A flaw was found in libblockdev. The bd_fs_ntfs_set_label() and bd_fs_vfat_set_label() functions pass caller-supplied label values as bare positional arguments to the ntfslabel and fatlabel utilities without end-of-options separators or input validation for values beginning with a dash. This allows a label starting with '-' to be interpreted as a command-line flag by the spawned tool, potentially bypassing safety checks or causing unintended operations. This is reachable by unprivileged local users through the udisks2 Filesystem.SetLabel() D-Bus method without administrator authentication.
There is no practical mitigation that does not affect normal desktop functionality. Restricting the polkit policy for org.freedesktop.udisks2.modify-device to require administrator authentication (changing allow_active from "yes" to "auth_admin") would block the attack surface but would also require a password for all legitimate filesystem label and UUID operations on user-owned devices.