Immutable backups for Essential Eight Maturity Level 3

Maturity Level 3 backups must resist modification and deletion during retention, even by backup administrators. This page explains what that requires, what evidence an assessor expects, and who does which part of the work.

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What immutable backups for Essential Eight Maturity Level 3 require

Your contract, customer or regulator now expects Essential Eight Maturity Level 3 backups, and the core demand is specific: backup administrator accounts must be prevented from modifying and deleting backups during their retention period (ISM-1708). The organization owns this control and the evidence that proves it. An implementation partner can prepare the systems, configuration and evidence, but only an independent assessor issues the assessment conclusion. Start by confirming in writing which backup administrator accounts exist today, because any account that can alter retention settings or purge a repository will fail this test regardless of how the backup product is labeled.

Mapping that requirement to a design means separating three roles in your backup estate: unprivileged users, privileged administrators and backup administrators. Under the Essential Eight baselines, unprivileged accounts and privileged accounts other than backup administrators must not access their own backups (ISM-1813, ISM-1706), and privileged accounts must not reach backups belonging to other accounts (ISM-1705). In a self-hosted design, this usually means a separate backup server identity, repository credentials held outside the production estate, and a retention lock enforced by the storage or tool rather than by an operator's good intentions. In Bareos, retention periods for jobs, files and volumes determine how long records stay in the catalog and how long a volume is guaranteed not to be reused. In restic, restic check with --read-data-subset can confirm that stored data has not been modified, which supports the integrity evidence an assessor will ask for.

Cost and effort depend on the size of the estate, data volumes, retention periods, the number of systems in scope and how much existing tooling you can reuse. One-time costs usually include design, separation of roles, immutable storage configuration and the first restore test; recurring costs include monitoring, periodic restore exercises, storage growth and support. Ask any partner for a written scope that names the systems in scope, the retention policy, the restore test plan and the evidence deliverables, and ask which parts they expect your team to perform. Restoration of data, applications and settings to a common point in time must be tested as part of disaster recovery exercises (ISM-1515), so budget for that test as a recurring activity, not a one-off.

Who is paid for what in an immutable backup project

This table compares each cost component of an immutable backup project for Essential Eight Maturity Level 3, showing who is paid, whether the cost is one-time or recurring, and what drives it. Read it when preparing a quote request, because the largest recurring costs often sit with the organization rather than the implementation partner. No prices or durations are stated, so ask each party to price its share against the written scope.

Cost componentWho is paid or who carries itOne-time or recurringWhat drives the cost
Internal backup and security staffThe organization's own staff, who own the backup policy, approve retention and hold the backup administrator roleRecurringNumber of backup administrators, separation of duties, time spent on retention reviews and access recertification
Storage and infrastructureThe organization, in its own cloud accounts, data centers or air-gapped networkRecurring, with a one-time setupData volume, retention period, growth rate, replication to a second location and the immutable storage design chosen
Backup software licencesOpen-source tools such as Bareos or restic carry no licence fee, so the cost is hosting and operationOne-time setup, then recurring operationHosting platform, patching of the backup server, monitoring and the staff time needed to run it
Implementation partner scoping and buildThe implementation partner, under a written, fixed-scope agreementOne-timeNumber of systems in scope, number of backup sources, integration with existing identity and storage, and documentation depth
Integrity checks and restore testingShared: staff run the tests, and the partner may design the proceduresRecurringFrequency of checks, amount of data read back, and how often a common point in time restore must be tested under ISM-1515
Retention and access auditsThe organization, with its internal audit or compliance teamRecurringNumber of accounts that can reach backups, number of retention policies, and how often access rights are reviewed
Independent assessorThe independent auditor or assessor, who reviews the evidence. DeployOpen does not issue assessment reportsUsually one-time per assessment, repeated on reassessmentScope of evidence, number of backup systems tested, and whether the assessor needs to observe a restore in person
Growth and ongoing supportThe organization, or a support retainer with the partner if one is agreedRecurringStorage growth, new systems added to backup scope, upgrades of backup tools, and changes to retention rules

What makes ML3 backup work cost more or take longer

Immutable backups at Maturity Level 3 are shaped less by the backup software than by how many systems, retention periods, administrator identities and restore tests the organization must carry. Each driver below changes the design, the recurring effort and the evidence an assessor will ask for, so scope them in writing before choosing tools.

