The broker said it had advised customers to reset their passwords but that the attack had not otherwise interrupted trading.
The firm refused to comment on whether this was the first attack of its kind on the firm, however experts say they are increasingly common.
Some brokers have adopted a two-step identification method to try and stop the attacks, according to Thomas Peterffy, chief executive of Interactive Brokers Group Inc.
Some hackers try to slow down the speed of the messaging system, which is key for a broker, by bombarding the system with messages.
“They bombard you...We don’t know whether messages are real or not,” Mr. Peterffy told the Wall Street Journal.
