Full definition
Common implementations: S3/MinIO Object Lock (Compliance mode blocks even root), Veeam Hardened Repository on Linux with chattr +i, Azure Blob Storage Immutable Vault, LTO tapes in WORM mode, ZFS snapshots with hold. The goal is to ensure that even if ransomware compromises the backup server, immutable data remains recoverable. Immutable backup is the most effective control against corporate ransomware in 2026. Groups like BlackCat, LockBit and Conti routinely target Veeam B&R and ESXi with elevated local permission. Without immutability, the attacker encrypts the backup along with production. Recommendation: mandatory immutability on at least one copy of the 3-2-1-1 rule.
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