Systems and data classes in scope

Every system whose data must be recoverable adds a backup job, a catalog entry, a restore path and a retention rule. Group systems by data class and recovery requirement rather than by server, so that the number of distinct policies stays manageable. Ask the implementation partner to list each system, its data classification and its owner before pricing begins. Confirm which systems are cloud-hosted SaaS, since those often need a separate export or vendor-native retention feature. A longer inventory means more jobs to build, more restore tests to run and more evidence to collect each cycle.

Retention periods and storage volume

Retention determines how long backups must remain unmodifiable, which in turn sets storage growth and the size of the immutable window. A longer period multiplies stored data across full and incremental copies. In Bareos, job, file and volume retention periods control how long catalog records remain and how long a volume is guaranteed not to be reused, so these values must be set deliberately for each data class. Ask for a written retention schedule, a storage growth estimate under that schedule and a plan for when retention expires and media is recycled.

Backup administrator accounts and separation

The ML3 rule that backup administrator accounts cannot modify or delete backups during retention is the most design-sensitive requirement. Each backup administrator account is a separate identity with its own controls, and the number of such accounts drives the work of provisioning, reviewing and monitoring them. Keep backup administrators apart from production identities and from the domain administrators who manage the systems being backed up. Ask whether the backup platform can enforce retention independently of the administrator who holds the keys, since a shared administrator model usually needs extra compensating work.

Existing backup tooling and immutability features

Costs fall sharply when existing tools already support write-once storage, object lock or enforced retention, and rise when they do not. Before procurement, document each current backup product, its version, its storage targets and whether retention can be changed by an administrator. Confirm these features with the vendor and in a test environment, not from marketing material. Open-source options such as restic can write to repositories that are then placed behind storage-level protection, but the storage layer must still enforce the immutability that the design depends on. Migrating from an incompatible tool adds a parallel run and a longer evidence period.

Offsite and air-gapped copies

A copy held offsite or in an air-gapped location protects against a compromise that reaches the primary backup environment, but it duplicates storage, transfer and access controls. Each additional location needs its own encryption keys, network path, retention settings and restore test. Decide early whether the offsite copy is a second immutable repository, a tape or disconnected medium, or a cloud bucket in a separate account. Ask the partner to show how the offsite copy is synchronized, how failed transfers are detected and who can reach it. Every extra location also adds a line to the evidence pack.

Existing restore evidence

Assessors look for proof that data, applications and settings can be restored to a common point in time as part of disaster recovery exercises, as the requirement in ISM-1515 expects. Organizations that already run documented restore tests with recorded timings, results and corrective actions need less preparation than those that have only backup job logs. Where no restore history exists, plan for a first full exercise, a written test plan and a remediation cycle before the assessment window. Ask for the last restore test report, the scope it covered and the open findings from it.

Which self-hosted tools can meet the retention and integrity rules

Bareos is an open-source backup system licensed under AGPL-3.0-only. Its retention settings are the main tool-level control over how long backup records and data stay in place. Job, file and volume retention periods work on different objects. Job retention controls how long the job record stays in the catalog, file retention controls how long file entries stay in the catalog, and volume retention controls how long a volume is guaranteed not to be reused after its last write. Those settings govern when Bareos may recycle media. They do not, by themselves, stop an administrator from editing the catalog, deleting volumes or changing the configuration.

  • Catalog records: Job and file retention determine how long Bareos keeps records of what was backed up, which affects restore planning and audit trails.
  • Volume guarantee: Volume retention sets the period in which Bareos will not overwrite a volume, but it is a scheduling rule rather than a write lock.
  • Admin separation: Backup administrator accounts can still change retention settings or remove volumes unless the organization restricts them outside Bareos.
  • Storage write protection: Immutability has to come from the storage layer, such as object lock or write-once media, configured and verified by the organization.
  • Restore testing: Retention settings do not prove that backups can be restored to a common point in time, so restore exercises remain a separate task.

Confirm with the Bareos project and your own storage platform which features prevent modification and deletion during retention, and document how those features are enforced for backup administrator accounts.

How an ML3 backup design moves from scope to assessor evidence

This sequence turns the Essential Eight regular backups strategy into a backup design that an assessor can test, starting from what data exists and ending with restore results and retained records. Each phase produces a written artifact that the next phase depends on, so gaps surface early rather than during the assessment.

  1. Scope systems and data classes

    The organization owns this step and its internal asset owners do most of the work, with an implementation partner helping to map systems, backup sources and data flows. List every system that holds data the organization must be able to recover, including Microsoft 365 or other SaaS data where it applies. Group the data into classes by business impact and retention need. The step is done when every in-scope system is assigned to a class and the owner has signed off the inventory.

  2. Define retention periods per data class

    The organization sets retention periods for each data class, and its risk, legal and records owners approve them. Retention drives how long a backup is protected from modification or deletion, so each period must be written down with the reason behind it. Confirm the retention rules in your backup tool match the approved periods, using the tool's documentation on job, file and volume retention. The output is a retention schedule that the backup configuration can be checked against.

  3. Separate backup administrator identities

    Create dedicated backup administrator accounts that are distinct from daily administrator accounts and from unprivileged user accounts. Restrict which accounts can read, write or delete backups, so that no ordinary privileged account can reach another account's backups. The identity team and the security team review the access matrix and confirm that backup administrator accounts are themselves protected by strong authentication. The output is an approved access matrix that names every account allowed to touch backup data.

  4. Configure write protection and retention enforcement

    Configure the backup repository so that backup administrators cannot modify or delete backups during their retention period. This is usually done through the tool's volume or repository controls, object-lock style features on the storage layer, or a separate storage account with its own deletion rules. Test the control by attempting a deletion with a backup administrator account before retention expires, and record the result. The output is a tested configuration with the failed deletion attempt documented.

  5. Set up encryption and integrity checks

    Enable encryption for backup data at rest and in transit, and store key material separately from the backup repository. Schedule integrity checks that verify repository structure and, on a regular cycle, read stored data back to confirm it has not changed. Confirm with the tool's documentation which checks are structural and which read data, because only the data-reading checks confirm the stored content. The output is an encryption configuration, a key custody record and a schedule of integrity check runs with their logs.

  6. Schedule restore tests within disaster recovery exercises

    Plan restore tests as part of disaster recovery exercises, so that data, applications and settings are restored to a common point in time. The recovery team runs the exercise, and the system owners confirm that restored systems behave correctly against the expected point in time. Record the recovery point used, the time taken and any gaps found. The output is a dated exercise report with the restore results and the actions raised from them.

  7. Collect evidence for the assessor

    Assemble the artifacts from each earlier phase into one evidence pack that maps to the ISM controls for backups at the target maturity level. The pack should include the scope inventory, retention schedule, access matrix, enforcement test results, integrity logs and exercise reports. Ask the independent assessor which samples they will select and confirm the pack covers them. The step is done when the assessor has accepted the evidence list, and only the assessor can state whether the design meets the level.

What must be in place before an assessor reviews the backups

An assessor testing regular backups at Maturity Level 3 looks for evidence that each control operates, not only that it is configured. The six items below are the records most likely to be requested, and each one can be checked internally before the assessor arrives.

