sophos.
He warned that Ameritrade clients should be on the lookout for phishing attempts, which try to steal users' log-ins and passwords by lulling them into believing the e-mail is being sent by the online broker.
Hackers may also try to use the information to run a pump-and-dump scheme, in which certain stocks are touted to clients, driving up the stock price before the attackers dump the stock.
Ameritrade said it hired ID Analytics to conduct a forensics test to ascertain what information, if any, has been compromised. It has also posted more information on its Web site.