At Dropbox, the security of your data is our highest priority. We have a dedicated security team using the best tools and engineering practices available to build and maintain Dropbox, and you can rest assured that we've implemented multiple levels of security to protect and back up your files. You can also take advantage of two-step verification, a login authentication feature which you can enable to add another layer of security to your account.
Dropbox uses Amazon's Simple Storage Service (S3) for storage, which has a robust security policy of its own. You can find more information on Amazon's data security from the S3 site or, read more about how Dropbox and Amazon securely stores data.
Some third party apps with Dropbox integration will ask for permission to access your Dropbox. To grant access, you will need to log in to your Dropbox account and explicitly press the Authorize button when prompted. You can visit the My Apps section of your account settings to review the apps you've granted permission to and revoke access at your discretion.