Hackers have breached part of Nasdaq's website, with the stock exchange group warning members of its community forum that email and password details have been compromised.
Nasdaq sent an email to users explaining that it had been made aware of the breach following "standard security monitoring", warning that account information could have been stolen following the breach on Tuesday.
However the firm said that its trading and commerce platforms had not been affected.
Security expert Graham Cluley said on his blog that it is likely that hackers had capitalised on a deficiencies in Nasdaq's security infrastructure.
"My guess is that the servers running the NASDAQ community message board software had not been properly configured or not kept updated against vulnerabilities, and this allowed hackers an open window to access sensitive information," he said.
The website is still unavailable, though an onsite message states that the forum is offline due to upgrades being made.
Cluley added that the stock exchange group should be more clear about the security risk posed, and warned over potential phishing attacks as a result of the breach.
"What also irks me is how NASDAQ is describing the issue on the (currently shut-down) community forum itself.
"Any member of the online NASDAQ community who has missed the email advisory, won't be any the wiser from that message that the site has been hacked, and their usernames, email addresses and passwords have been compromised."
It is not the first time that the stock exchange's security has come under the spotlight following a breach. In 2011 an FBI investigation pointed to Nasdaq's ageing software and out of date security patches as part of the reason for servers being hacked.
Investigators found some PCs and servers with out of date software and uninstalled security patches, including Microsoft Windows Server 2003, while some of its firewalls ahd been incorrectly configured.