Backup security has changed. For years, many organizations treated backups as a storage and scheduling problem: pick a target, set a retention policy, and ensure the jobs complete. Ransomware and credential theft turned that assumption into a liability. Attackers don’t just encrypt production data; they target backup servers, backup catalogs, and backup repositories so you can’t restore without paying.
Immutable backups are one of the most effective countermeasures because they change the attacker’s math. If a backup copy is truly immutable—meaning it cannot be modified or deleted until a defined retention period expires—then even an attacker with administrative access to a backup server has a much harder time destroying recovery options. Immutability is not a single product feature; it is a property you design and verify across storage, identity, backup software, and operational process.
This guide focuses on practical design for IT administrators and system engineers. It explains what immutability actually means in real systems, how immutable storage differs from air gapping, where teams accidentally undermine immutability with convenience shortcuts, and how to validate that your “immutable” backups would survive a real incident.
What “immutable backups” actually means (and what it doesn’t)
In backup security, “immutable” means write-once-read-many (WORM) behavior for backup data: once written, data cannot be altered or deleted until retention expires. Immutability is typically enforced by the storage layer (for example, object storage with retention locks) rather than by the backup application alone.
It’s important to distinguish immutability from several related concepts:
Immutability is not just “read-only permissions.” Filesystem ACLs or share permissions can be changed by a sufficiently privileged account. If an attacker compromises a domain admin or backup admin, discretionary permissions are rarely a strong boundary.
Immutability is not the same as offline copies. Offline media can be immutable, but you can also have immutable data that is online and accessible. Online immutability is valuable because it supports frequent backups and operational restores, but it must be designed to withstand credential compromise.
Immutability is not a guarantee that backups are usable. You can have perfectly immutable backups that are encrypted with lost keys, captured from already-corrupted data, or missing critical configuration. Backup security needs immutability plus restore integrity.
A working definition that’s useful for engineering is: immutable backups are backup copies stored on media or services that enforce retention at the storage layer such that deletion or modification is blocked—even for administrators—until the retention period ends.
That last clause (“even for administrators”) is where the real work is. Many environments implement “immutable” in name only, because there is an administrator path that can disable the setting or delete data anyway.
Why immutability matters in modern incidents
Traditional backup risk assumed hardware failure, accidental deletion, or a localized disaster. Ransomware incidents are different: they are adversarial, intentional, and often include weeks of stealthy access. Attackers commonly perform reconnaissance to locate backup infrastructure, then use stolen credentials to delete backup jobs, remove restore points, or wipe repositories.
Immutability directly addresses two common failure modes:
First, it reduces the blast radius of compromised administrative credentials. If an attacker gains access to your backup server, they may be able to stop jobs and delete catalogs, but they should not be able to delete the immutable data copy itself.
Second, it reduces time pressure during incident response. When teams know they have an undeletable recovery point, they can prioritize containment and forensic clarity instead of racing against an attacker who is destroying restore options.
Immutability also helps with insider threats. Not every destructive event is an external attacker; sometimes it’s a disgruntled employee or accidental action by an over-privileged admin. Retention enforcement provides a backstop against both.
That said, immutability is not a standalone ransomware solution. If attackers can encrypt production and also wait out a short retention window, or if backups are immutable but captured after data was already encrypted, you still lose. This is why immutability must be paired with retention design, backup frequency, and detection.
Threat model: how attackers actually target backups
Designing immutable backups is easier when you start with an explicit threat model rather than a product checklist. In most ransomware playbooks, backup destruction happens after privilege escalation and lateral movement.
A realistic attacker path looks like this:
They compromise an endpoint or user, then obtain higher privileges via credential dumping, token theft, misconfigurations, or exploitation.
They locate backup infrastructure using AD queries, DNS, vCenter inventory, cloud subscriptions, documentation shares, or common hostnames like “veeam,” “backup,” or “commvault.”
They target the backup control plane: backup servers, management consoles, API keys, and service accounts. If they can disable jobs, delete restore points, or erase catalogs, you may not even know which recovery points exist.
They target the backup data plane: NAS shares, SAN LUNs, object storage buckets, dedupe appliances, and snapshot repositories. If they can delete or encrypt stored backup data, recovery becomes impossible.
Finally, they attempt to block restore operations. This might include deleting hypervisor snapshots, corrupting DNS/PKI, changing firewall rules, or encrypting staging servers.
