Hackers have targeted the public websites of the operators of the Nasdaq and Bats stock exchanges over the past two days with cyberattacks that disrupted the sites but had no impact on trading.
The Nasdaq.com website was briefly inaccessible at times on Tuesday although it was back online and functioning normally late in the day.
Nasdaq did not immediately reply to an inquiry from AFP but The Wall Street Journal quoted a spokesman as saying that "during the past 24 hours, Nasdaq OMX has experienced intermittent service disruptions on our corporate websites."
The Kansas-based Bats, which operates the BZX and BYX exchanges, said the Bats public website, "along with other securities industry websites," was hit by a Distributed Denial of Service (DDoS) attack on Monday.
"Our trading systems were not affected and there were no Exchange customer disruptions associated with the incident," a Bats spokeswoman told AFP.
"Our website is used to display general information about our market activity for the casual user, and there are no critical functions supported through our general public site," she said.
"We worked with our Internet Service Provider and swiftly returned our website to a normal operating state," the Bats spokeswoman said. "Our trading systems continue to operate normally today."
In a DDoS attack, a large number of computers are commanded to simultaneously visit a website, overwhelming its servers and knocking it offline.
The hacker group Anonymous claimed Friday to have rendered the website of the Central Intelligence Agency briefly inaccessible with a DDoS attack.
Anonymous last month also briefly knocked the websites of the US Justice Department and the Federal Bureau of Investigation offline.
There was no immediate claim on Twitter feeds used by Anonymous for the attacks on the Nasdaq and Bats websites.
Explore further: Facebook abuses 'quasi-monopoly' on user data, EU lawmaker says