Azure workloads often need to call another service without a human signing in. Microsoft Entra provides several workload-identity patterns for that problem, and the terminology can be confusing because managed identities are implemented as special service principals even though administrators operate them differently from ordinary application service principals.

The current SC-300 objectives cover application and workload access, making the distinction operationally important. The choice affects credential management, lifecycle ownership, portability, authorization, and how easily a workload can move beyond Azure-hosted resources. A useful SC-300 workload-identity practice question distinguishes an Azure-hosted application that can use managed identity from a portable application that may need a carefully managed service principal.

Start with the identity-object relationship

An application registration represents an application definition in Microsoft Entra. A service principal is the tenant-local identity through which that application operates in a directory. Service principals can receive permissions, roles, and consent and can authenticate as a workload.

A managed identity is also represented by a service principal, but Azure manages the credential material and exposes the identity to supported Azure resources. Administrators do not copy a client secret or private key into the application. The resource requests a token from the managed-identity endpoint or a supported SDK credential chain, and Azure handles the underlying credential lifecycle.

That relationship explains why managed identities appear in enterprise applications and why both patterns ultimately obtain Microsoft Entra tokens. The operational difference is not whether a service principal exists; it is who creates, protects, rotates, and exposes the credential needed to use that identity.

System-assigned identity follows one resource

A system-assigned managed identity has a one-to-one lifecycle relationship with the Azure resource on which it is enabled. If the resource is deleted, the identity is deleted with it. That behavior is convenient when the identity exists only to serve that resource.

A virtual machine, Function App, App Service, or another supported resource can use the identity to request a token for a downstream service that supports Microsoft Entra authentication. Authorization remains separate: enabling the identity does not automatically grant access to Storage, Key Vault, SQL, or another target. The administrator still assigns the appropriate role or application permission.

System-assigned identity reduces orphaned credential risk because there is no independent secret to rotate and no separate identity to remember after the resource disappears. Its trade-off is that the identity cannot simply be moved to another resource because the lifecycle is intentionally coupled.

User-assigned identity is reusable and independent

A user-assigned managed identity is an Azure resource with a lifecycle independent of any one compute resource. It can be created in advance and assigned to multiple supported Azure resources.

That makes it useful when several workloads should present the same identity to a downstream service, or when the identity must survive replacement of the compute resource. For example, a deployment can rebuild an application host while preserving the same user-assigned identity and its downstream authorization.

The flexibility also introduces governance work. Because the identity outlives individual resources, administrators must track where it is attached, what permissions it holds, and whether it is still needed. A reusable identity should not become a convenient shared super-account. Scope permissions narrowly and document the workloads that legitimately depend on it.

Ordinary service principals provide broader portability

A standard application service principal is often appropriate when the workload is not hosted on a supported Azure resource, when software must operate across environments, or when the application identity is part of a broader application-registration design.

Historically, service principals frequently authenticated with client secrets or certificates. Those credentials create lifecycle obligations: secure storage, rotation, expiration monitoring, and incident response if material is exposed. Modern workload identity federation can reduce that burden by allowing trusted external identities to exchange their existing credentials for Microsoft Entra tokens without storing a long-lived secret in the workload.

The right comparison is therefore not “managed identity secure, service principal insecure.” A well-designed service principal can use strong federation and narrow permissions. The important difference is whether Azure can manage the workload identity directly or whether the application needs an identity model that extends beyond that managed boundary.

Federated identity credentials are an important alternative when a workload cannot use a managed identity but can present a trusted token from another platform. CI/CD systems and Kubernetes workloads can often exchange that external token for a Microsoft Entra token without storing a long-lived Entra client secret. The trust still needs narrow issuer, subject, and audience rules; an overly broad federation subject can let more workloads impersonate the service principal than the design intended.

Authentication and authorization stay separate

Managed identity solves how a workload authenticates to Microsoft Entra; it does not decide what the workload may do. The same applies to a service principal.

