Security researcher files FTC complaint over Dropbox’s claims over encryption. Claims that users’ files are hash-tagged each time they are uploaded. This means that, if user A uploads, for example, pictures with a given hash tag and then user B uploads the same name/sized file – with the same hash tag – their version is not actually uploaded. Could also be possible to upload an infected version of a popular file on other services and, when other users `upload’ the legitimate file – which would generate the same Hash-tag as the infected file – their Dropbox folder would be populated with the infected version.
Source: https://thehackernews.com/2011/05/dropbox-potential-security-flaw.html