System and method for secure multi-tenancy in datadomain operating system (DDOS), a purpose built backup appliance (PBBA) operating system

A first request is received from a first user to access a first file system management objects (FSMOs) of a first tenant unit (TU), the first TU being one of a plurality of TUs. The first FSMO is one of a plurality of FSMOs, each FSMO containing namespace information of a particular tenant. In response to the first request, it is determined whether the first user is authorized to access the first FSMO, including identifying TUs that are associated with the first user, determining whether the first TU is included in the TUs, and if so, determining whether the first FSMO is included in FSMOs associated with the first TU. In response to determining that the first FSMO is included in a set of one or more FSMOs that are associated with the first TU, the first user to access the first FSMO is allowed.