There is only one file type that uses the .KDBX extension:
KeePass Password Database
File created by KeePass Password Safe, a free password manager for Windows; stores an encrypted database of passwords that can only be viewed using a master password set by the user; used to securely store personal login credentials for Windows, email accounts, FTP sites, e-commerce sites, and other purposes.
The KDBX format was introduced with version 2 of KeePass. The previous version used the .KDB format. KeePass 2 can import older KDB files as well as export to the KDB format.
NOTE: A Mac and Linux edition of KeePass exists called KeePassX. However, this edition does not yet support the KDBX format.
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Updated: October 9, 2014