What is the principle of least privilege and how do you apply it?

The principle of least privilege says every user, service and process gets only the permissions it needs for its job, and only for as long as it needs them. If an account is compromised or buggy, the damage stays small.

How to apply it in a backend:

• Database: the application connects with a role that can SELECT, INSERT and UPDATE its own tables, not DROP or superuser. Use a separate role for migrations and a read-only role for reporting.
• Cloud and services: give each service its own identity and an IAM policy listing exact actions and resources, not *.
• Users: start with no access and grant roles by job; review them regularly and remove them when people change teams.
• Tokens and keys: narrow scopes, short lifetimes, no shared all-powerful credentials.
• Runtime: run containers as non-root, drop capabilities and mount file systems read-only where possible.
• Time: prefer temporary elevated access with approval over permanent admin rights.

Takeaway: start from deny, add the minimum, and revisit it often, because permissions only tend to grow.