Isolation and least privilege
Beyond who may enter, what matters is how far each access reaches.
Separating applications, data and roles so every identity receives only the access it needs and failures have a limited reach.
One identity for each role
Sharing one identity across different tasks broadens what each one can do. Separating roles makes it possible to grant only the required operations and revoke access without dismantling the entire system.
Least privilege also improves investigation because an action remains associated with the component or role that should perform it.
Isolation limits the reach of a failure
Separate applications should not share sessions, temporary areas or data without a reason. Execution boundaries reduce the chance that a local fault crosses into another project.
Isolation contains part of the impact. Updates, validation, authorization and recovery answer for the rest.
Data and secrets follow the same separation
An application account accesses only its own data domain. Administrative operations, public reads and maintenance carry different risks and do not need the same reach.
Credentials, keys and tokens stay outside public code and HTML. Inventory, restricted access and planned replacement complete their lifecycle.