Cyberduck is an open source client for FTP and SFTP, WebDAV, OpenStack Swift, and Amazon S3, available for Mac OS X and Windows (as of version 4.0) licensed under the GPL. Cyberduck is written in Java and C# using the Cocoa user interface framework on OS X and Windows Forms on Windows. It supports FTP/TLS (FTP secured over SSL/TLS), using AUTH TLS as well as directory synchronization. The user interacts with the user interface, including file transfer by drag and drop and notifications via Growl. It is also able to open some files in external text editors.