An issue was discovered in the cantata-mounter D-Bus service in Cantata through 2.3.1. A regular user can inject additional mount options such as file_mode= by manipulating (for example) the domain parameter of the samba URL.
https://github.com/CDrummond/cantata/commit/afc4f8315d3e96574925fb530a7004cc9e6ce3d3