Documented retention schedule

Write down which data sets are backed up, how long each backup set is kept, and when a volume or snapshot may be expired. An assessor compares this schedule with the configured retention settings, so any mismatch becomes a finding. Check that the schedule names an owner, a review date and the backup tool that enforces each period. For Bareos, the job, file and volume retention values should each match the document. Keep the schedule under change control so the version in force on the test date is clear.

Backup administrator account separation

Produce an account inventory that lists every account able to reach the backup platform, its role and its privilege level. The ML3 rules separate backup administrators from other privileged accounts and from unprivileged users, and the assessor will check that no one account holds more than its role allows. Confirm that backup administrators cannot change or delete backups during retention, and that other privileged accounts cannot read another account's backups. Review the inventory against the directory each quarter and record who approved each entry.

Evidence of retention enforcement

A configured retention period is not enough on its own. The assessor will ask for evidence that the platform blocks a deletion attempt before the period ends, such as a test where a backup administrator tries to remove a protected backup and the attempt is refused and logged. Store the result of that test with the date, the account used and the platform version. Repeat the test after upgrades, since a change to the backup software can alter how locks behave.

Integrity check logs

Keep dated logs from integrity checks run on the repository. For restic, the check command verifies the structure of snapshots, trees and pack files, and the --read-data or --read-data-subset options read pack files back to confirm the stored data is unmodified. Record the command used, the scope of the read (full or subset), the result and who reviewed any error. An assessor expects a regular cadence, so show the schedule and the logs that fulfill it, including any failed run and how it was resolved.

Encryption configuration

Document where encryption keys are created, who holds them, and where recovery copies are stored. For restic, all repository data is encrypted with AES-256 in counter mode and authenticated with a Poly1305-AES message authentication code, so the assessor will focus on key handling rather than the algorithm. Confirm that keys are not stored only on the backup server, that the key holders are separate from backup administrators where practical, and that a key recovery test has been run and recorded.

Recent restore test with a common point in time

ISM-1515 requires restoration of data, applications and settings to a common point in time to be tested as part of disaster recovery exercises. Prepare a dated record showing that the restore used one recovery point for all systems in scope, the time taken, the systems involved and the checks that confirmed the restored data was usable. Note any gap between the common point and the last good backup. Confirm the test was performed by someone other than the backup administrator who set up the job.

Who owns the controls and who prepares the evidence

The organization owns its backup controls, its retention decisions and its restore capability. Under the Essential Eight, the mitigation strategy is implemented and operated by the organization, so no outside party can accept responsibility for whether backups are immutable during retention. Internal staff also decide who holds backup administrator accounts and how those accounts are separated from everyday administration.

  • Retention decisions: Business and legal owners set how long each data set is kept and approve any change that shortens or extends retention.
  • Access design: The security team decides which accounts are unprivileged, privileged and backup administrator, and confirms that no account can both write and delete its own backups.
  • Restore exercises: Operations staff schedule and run restore tests, including the common point in time restoration tested in disaster recovery exercises (ISM-1515).
  • Evidence ownership: The organization keeps the configuration exports, job logs, access reviews and restore test records that an assessor will request.
  • Sign-off: A named executive accepts residual risk where a control cannot be met as written.

Before engaging anyone, confirm internally who holds each of these decisions and who will sign the restore test records.

Questions buyers ask about Essential Eight ML3 backups

Can an open-source backup tool alone meet Essential Eight ML3 for backups?

No tool alone meets a maturity level, because the Essential Eight is a set of mitigations that the organization implements and evidences, not a certification a product can earn. Open-source tools such as Bareos and restic can supply retention controls, encryption, integrity checks and restore logs, but the organization still has to configure account separation, storage access rules and restore exercises so that backup administrators cannot modify or delete backups during retention. Confirm each ISM control in the ML3 profile against the design, then have an independent assessor test it.

What does the work need from our internal staff?

Internal staff remain responsible for the decisions that assessors will ask about: which systems are in backup scope, how long each data set must be kept, who holds backup administrator rights, and who approves a restore. Your team also needs to provide access to identity systems, storage accounts and change records so the implementation can be verified. An implementation partner can prepare the configuration and documentation, but the organization owns the retention policy and its operation after handover.