Immutable backups are mainly about protecting the data plane. But if the control plane is compromised, you may still face operational disruption. That’s why a complete design includes secured management access, audit logging, and a restore path that works even if the primary backup server is unavailable.
Immutability vs air gap: where each fits
Air gapping means isolating a backup copy from the production environment so it is not reachable through normal network paths. Historically that meant tape vaulted offsite. Modern “logical air gaps” can be achieved with separate accounts, separate credentials, restricted network paths, and one-way replication.
Immutability and air gapping solve overlapping but distinct problems:
Immutability prevents deletion/modification for a time window. It is excellent for defending against credential compromise because retention is enforced by storage policy.
Air gapping reduces reachability. It is excellent for defending against widespread malware and misconfigurations because the backup target is not accessible from the compromised environment.
In practice, most organizations benefit from both. A common pattern is to keep a local immutable copy for fast restores and also maintain a separate, logically isolated copy (often in cloud object storage or tape) for disaster recovery.
When teams choose only one, they often choose immutability because it is operationally simpler than building a true air gap. The risk is that “immutable” targets still require network access for backups and restores, and if identity boundaries are weak, an attacker can sometimes change retention settings or delete the entire container.
A useful way to frame it is: immutability is a control; an air gap is an architectural separation. Controls can fail when identity is compromised; separations can fail when networking and trust boundaries are too permissive. Combining them reduces correlated failure.
Common technologies that provide immutable backup storage
Immutability can be implemented in several ways. The details matter because each option has different administrative bypass paths.
Object storage with WORM/retention locks
Object storage is the most common modern approach. The key feature is retention enforcement at the object level: once an object is written with a retention timestamp, deletion is blocked until that timestamp is reached. Some platforms also provide a “legal hold” capability that prevents deletion regardless of retention expiry until explicitly removed.
In AWS S3, this is implemented via S3 Object Lock, which operates in either Governance mode or Compliance mode. In Governance mode, privileged users with the correct permission can bypass retention. In Compliance mode, retention cannot be bypassed—even by the root account—until expiry. This distinction is critical when you’re designing against administrative credential compromise.
Other providers implement similar capabilities under different names. The architectural pattern is consistent: immutable objects live in a bucket/container with versioning and retention policies that are enforced by the service.
The practical advantage of object storage is durability, geographic options, and the ability to keep long retention without managing on-prem media. The downside is that you must treat the object store account as part of your security boundary. If the same IAM identities that run production also manage the backup bucket, you’ve built a soft target.
Backup appliances and hardened repositories
Some backup appliances and software-defined storage platforms implement immutability by controlling deletion at the filesystem or object layer, sometimes combined with hardened Linux repositories. The strongest implementations prevent remote deletion via standard admin protocols, require local console access for destructive actions, and log immutability policy changes.
However, the details vary widely. Some solutions rely on application logic (“the backup server won’t delete data if it is flagged immutable”) which can fail if the backup server itself is compromised. Others enforce immutability at a storage layer that is not trivially bypassed by the backup server.
When evaluating an appliance approach, focus on whether the backup administrator can disable immutability remotely and whether the repository is on a general-purpose OS that shares identity with the rest of the environment. If you can SSH in using domain credentials and delete files, that’s not immutability.
Snapshot-based immutability (array or hypervisor)
Storage snapshots (for example, on SAN/NAS arrays) and hypervisor snapshots provide point-in-time recovery. Some platforms offer immutable snapshots where deletion is restricted by policy.
Snapshots are valuable because they can provide very fast restores and frequent recovery points. They are also commonly targeted by attackers: if the attacker compromises array management or vCenter, they may delete snapshots or replicate encrypted data.
Snapshots are best treated as a layer, not the only layer. Use them for short retention and rapid operational recovery, but still maintain immutable backups on separate storage with longer retention.
Tape and offline media
Tape remains a strong air-gapped option. A tape cartridge vaulted offsite is effectively offline and cannot be deleted by an attacker over the network. For long-term retention and regulatory needs, tape can be cost-effective.
The operational tradeoffs are obvious: longer restore times, handling logistics, and the need for periodic restore tests. Tape also doesn’t solve the “last hour” recovery problem unless you have frequent exports.
A modern design often uses disk/object immutability for short-to-medium retention and tape for an additional isolated copy or archival tier.
