Dropbox users in Europe started receiving spam this week at their e-mail addresses associated with their Dropbox account, ZDNet reported Wednesday.
The spam messages appeared even if the users had created the e-mail account exclusively for the storage site and used it nowhere else.
"We wanted to update everyone about spam being sent to e-mail addresses associated with some Dropbox accounts," a Dropbox employee posted to the Dropbox Forum. "We continue to investigate and our security team is working hard on this.
"We've also brought in a team of outside experts to make sure we leave no stone unturned."
The spam messages being sent to different countries in Europe have been arriving in the user's native language, suggesting a very expert and coordinated attack.
The spam e-mails advertise different domain names, all of them created very recently, and all seeming to advertise online casino.
The spam messages use Russian DNS servers, with the domain names registered at a Chinese site, Bizcn, ZCNet reported.