The tip-off was in the form of e-mails from some users, who complained about spam they were receiving at e-mail addresses they used only for their Dropbox accounts. The company said it has contacted the users and helped them protect their accounts.

Two-Factor Authentication

The sequence, according to a posting Tuesday by Aditya Agarwal on the corporate blog, was that one of the stolen passwords had been used to gain access to a Dropbox account of a company employee, which contained a project document with user e-mail addresses.

Agarwal apologized for the slipup, which the company believes led to the spam, and said that "additional controls" have been put in place to prevent such a recurrence by an employee.

Additionally, steps are being taken to improve the security of accounts. Within a few weeks, Agarwal said, two-factor authentication will be implemented. This will require two proofs of identity to sign in, such as the password and a temporary code sent to one's phone.

New automated mechanisms will be used to help ID suspicious activity, and a new page will allow users to see all active logins to an account. The company may also require a user to change a password, if, for example, it's a common password or hasn't been changed in awhile.

The company also "strongly" recommends that a unique password be set for each Web site. Otherwise, Dropbox noted, if one site's password is compromised, all are.

Dropbox for Teams

Dropbox's investigation following the spam complaints initially found that no security breaches had taken place, although that was later revised.

A key question is whether this breach, and the company's response to it, will affect its continuing efforts to market its services for businesses. While competitor Box is more business-oriented, Dropbox has been targeting companies as part of its strategy for growth.

One of the main services it markets to businesses is Dropbox for Teams, launched in October of last year. The service is priced at about $800 annually for five years, and $125 for each additional user. It grows out of the wide use of Dropbox accounts for business purposes and goes beyond the consumer-oriented ones, which are not optimized for more active, version-control-obsessed business users.

Dropbox for Teams provides a generous, although unspecified, amount of storage. It also supports mobile devices and automatic syncing between devices, comes with phone support, and provides administrative tools for monitoring activity and storage usage for each person.

In the face of increasing competition, Dropbox is busily adding new features and pruning older ones. Last month, for example, it dropped support for public folders and updated its app for Apple's iOS platform.

The growing list of cloud -based sharing and storage services competitors includes Microsoft 's SkyDrive, Google's Drive, Samsung's S-Cloud service, Apple's iCloud, Amazon 's Cloud Drive, and the aforementioned Box.

Tech Marketer:

Jackson Shaw: