Earlier quoted context omitted.
> You're only safe if you have offsite backups with drives that didn't mount to your computer recently. Or if your backup solution is—from the perspective of the computer being backed up—an append-only store. Like a box of tapes, or Tarsnap using restricted keys, or Arq pointed at a versioned S3 bucket, or a NAS exposing an iSCSI target backed by an LVM thin pool LV with automatic daily snapshots. Sadly, as far as I…
I've contemplated setting up a small home server with write-only shares for backups, but ended up not doing it because of the cost and time. If there were a reasonably priced off-the-shelf product for this, I'd recommend it to everyone I know. On the other hand, if there were an off-the-shelf product for this, it would probably have unpatched security issues two weeks after you bought it, and if it were in common use…
I'm waiting for the NAS "appliance" that's actually running CoreOS, and then just relies on running the :latest tag of some popular Docker image (and not a fork of it that they'll forget to update eventually; the original upstream image), plus a bit of config-file glue generated into a shared volume from a web-UI service running in another container. (Bonus points if the second container is only started up, for an hour at a time, when you press a button on the NAS, WPS-style.)
Such a design is essentially the same as shipping the device's OS as "firmware" with auto-updates, but for the fact that the vendor themselves isn't anywhere within the path of creating or distributing those updates. Which, in the end, makes all the difference.