Secrets are stored on Mitro's servers. Presumably -- hopefully -- the passwords themselves are encrypted. Edit: Ah, yes:

    Mitro is distinctive amongst free/open source password managers
    in that it's architected around cloud storage. For security,
    the online password databases are encrypted with client-side
    keys derived from your master password. For availability, they
    are mirrored across three cloud storage providers. With this
    design ... passwords can be synchronized across all of your
    computers and devices with minimal effort.