After authentication, authorization comes from the target resource and permission model. Azure RBAC may grant a managed identity access to a Key Vault, storage account, or subscription resource. Microsoft Graph or another API may require application permissions and administrative consent. A custom API may expose application roles or scopes that the service principal can receive.

This separation matters in troubleshooting. If token acquisition fails, investigate identity availability, audience, federation, credential configuration, or the managed-identity endpoint. If token acquisition succeeds but the API returns forbidden, investigate authorization. The existence of a valid token does not prove the workload has the required role.

Also identify which authorization system the target uses. Azure resource access commonly relies on Azure RBAC at a management- or data-plane scope. Microsoft Graph and many SaaS APIs use delegated or application permissions on the service principal. A custom API may expose application roles. Granting an Azure role will not satisfy a missing Microsoft Graph application permission, and consenting to a Graph permission will not grant access to a storage account.

Token handling still deserves attention

A managed identity removes the need to store a secret, but applications still need to request tokens for the correct resource and handle failures correctly. Azure SDKs can simplify token acquisition, while lower-level implementations may call the managed-identity endpoint directly.

For ordinary service principals, applications also obtain OAuth 2.0 access tokens for the intended audience. A token issued for one resource should not be treated as a universal credential for every API.

Operational logs should distinguish identity failures from downstream service failures. Track which client or managed identity made the request, what target resource was requested, and whether the failure occurred before or after token issuance. That evidence is much safer than responding to errors by broadening permissions or creating another secret.

Prefer credential elimination when Azure can provide it

If an Azure-hosted workload can use a managed identity to reach a downstream service that supports Microsoft Entra authentication, that is usually simpler than creating and distributing a client secret. The application no longer needs a secret store entry merely to prove its own identity, and administrators cannot accidentally paste the managed credential into code because they do not have access to it.

This is a security improvement, but it does not eliminate the need for least privilege. A managed identity with Owner at subscription scope can be more dangerous than a tightly scoped service principal with a certificate. Credential hygiene and authorization scope are separate dimensions of the design.

Use managed identity to remove avoidable credential management, then apply the same role-minimization, monitoring, and lifecycle discipline you would apply to any other nonhuman identity.

Managed identities are not a universal replacement for every application credential. They are designed for supported Azure resources and Microsoft Entra token-based access. A legacy dependency that accepts only a static API key will not suddenly understand managed identity. In that design, the workload can still use managed identity to retrieve the external secret from a protected store, reducing exposure even when the final dependency requires a non-Entra credential.

Govern service identities as first-class accounts

Workload identities deserve owners, purpose statements, permission reviews, and lifecycle controls. Service principals can accumulate application permissions over time, and user-assigned managed identities can remain after the workloads that originally needed them have changed.

Document who owns the workload, which resources it should access, which environments use the identity, and how permissions are reviewed. Monitor changes to role assignments and application permissions. Where long-lived secrets or certificates still exist, track expiry and rotation; where workload identity federation is used, govern the trust relationship and subject mapping.

Microsoft Entra treats managed identities, service principals, and other workload identities as security principals. Operational governance should do the same rather than considering them invisible plumbing behind an application.

Choose by hosting, lifecycle, and trust boundary

A useful decision sequence is:

1. Is the workload running on an Azure resource that supports managed identity?
2. Does the downstream service support Microsoft Entra authentication?
3. Should the identity live and die with one resource? Use system-assigned identity.
4. Should the identity survive resource replacement or be reused across several supported resources? Consider user-assigned identity.
5. Does the workload operate outside Azure or require a broader application identity? Use an application service principal, preferably with federation where it fits.
6. In every case, assign only the permissions the workload needs and review them over time.

The strongest design is the one that minimizes secret management without confusing identity lifecycle with authorization. Managed identities simplify Azure-hosted authentication; service principals remain the broader application identity construct. Both still require deliberate permission scope and governance.