AOL has launched an investigation with US authorities into a security breach that allowed hackers to gain access to around two per cent of its email accounts.
The probe began "following a significant increase in the amount of spam appearing as `spoofed emails' from AOL mail addresses," the internet company said in a statement.
Spoofing is a tactic used by hackers to make it appear that the message is from an email user known to the recipient in order to trick the recipient into opening an attachment which may contain malware, AOL said.
"AOL's investigation is still underway, however, we have determined that there was unauthorised access to information regarding a significant number of user accounts," the statement said.
"This information included AOL users' email addresses, postal addresses, address book contact information, encrypted passwords and encrypted answers to security questions that we ask when a user resets his or her password, as well as certain employee information."
The company added that "we believe that spammers have used this contact information to send spoofed emails that appeared to come from roughly two per cent of our email accounts."
AOL said it was working with "best-in-class external forensic experts and federal authorities to investigate this serious criminal activity."
The statement said it was not known whether the encryption on passwords or answers to security questions was broken, but that "as a precautionary measure, we nevertheless strongly encourage our users and employees to reset their passwords used for any AOL service."