Designing immutable backups using a layered model
A robust design is easier when you separate the system into layers: data sources, backup control plane, backup data plane, identity and keys, and operational process. Each layer can fail independently, and your goal is to avoid a single compromise taking out all recovery options.
Start by deciding what you are protecting and how quickly you must restore it. Recovery time objective (RTO) is the acceptable downtime, and recovery point objective (RPO) is the acceptable data loss window. These objectives drive how frequently you take backups, where you store them, and how long you retain them.
Then map those objectives to at least two independent recovery mechanisms. For many organizations, that means a fast local restore mechanism and a resilient immutable copy that’s designed to survive a hostile actor.
Where immutable copies fit in 3-2-1-1-0
Many engineers use the “3-2-1” backup rule as a baseline: three copies of data, on two different media types, with one copy offsite. An evolution of that guidance is 3-2-1-1-0: add one immutable or offline copy (“1”), and zero errors verified (“0”) through automated verification.
Immutability directly supports the extra “1.” But if the immutable copy is the only offsite copy, you may still be exposed to account compromise or region-wide outages. Conversely, if you have an offsite copy that is not immutable, an attacker may still delete it.
A practical approach is to treat immutability as a property of at least one offsite copy, and ideally also one local copy if your platform supports it without weakening security boundaries.
Define retention in terms of attacker dwell time
Retention is often set by compliance (“keep 30 days”) or convenience. In ransomware scenarios, retention needs to consider dwell time—the period attackers may be present before detonating encryption.
If your immutable retention is only seven days and attackers were inside for two weeks before encryption, all immutable restore points might already contain encrypted or exfiltrated data. This is why longer retention and multiple restore points matter.
A reasonable baseline for many environments is to keep at least 14–30 days of immutable restore points, with longer retention for critical systems and monthly/quarterly copies for rollback beyond typical dwell times. Your storage costs and business requirements will shape the final numbers, but the threat model should be explicit.
Identity and access: preventing the “backup admin = bucket admin” trap
Most failed immutability deployments fail at identity boundaries. Storage-layer retention is strong only if attackers cannot change the retention policy or delete the container holding the objects.
A common anti-pattern is to let the same backup service account that writes backups also have permissions to change bucket policies, disable object lock, or delete the bucket. Another is to manage backup storage using the same cloud account used for production workloads.
A better model is to separate duties:
The backup writer identity can write objects and list/read what it needs for restores, but it cannot delete objects, shorten retention, or change object lock settings.
A separate security/admin identity can change retention policies, but it is tightly controlled, MFA-protected, and ideally not used for day-to-day operations.
An audit/logging system receives immutable logs of bucket policy changes, object lock configuration changes, and access events.
In cloud environments, consider a dedicated backup account/subscription that production admins do not administer routinely. This creates a logical air gap: compromising a production subscription does not automatically grant control of the backup repository.
When you design IAM, assume credential theft. Favor short-lived credentials, strong MFA for human admins, and explicit deny policies for destructive actions.
Implementing immutable backups in AWS S3 (Object Lock)
AWS S3 Object Lock is a common target for immutable backups because many backup products can write directly to S3-compatible object storage. The key engineering tasks are enabling the right bucket configuration at creation time, choosing the correct lock mode, and ensuring IAM permissions do not allow bypass.
Object Lock requires bucket versioning and must be enabled when the bucket is created. You cannot enable Object Lock on an existing bucket retroactively. This impacts migrations: you may need a new bucket and a plan to transition backups.
Governance vs Compliance mode
Governance mode allows authorized users to bypass retention with special permissions. Compliance mode does not allow bypass, even for the root account. If your threat model includes compromised cloud admin credentials, Compliance mode is usually the safer choice.
The tradeoff is operational flexibility. In Compliance mode, you cannot delete objects early to fix a mistake or reduce costs. This is why retention policy design and environment separation matter.
Creating an Object Lock bucket with AWS CLI
The following example creates a bucket with Object Lock enabled. Adjust region, bucket name, and any required settings (such as SSE-KMS) to match your environment.
