Why Your Cloud Migration Security Plan Must Start with Shared Responsibility

84% of organizations say managing cloud spend is their top challenge, but that's not the scariest number in the Flexera 2025 report. The scariest one is hiding in plain sight: under the shared responsibility model, you—not your cloud provider—are on the hook for your data, your access management, and your compliance. Yet most migration plans I see treat security as an afterthought, a checklist item to be bolted on after the cutover. That's backwards.

So here's the question that should be driving every cloud migration decision you make: Who actually owns security in your migration, and what does that mean for how you plan it? The answer isn't a vendor slogan. It's a concrete division of labor that, if you get it wrong, will turn your 'successful' migration into a compliance nightmare.

The Shared Responsibility Model Is Not a Slogan

Both AWS and Azure are explicit about this. AWS is responsible for protecting the infrastructure that runs its services—the hardware, software, networking, and facilities. You're responsible for what you put on top of that infrastructure, and that responsibility shifts depending on the service you choose. With IaaS like EC2, you manage the guest OS, security patches, installed software, and the security group firewall. With abstracted services like S3 or DynamoDB, AWS handles the infrastructure and platform, but you still own your data, encryption choices, asset classification, and IAM permissions (AWS Shared Responsibility). Azure's model is even more direct: you always retain responsibility for your data—classification, protection, encryption decisions, compliance—plus your endpoints, accounts, and access management (Microsoft Learn, security).

That's not a legal disclaimer. It's the operational reality of every migration. When you lift-and-shift a legacy app to EC2, you're not handing security to AWS—you're just moving your old unpatched Windows Server into a new building. The building is safer, but your guest OS is still your problem. And when you replatform to a managed database, you gain some relief, but you still own the data inside it.

Here's where most teams trip up: they assume that because the cloud is 'more secure,' their security burden shrinks. It doesn't. It shifts. In some ways it grows, because now you have to understand a new shared boundary, configure IAM correctly, and monitor a sprawling environment. That's why the first step in any migration security plan isn't choosing a firewall—it's mapping out who owns what for every single workload you're moving.

Security Can't Be an Afterthought in Your Migration Wave

Most migration frameworks—whether AWS's three-phase assess, mobilize, migrate, or Azure's four-stage assess, migrate, optimize, monitor—put assessment first. But too often, 'assessment' means inventorying servers and mapping dependencies, not assessing security ownership. You can't wave that away. If you skip the security assessment, you'll discover mid-migration that a legacy app has hardcoded credentials or that your compliance officer requires data residency that your chosen region doesn't provide. Then you're stuck replatforming something you planned to rehost, or worse, you're retaining an app on-premises that you thought you'd retired.

Retain is a legitimate strategy—AWS lists data residency compliance as a valid reason to keep an application on-premises (AWS Prescriptive Guidance). But you need to know that before you start, not after you've already migrated half your portfolio. The same goes for refactoring for compliance reasons. AWS notes that refactoring can mean splitting a database so some tables stay on premises to meet compliance requirements (AWS Prescriptive Guidance). That's a deliberate architectural decision, not an accident. You can't make it if you haven't mapped your data's compliance obligations to the cloud provider's regions and services.

Concrete example: Suppose you're migrating a healthcare application that stores patient records. Under the shared responsibility model, you own the compliance of that data, regardless of whether it sits in an RDS instance or an S3 bucket. If your compliance framework requires data to stay within a certain geographic boundary, you need to pick a region that satisfies that—and you need to ensure your IAM policies prevent accidental cross-region replication. That's not a 'cloud feature.' That's your job.

Your Security Team Must Be Part of the Migration Team, Not a Reviewer

In too many organizations, security is a gate at the end of the process. The migration team builds, then throws it over the wall for a security review. That's a recipe for disaster. Security has to be embedded in every phase, from assessment through optimization. That means your security engineers are in the room when you decide which of the 7 Rs to apply to each application (AWS Prescriptive Guidance). They're the ones who can tell you that rehosting that legacy app will require you to patch the OS yourself, while replatforming to a managed service would shift that burden to the provider. They're the ones who can flag that a particular workload has data residency requirements that force a retain strategy.

This isn't just about avoiding breaches. It's about avoiding the cost of rework. Flexera found that security and compliance is the top scaling challenge for 53% of organizations running AI workloads (Flexera 2026). If you're just starting to migrate AI workloads, you need security in the design phase, not as an afterthought.

And it's not just about the technical side. The shared responsibility model has a human dimension. Your developers need to understand how to configure IAM roles correctly, your operations team needs to know how to respond to a security incident in the cloud, and your compliance officer needs to know what evidence they can request from the provider. That's why the AWS Cloud Adoption Framework groups capabilities into six perspectives, one of which is Security—not as a separate silo, but as part of the overall business and governance structure (AWS Cloud Adoption Framework).

Practical Steps to Operationalize Shared Responsibility in Your Migration

So what do you actually do? Here's a starting point:

• Map each workload to a specific service model (IaaS, PaaS, SaaS) and document which party owns security for each layer—OS, runtime, data, access. Use the AWS or Azure shared responsibility models as your template.
• Do a security assessment in the Assess phase, not just a technical one. Identify data classification, compliance requirements, and encryption needs for every application you plan to move.
• Involve security in the strategy selection for each workload. If you choose rehost, you're taking on more security ops. If you choose replatform, you're shifting some of that to the provider, but you still own the data.
• Define clear IAM policies before you migrate, not after. Test them with a small pilot workload first.

One more thing: don't forget the data transfer itself. The Azure Cloud Adoption Framework lists four data migration paths—ExpressRoute, VPN, Azure Data Box, and the public internet—and explicitly notes that the public internet is the least secure (Microsoft CAF). If you're moving sensitive data, that's not a decision you make casually. You choose the path based on the data's classification, not just the cost.

Bottom Line

Your single best move is to conduct a shared responsibility audit for every workload before you choose a migration strategy. That means documenting who owns security for each layer, identifying compliance requirements, and involving your security team in the strategy selection. Do that, and you'll avoid the most common migration security failures—and you'll be able to sleep at night after the cutover.

Sources

• AWS Shared Responsibility - https://aws.amazon.com/compliance/shared-responsibility-model/
• Microsoft Learn (security) - https://learn.microsoft.com/en-us/azure/security/fundamentals/shared-responsibility
• AWS Prescriptive Guidance - https://docs.aws.amazon.com/prescriptive-guidance/latest/large-migration-guide/migration-strategies.html
• Flexera 2026 - https://www.flexera.com/blog/finops/the-new-era-of-cloud-what-2026-data-tells-us-about-spend-scale-and-strategy/
• Microsoft CAF - https://learn.microsoft.com/en-us/azure/cloud-adoption-framework/get-started/migrate