EBay shares fell as much as 3.2 percent after the latest high-profile hacking attack on a U.S. company.
"For the time being, we cannot comment on the specific number of accounts impacted. However, we believe there may be a large number of accounts involved and we are asking all eBay users to change their passwords," eBay spokeswoman Kari Ramirez said.
The attack was made through compromised employee accounts that allowed unauthorized access to its corporate network, the company said in a statement. It said the breach was first detected about two weeks ago.
The company said it found no evidence of unauthorized access to personal or financial information for users of its online payment service, PayPal.
"Working with law enforcement and leading security experts, the company is aggressively investigating the matter and applying the best forensics tools and practices to protect customers," the company said in a press release.
EBay earlier issued a notice on its PayPal website asking users to change their passwords, but took down the message a short time later without explanation.