# Create bucket (region-specific syntax may vary)
aws s3api create-bucket \
--bucket mycompany-backup-immutable-prod \
--region us-east-1
# Enable versioning
aws s3api put-bucket-versioning \
--bucket mycompany-backup-immutable-prod \
--versioning-configuration Status=Enabled
# Enable Object Lock configuration (must be enabled at creation time for the bucket)
aws s3api put-object-lock-configuration \
--bucket mycompany-backup-immutable-prod \
--object-lock-configuration "ObjectLockEnabled=Enabled,Rule={DefaultRetention={Mode=COMPLIANCE,Days=30}}"
In practice, you should create the bucket with Object Lock enabled from the start (for example, with CloudFormation/Terraform or the console), because S3 enforces that Object Lock must be enabled at creation. Treat the CLI snippet as illustrative of the configuration you’re aiming for.
IAM: allow writes, deny deletions
For a backup writer role, focus on least privilege: allow PutObject, AbortMultipartUpload (if your product needs it), and reads required for restore verification. Explicitly deny DeleteObject, DeleteObjectVersion, and configuration changes.
Because IAM policy design is nuanced and environment-specific, the key principle is to separate these capabilities. Even if you use Compliance mode, you still want IAM denials to reduce accidental changes and limit blast radius.
Logging and detection
Immutability is strengthened by visibility. Enable CloudTrail data events for S3 (cost-aware, but valuable for sensitive buckets), and ensure bucket policy changes, Object Lock changes, and access events are forwarded to a SIEM. The operational goal is to detect attempted deletions, unexpected reads, and policy changes quickly.
Implementing immutable backups in Azure (immutability policies)
Azure provides immutability features primarily for Blob storage. Immutability policies can enforce time-based retention and legal hold behavior on containers. The exact feature set and constraints depend on the account type and configuration.
For backup use cases, the engineering concerns mirror S3: retention enforcement must be applied at the storage layer, and identities that write backups must not be able to reduce retention or remove immutability.
Creating a storage account and container
A typical deployment uses a dedicated storage account for immutable backups, with private endpoints or restricted network access and Azure AD-based control.
bash
# Create a resource group
az group create -n rg-backup-immutable -l eastus
# Create a storage account
az storage account create \
-g rg-backup-immutable \
-n mystorageimmutable01 \
-l eastus \
--sku Standard_RAGRS \
--kind StorageV2
# Create a blob container
az storage container create \
--account-name mystorageimmutable01 \
-n backups-immutable
From here, you would apply an immutability policy to the container using supported Azure commands for your environment and feature availability. In production, automate this via IaC and ensure policy changes require privileged roles with MFA and approvals.
Network controls
Do not treat immutability as a replacement for network segmentation. Restrict storage access using private endpoints, firewall rules, and “trusted services” exceptions only where required. The fewer networks that can reach the storage endpoint, the fewer paths an attacker has to attempt credential abuse.
On-prem immutable repositories: what to validate
On-prem implementations vary more than cloud services, which makes validation more important. You will encounter terms like “immutable repository,” “hardened repository,” or “WORM mode,” but the operational meaning differs.
When assessing an on-prem immutable target, validate these properties:
The immutability policy is enforced below the backup application (for example, by the filesystem or storage platform), not just by the backup software’s catalog.
Administrative bypass requires a different trust boundary, such as local console access or a separate management plane. If a domain admin can remotely log in and delete the data, your threat model isn’t addressed.
Retention cannot be reduced retroactively for existing data without a privileged process that is auditable.
All policy changes are logged to an independent log system.
Because there is no universal CLI for all on-prem solutions, your validation steps should focus on reproducible tests: attempt deletion as the backup service account, attempt deletion as a backup admin, and attempt retention reduction. Document the expected failures and ensure they happen.
Encryption, key management, and immutability
Immutability prevents deletion; it does not prevent reading. Backups often contain the most sensitive data in your environment, including database contents, file shares, and possibly credentials embedded in system state.
Encrypt backup data in transit and at rest. In cloud object storage, server-side encryption is common. For some organizations, customer-managed keys (CMKs) are required to meet compliance. But CMKs add a new failure mode: key loss or malicious key deletion can render immutable backups unrecoverable.
This is a subtle but common issue: you can make data undeletable, but if an attacker can delete or disable the encryption keys, the effect is similar to deletion. Therefore, key management must be part of the same security boundary as immutability.
Practical guidance:
Use separate key management scopes for backup repositories, ideally in a separate account/subscription.
Restrict key deletion and key policy changes with MFA and approvals.
Log all key operations and alert on unusual access.
Document and test key recovery procedures.
When you discuss “backup security,” include keys in your inventory. Immutable data encrypted with inaccessible keys is not a recoverable backup.
Backup control plane hardening: protecting the orchestrator
Even with immutable storage, attackers can disrupt recovery by attacking the backup control plane: the backup server, database/catalog, and management console.
Hardening steps typically include:
Separate admin identities for backup management. Avoid using domain admin accounts for backup operations. Use just-enough administration and time-bound privilege elevation where possible.
Multi-factor authentication for management interfaces, especially if they are reachable from user networks.
Network segmentation: backup servers should not be directly reachable from general workstation subnets. If an attacker compromises a workstation, you do not want direct RDP/SSH paths to backup infrastructure.
Patch management: backup servers often run complex software stacks and plugins. Treat them as critical infrastructure.
Immutable logging: forward logs from backup servers to a log system the attacker cannot easily tamper with.
Catalog protection: if your backup product relies on a database, ensure that database is backed up and that those backups are also protected (ideally immutably). A surprising number of recovery failures happen because the backup data exists but the catalog describing it is lost.
As you harden the control plane, keep the operational reality in mind: during an incident, engineers need a way to run restores. The design goal is not “no one can access backups,” but “access is controlled, audited, and resilient to common compromise paths.”
Restore integrity: immutability is worthless without verified restores
Immutability ensures the bits remain, not that they are the right bits. Restore integrity requires ongoing verification.
A practical restore validation program includes:
Periodic restore tests of representative workloads (VMs, databases, file shares) to an isolated network. Do not restrict testing to small files.
Verification of application-consistent backups where needed, especially for databases. Crash-consistent backups may be acceptable for some workloads and unacceptable for others.
Validation that the restore process works without relying on production identity systems that might be down (for example, if AD is compromised).
Measurement of actual RTO and RPO against objectives.
It’s helpful to tie this back to 3-2-1-1-0: “0 errors” implies you have an automated or scheduled way to prove backups are usable. Many organizations discover in their first serious test that backup jobs were “green” but the restored system fails to boot or the database logs are missing.
Real-world scenario 1: Ransomware hits the domain, but immutable object storage saves recovery
Consider a mid-sized enterprise with VMware vSphere, Windows file servers, and a central backup server joined to Active Directory. The backup target is a NAS share. Backups run nightly and are retained for 14 days.
An attacker compromises a helpdesk account, escalates to domain admin, and deploys ransomware widely. Before triggering encryption, they locate the backup server and use domain admin to access the NAS share. They delete backup files and snapshots. When encryption begins, the organization has no usable backups.
After the incident, the organization redesigns backup storage to use immutable object storage with retention set to 30 days and places the object storage in a separate cloud account. The backup server uses a write-only role that cannot delete objects or change retention. During a later incident, the attacker again compromises AD and the backup server. They stop jobs and delete the local catalog, but they cannot delete the immutable objects in the separate account. The organization rebuilds a backup server in a clean environment, reconnects to the object repository, and restores critical VMs.
The key lesson is not just “object storage is good,” but that separation of identity and retention enforcement blocked the attacker’s most common tactic: deleting backups using stolen admin credentials.
Real-world scenario 2: Immutable retention too short to beat dwell time
A SaaS company implements immutable backups with seven-day retention to control costs. They back up databases every four hours and keep daily copies immutably for a week.
Attackers gain access through a vulnerable public-facing service and remain undetected for three weeks, slowly exfiltrating data and planting persistence. When they finally trigger ransomware, the team restores from the most recent immutable backups—only to find that several restore points already contain malicious changes and encrypted files.
The company ultimately needs to roll back to a clean state, but the immutable retention window does not reach far enough. They rebuild from older archives and partial exports, losing significant operational data.
Afterward, they adjust retention strategy: keep 30 days of immutable backups for tier-0 systems (identity, billing, core databases), and maintain monthly immutable copies for a year. They also add detection for unusual access patterns to the backup repository.
The lesson is that immutability does not replace retention design. The retention window must be selected with dwell time and business risk in mind, not only with storage cost in mind.
Real-world scenario 3: Encryption keys become the weak point
A regulated financial organization stores immutable backups in cloud object storage with server-side encryption using customer-managed keys. They correctly configure Compliance-mode retention and restrict bucket deletion. However, the key management system is administered by the same cloud admin group that manages production.
An attacker compromises an admin workstation, obtains cloud admin privileges, and cannot delete the immutable backup objects. Instead, they disable or delete the encryption key used for the backup bucket. The backup data remains intact but becomes unreadable. Recovery fails.
