Due to not verifying whether an user has been granted access to add external storages an authenticated user could even mount external storage (e.g. SMB/FTP/etc.) without permission.
We reviewed the access-control of the files_external application and ensured that permissions are checked properly for every action.
Additionally our next major release ownCloud 7 will give administrators the possibility to define which external storage type an user is allowed to mount.
The ownCloud team thanks the following people for their research and responsible disclosure of the above advisory: