Glossary
What is: Immutable backup
An immutable backup is a backup copy that cannot be changed or deleted for a period — not by ransomware, not by an administrator — so a clean copy always exists to restore from.
The big mistake ransomware exploits: backups on the same network get encrypted together with the data. An immutable (or offline) copy breaks that chain.
It is combined with the 3-2-1 rule and regular restore tests — because a backup that has never been tested is not a guarantee.
FAQ
Why isn't a simple backup enough?
Because if it is on the same network, ransomware encrypts it too. You need a copy that is isolated (offline/immutable) and tested.