How do we evidence that restore testing actually works?

Evidence is a dated record of a restore to a common point in time, covering data, applications and settings, performed as part of a disaster recovery exercise. ISM-1515 asks for this restoration to be tested, so keep the exercise plan, the job logs, the restored system's validation results and the signed-off outcome. Ask your auditor which records they will sample, and store them where backup administrators cannot alter them.

Can DeployOpen run an ML3 backup project under a fixed scope?

Yes, most DeployOpen engagements are fixed-scope, with the scope, deliverables and timelines agreed in writing before work starts. A typical written scope for this work would name the systems in backup, the self-hosted tools deployed in your environment, the account and storage design, the restore test plan and the evidence pack. DeployOpen prepares systems and evidence for your assessment, but it does not issue audit reports or certifications, and it does not publish a price list. A quote follows from the written scope.

What changes for Microsoft 365 data under the shared responsibility model?

Microsoft 365 data remains your responsibility, even though the service is hosted by Microsoft. Microsoft's Essential Eight backup guidance states that customers always retain responsibility for their information and still need to protect it, with in-place retention policies and labels as one way to do so. Your design must decide whether those retention features, a separate self-hosted backup, or both will meet the ML3 access rules, and you must show that backup administrators cannot delete held data.

How do we confirm which ML3 requirements apply to our backups?

Start from ASD's machine-readable profiles, which list the ISM controls for Essential Eight ML1, ML2 and ML3 in OSCAL form. Download the ISM OSCAL release and open the ML3 baseline, then filter the regular backup controls, including ISM-1813, ISM-1706, ISM-1708, ISM-1705, ISM-1707 and ISM-1515. Read each control's text in the catalog, map it to a configuration or procedure you own, and record the gap. Confirm the release version your assessor uses, because profiles are updated.

Which backup rules are hardest to satisfy with self-hosted tools?

The hardest rule is preventing backup administrators from modifying or deleting backups during retention, because the same people who run the tool often hold the credentials that can remove data. Self-hosted designs usually rely on separate storage accounts, write-once or object-lock features where the storage platform offers them, and retention settings enforced outside the administrator's direct control. Confirm that your chosen storage backend supports these features before selecting the tool, and test that a privileged deletion attempt fails.

How DeployOpen scopes and prepares ML3 backup evidence

DeployOpen plans, builds and documents self-hosted backup platforms inside your own infrastructure, so the access controls, retention settings and restore records that Maturity Level 3 asks about are in place before an assessor arrives. The organization owns its controls, and an independent assessor reaches the conclusion about maturity.

How we scope

  • We agree a written, fixed scope listing the backup tools, storage targets and systems covered for Immutable backups for Essential Eight Maturity Level 3.
  • The scope names which account types exist, including backup administrator accounts, and which accounts must be denied access to backups.
  • Scope documents the retention periods, restore test cadence and evidence owners that the written scope commits to delivering.

How we build

  • We configure backup software so retention periods stop modification and deletion of backups by backup administrator accounts during retention.
  • We set up integrity checks and encryption, then document how each check runs and where its output is stored for review.
  • We build and document restore tests for a common point in time, so disaster recovery exercises produce evidence an assessor can follow.

How we hand over

  • You receive configuration records, access role mappings and retention settings mapped to the relevant ISM controls for your assessor.
  • Handover includes runbooks for restore testing and a log of past test results, with the named owner for each recurring task.
  • We walk your team through the evidence set so your staff can reproduce checks and explain them to an independent assessor.

How we support

  • Ongoing support, upgrades and continuous testing are available as a retainer after handover, covering backup software updates and repeated restore tests.
  • A retainer can keep retention settings, access reviews and integrity check outputs current as your environment changes.
  • Confirm with us in the quote which post-handover tasks sit inside the retainer and which remain with your internal team.

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