Hackers have hit the websites of the NASDAQ and BATS stock exchanges.
NASDAQ and BATS saw their sites disrupted during the day on Monday. The sites host company news and share price data, as well as vital information on live service status on the exchanges.
It is understood, however, that while the websites were affected, the stock exchanges continued to trade as normal with no change to trading.
A spokesperson at BATS said the exchange's site had been hit with "an external Distributed Denial Of Service (DDoS) incident".
"Our trading systems were not affected and there were no exchange customer disruptions associated with the incident," she added. "We worked with our internet service provider and swiftly returned our website to a normal operating state. Our trading systems continue to operate normally today."
BATS said it would continue to be "vigilant in protecting our systems from future threats".
NASDAQ told the Wall Street Journal that on Tuesday it experienced "intermittent service disruptions on our corporate websites".
It is not known who initiated the attacks.
In 2010, NASDAQ's Directors Desk online scheduling application was compromised by hackers. An FBI investigation found that the stock exchange's ageing software and out of date security patches played a key part in the problems.
Last month, in separate incidents, the Saudi Arabia and Abu Dhabi stock exchanges were targeted by hackers, slowing trading.