The remediation is to treat keys as part of the backup security boundary: move backup keys to a separate administrative domain, restrict key deletion with privileged access workflows, and create alerting on key state changes. They also test restores that include key access as part of the process.
The lesson is that “undeletable” is not the same as “recoverable.” Key management must be designed so that an attacker cannot achieve denial of recovery by attacking the cryptographic dependencies.
Operational patterns that make immutable backups workable
Immutable backups can introduce friction: you can’t just delete old chains to reclaim space, and misconfigurations can have long-lived cost impact. Operational design makes the difference between a resilient system and an expensive, brittle one.
Use tiered repositories for cost and performance
A common pattern is a fast local repository for short retention and operational restores, combined with an immutable object repository for longer retention. The local tier handles frequent restore requests efficiently (for example, a user deleted a file), while the object tier is your ransomware-resilient safety net.
This also allows you to optimize costs. Object storage is economical for long retention, but frequent small restores may be slower and may incur egress costs depending on your provider.
Plan for catalog and metadata recovery
If your backup product uses a catalog, you need a plan for rebuilding it. Some products can re-scan repositories; others require database restores. In either case, ensure that catalog backups are included in your immutable strategy or that you can rebuild the catalog from the repository without trusting compromised infrastructure.
A practical approach is to maintain infrastructure-as-code for the backup server build, store configuration backups immutably, and document the minimum secrets required to reconnect to repositories.
Separate restore credentials from backup credentials
Backup jobs often run under service accounts with broad read access to production systems. If those accounts are compromised, an attacker can potentially access production data and backup infrastructure.
Where possible, separate identities: one set of credentials for reading production data during backups, another for writing to the immutable repository, and a controlled process for restore operations that requires additional approvals or privileged access.
This reduces the chance that a single credential theft grants both “read everything” and “destroy backups.”
Maintain clean restore environments
Restores during incidents should be performed into a clean, isolated environment to avoid re-infection. This means having network segments and staging infrastructure ready, including DNS/DHCP strategies and credential management.
If your restore process assumes the compromised environment is still trustworthy (for example, restoring a domain controller into the same network with the same compromised credentials), you may reintroduce the attacker.
This topic connects back to why control plane hardening and identity separation matter: during a real incident you may need to rebuild your identity systems from backups, which requires a restore path that does not depend on the compromised identity system itself.
Governance and change control for immutability policies
Immutability is powerful enough that governance matters. A misconfigured retention policy in Compliance mode can lock you into excessive storage growth, and a misconfigured short retention can provide false confidence.
Treat immutability settings as security controls subject to change control:
Document retention requirements per data class (tier-0 identity systems, core databases, file servers, endpoint backups).
Review retention periodically against business requirements and threat intelligence.
Use infrastructure-as-code where possible, with peer review and approvals.
Ensure that policy changes generate alerts and are visible to security monitoring.
From an operational perspective, this is similar to firewall rule governance: you want controlled change, auditable history, and rapid detection of unauthorized modifications.
Monitoring and alerting: what you should watch
Monitoring for immutable backup environments focuses on two categories: backup health and security signals.
Backup health monitoring includes job success, backup duration anomalies, repository capacity trends, and restore verification results. These are operational signals that indicate whether you can meet RTO/RPO.
Security monitoring includes:
Unexpected access to backup repositories (especially reads at unusual times or from unusual IP ranges).
Policy changes to retention, object lock, bucket/container policies, and encryption settings.
Attempts to delete objects, delete versions, or delete buckets/containers.
Changes to IAM roles and keys associated with backup operations.
A practical way to integrate this is to forward cloud audit logs (CloudTrail in AWS, Activity Logs in Azure) and backup server logs to a SIEM, then build alerts tied to your incident response playbooks. The goal is to detect a backup-targeting phase early, before encryption begins.
Validating immutability: prove it with controlled failure tests
Because “immutable” is often misunderstood, teams should validate with controlled tests. The test should simulate the capabilities an attacker would likely have during an incident.
Start with the identities that perform backups. Attempt to delete backup objects or files using those credentials. The expected result is failure due to retention enforcement and/or explicit deny policies.
Then test with backup administrator credentials. If a backup admin can delete immutable data, you need to decide if that matches your threat model. In many organizations, backup admin compromise is a realistic risk; designs that rely on “backup admins won’t be compromised” are fragile.
