The company said Wednesday that there is no evidence that any financial or credit card information was stolen and no sign that the breach has resulted in unauthorized activity for its 145 million active users.
EBay says its investigation is active and it can't comment on the specific number of accounts affected, but says the number could be large.
Cyberattackers stole a small number of employee log-in credentials that gave access to eBay's corporate network, the company said. The San Jose, California-based company is working with law enforcement to investigate the attack.
The database was hacked sometime between late February and early March, but compromised employee log-in credentials were first detected two weeks ago.
EBay owns electronic payment service PayPal, but eBay says there is no evidence PayPal information was hacked, since that data is stored separately.
The attack follows several other high-profile data security incidents, including a massive breach at Target stores and the recent discovery of the "Heartbleed." computer security flaw. Heartbleed is a point of weakness in a key piece of security technology used by more than 500,000 websites that had been exposing online passwords and other sensitive data to potential theft for more than two years.
And during Target's data breach last year, hackers stole about 40 million debit and credit card numbers and personal information for 70 million people.
Shares of eBay Inc. fell 8 cents to $51.88 in Wednesday's trading.