Immutable backup refers to a data backup that cannot be altered or deleted after writing — even with administrator privileges. The concept specifically protects against ransomware that attempts to encrypt or delete backups alongside production data. Technically, immutability is implemented via WORM storage (Write Once Read Many), Object Lock at cloud providers, or air-gapped media. In your ISMS, define which backup generations are stored immutably and how long the protection period lasts. Regularly test whether your immutable backups can actually be restored.