Immutable backup
Also known as: immutable backups, immutable storage, WORM backup, write-once backup
An immutable backup is a copy of data that, once written, can't be altered, encrypted, or deleted until a set retention period expires, not even by someone with administrator access.
It protects against ransomware gangs, which now routinely hunt for and destroy backups before encrypting systems. While immutable backups are one of the strongest ransomware defences, they only help if you've tested that restores actually work.
Immutable backup, explained at three levels
At home
An immutable backup is a copy of your files locked in a safe that nobody can open or change for a set time, so even if a hacker breaks in, the backup stays intact.
At work
Immutable backups give organizations a guaranteed clean copy to recover from after a ransomware attack.
At home
Some cloud backup services keep older versions of your files that can't be deleted for a period. If ransomware scrambles your files, you can restore a version from before the attack.
At work
Immutability can be enforced by object lock in cloud storage, hardened Linux repositories, WORM tape, or backup appliances. 'Compliance mode' locks prevent even administrators from deleting data early. Pair with separate credentials and MFA for backup systems.
At home
A backup you've never tried restoring is just a hope. In my opinion, testing a restore once a year is the most valuable hour you can spend on your digital life.
At work
Design with retention long enough to cover attackers' dwell time, isolated backup administration, monitoring for backup deletion attempts, regular restore tests with defined recovery time objectives, and cost planning for retained storage.
Real-world examples
At home
Nadia's laptop was hit by ransomware. Her cloud backup kept locked previous versions of her files, so she restored everything from the day before the attack.
At work
Jerome is IT director at a Sudbury municipality. After attackers deleted the town's backups in a neighbouring community's incident, he moved to immutable cloud backups with a 30-day lock and quarterly restore tests.
How it compares
| feature | Immutable backup | Regular backup |
|---|---|---|
| Can be deleted by admins | No, until retention ends | Yes |
| Ransomware resistance | High | Low if reachable |
| Storage cost | Higher, data must be kept | Lower |
| Flexibility | Less | More |
Regular backups can be destroyed by attackers. Immutable ones can't.
For analysts
I'd ask one question about backups: could an attacker with admin access delete them? If the answer is yes, immutable or offline copies should be the next investment.
History & origin
Write-once storage has existed for decades, from optical discs to 'WORM' (write once, read many) tape used for regulatory record keeping. As ransomware gangs began deliberately deleting backups in the late 2010s, immutability became a key feature. Amazon's S3 Object Lock, launched in 2018, let cloud storage enforce retention periods, and backup vendors such as Veeam, Rubrik, and Cohesity added immutable repositories and hardened storage. Cyber insurers increasingly asked whether organizations had immutable or offline backups when setting premiums. In my view, immutable backups are the closest thing to a ransomware insurance policy that actually pays out.