Finally, test with cloud/storage admin credentials. In cloud services, verify whether retention can be bypassed (Governance mode) or not (Compliance mode). If you rely on Governance mode, ensure the bypass permission is tightly controlled and monitored.
For object storage, you can validate by attempting deletion of a specific object version and checking the error responses, then confirming the object remains accessible for reads.
The important point is that validation should be repeatable and documented. This turns “we think we’re immutable” into “we have evidence that our retention and permissions block deletion.”
Integrating immutable backups into incident response
Immutable backups change how you respond to incidents because they provide a stable recovery anchor. However, incident response still needs to account for compromised control plane components.
Document a recovery runbook that assumes:
The domain is compromised.
The backup server may be compromised or destroyed.
Some credentials are untrustworthy.
The immutable repository is intact.
This runbook should describe how to provision a clean restore environment, how to authenticate to the immutable repository using break-glass credentials, and how to perform restores without reusing compromised secrets.
Break-glass access is a controlled emergency access method, typically using an identity that is not used day-to-day, protected by strong MFA and stored in a secure vault. The break-glass identity should be able to read the immutable repository and perform necessary administrative actions, but its use should generate high-priority alerts.
By connecting incident response planning to immutable backup design, you avoid a common gap: the backups survive, but no one can access them quickly under emergency conditions.
Cost and capacity planning for immutable retention
Immutability changes storage lifecycle management because you cannot delete data early. That makes capacity planning more important.
Estimate growth based on:
Backup frequency and change rate (incremental size trends).
Retention windows per workload class.
Compression and deduplication behavior (especially if your backup format supports it before object storage ingestion).
Replication factors (cross-region replication, secondary copies).
Also consider provider billing models. Object storage costs include capacity, requests, and potentially retrieval/egress. Some tiers have minimum storage durations. If you use cross-region durability, factor in replication costs.
A practical operational pattern is to set retention based on risk for tier-0 and tier-1 systems and use shorter retention for less critical data, while still maintaining at least one immutable copy for the critical set. This avoids a one-size-fits-all policy that either becomes too expensive or too weak.
Common design pitfalls to avoid
Several recurring mistakes undermine otherwise good immutable backup designs.
One pitfall is creating an immutable bucket but allowing the backup server to manage it fully. If the backup server’s role can change bucket policy, disable versioning, or remove object lock configuration (where possible), immutability can be bypassed.
Another pitfall is relying on a single immutable copy without any architectural separation. If the same identity provider, same admin group, and same network paths control both production and backup storage, then a single compromise can still cascade.
A third pitfall is ignoring the catalog and restore workflow. Teams often assume that if the data exists in object storage, restore is straightforward. In reality, you may need a clean backup server, the correct plugins, credentials, network access, and keys.
Finally, teams sometimes treat immutability as an excuse to reduce other controls, such as patching backup servers or monitoring access. Immutability is strong, but it does not remove the need for defense-in-depth.
Putting it together: reference architectures that work
A reference architecture is not a single diagram; it’s a set of design choices that remain valid across different products.
Hybrid enterprise pattern (local + immutable cloud)
In a typical hybrid design, backups land first on a local repository for fast restores, then a secondary copy is sent to immutable object storage in a separate cloud account/subscription. The backup server writes to cloud storage using a restricted role that cannot delete or change retention.
This pattern provides operational efficiency (local restores) while ensuring that even a full compromise of the local environment does not automatically permit deletion of the offsite immutable copy.
Cloud-first pattern (immutable primary repository)
Some organizations back up directly to object storage, using immutability as the primary repository. This can work well when workloads are already in cloud and network bandwidth is sufficient.
The key is to keep identity boundaries: a dedicated backup account, restricted writer roles, and centralized logging. For performance, consider caching or local staging if your backup software supports it.
Regulated pattern (immutable + offline)
For high assurance, pair immutable object storage with an offline/air-gapped copy, such as tape or a logically isolated secondary object store with one-way replication and separate credentials. This addresses scenarios where cloud credentials are compromised or where administrative control is coerced.
This pattern is operationally heavier, but for certain regulatory environments it aligns with requirements for long retention and strong separation.
Each pattern still depends on the same fundamentals discussed earlier: enforce retention at the storage layer, separate identities, protect keys, harden the control plane, and verify restores.