Build vs. Buy Authentication: Custom Build or Identity Server?

Building authentication logic internally appears straightforward initially: databases contain user records, password hashing functions exist in core libraries, and issuing basic JWTs requires minimal code. However, the hidden challenge lies in the security and protocol overhead that follows: key rotation, secure session management, token refresh revocation, edge-case vulnerability patching, and compliance with evolving specifications like PKCE and DPoP. The alternative is adopting an identity framework SDK to handle standard protocols out of the box. Because both options run on your infrastructure, the choice centers on whether your team wants to take on continuous security protocol engineering or offload it to a maintained framework.

A custom authentication build suits specific single-application use cases. However, full protocol implementations require managing a large surface area of security requirements that emerge after launching to production.

This page lays out that trade honestly. A custom build is genuinely the right answer for some narrow cases, and pretending otherwise would not help anyone. But the surface area of "real" auth is much larger than the first sign-in flow suggests, and most of that surface only becomes visible after you have shipped.

Custom Authentication Implementations Explained

Custom authentication spans three primary development approaches:

• Hand-rolled authentication: cookie-based sign-in against your own user store, no standard protocol involved. Fine for a single application with no third-party clients, until you need to share identity across services.
• Assembling libraries: wiring together OSS building blocks (token libraries, crypto primitives, an ASP.NET Core authentication handler) into something that speaks OpenID Connect, OAuth, or SAML, with you authoring the endpoints and gluing the pieces.
• Implementing the protocol yourself: writing your own authorization, token, and related endpoints to the OAuth, OpenID Connect, and SAML specifications.

Writing custom authorization and token endpoints requires replicating protocol logic supplied by existing frameworks. OpenIddict provides lower-level protocol building blocks, whereas full framework SDKs handle protocol endpoints automatically.

Security and Maintenance Requirements of Custom Auth

Production authentication platforms must support complex security routines beyond simple sign-in handling:

• Token validation and issuance done to spec, including signature verification, audience and issuer checks, expiry, and clock-skew handling.
• Signing key management, including rotation without downtime and publishing keys so clients can validate tokens.
• Session management, including server-side sessions, sign-out across applications, and revocation.
• Modern protocol features such as PKCE, PAR, DPoP, and CIBA, which move from "nice to have" to "required" as your security and regulatory needs grow.
• Refresh token handling, including rotation, reuse detection, and sane lifetimes.
• Standards conformance, so that third-party clients, APIs, and libraries interoperate with your server without special-casing.
• Ongoing security response, including tracking spec changes and patching promptly when a vulnerability is disclosed in a protocol or a dependency.

Implementing these features internally requires ongoing development and security reviews to prevent vulnerabilities.

Custom Build vs. Self-Hosted Framework Comparison

Building custom authentication provides total initial control, but requires complete responsibility for ongoing security patches and protocol updates. Using a framework retains code and database ownership while offloading core protocol engineering to a dedicated software library.

Long-Term Maintenance Overhead of DIY Auth

Long-term protocol maintenance represents the primary hidden expense of custom builds. Security specifications evolve, new vulnerability classes emerge, dependencies require CVE patches, and client applications demand modern protocol support (such as DPoP or PAR). Custom implementations require internal engineering teams to track, develop, and test fixes for these updates continuously. When a critical CVE is disclosed in a core cryptographic dependency, or an integration requires supporting complex modern specs, engineers must pause planned product work to analyze evolving RFCs and build compliant updates.

Adopting an identity framework shifts security and specification maintenance to specialized maintainers, turning protocol updates into standard library upgrades.

When to Build Custom Authentication

Building custom authentication is suitable in specific scenarios:

• Truly narrow scope: a single application, your own users only, no third-party clients, no need to share identity across services, and no plausible near-term path to any of those. Simple cookie authentication may be all you need, and a full identity server would be overkill.
• Learning and exploration: building an OAuth, OpenID Connect, or SAML server to understand the protocols is genuinely valuable, as long as the result does not quietly become production infrastructure.
• A constraint no product satisfies: a genuinely unusual requirement that no framework supports and cannot be extended to support, which is likely to be very rare given customization and extensibility in other products.

When applications remain within these boundaries, simple custom cookie authentication is a practical choice.

When to Use a Self-Hosted Identity Framework

Growing software products benefit from self-hosted identity frameworks. Frameworks deliver full infrastructure control, local data storage, and source code inspection without forcing development teams to implement and maintain security protocol specifications from scratch.

Framework SDKs like Duende IdentityServer provide token endpoints, security routines, key rotation, and advanced specification support out of the box, allowing engineering effort to focus on custom business logic and user interface design.

Responsibilities When Using an Identity Framework

Framework SDKs implement protocol engines and token generation while relying on your application for three specific integrations: user interface, data persistence, and user identity management. Standard templates and providers simplify these integration points.

• User interface: IdentityServer delegates UI rendering to your application. You can build custom workflows or start from ready-made IdentityServer templates for login, logout, and consent screens.
• Data storage: Client definitions, resources, and operational grants store in any database using existing Entity Framework Core providers or custom repository interfaces.
• Identity management: User data connects via standard store integrations like Duende User Management, ASP.NET Identity, or custom IProfileService implementations.

Framework SDKs handle protocol logic, token security, and key management while leaving user experience and data architecture under your direct control.

Choosing Between Framework Assembly and Configuration

Frameworks offer varying levels of protocol abstraction. OpenIddict requires manual endpoint construction using library building blocks. Duende IdentityServer supplies pre-built protocol endpoints alongside commercial support.

Where This Connects

• Deciding whether to run identity yourself at all? See self-host vs. SaaS identity.
• Weighing a self-built host against a ready-made containerized server? See build your own vs. a containerized identity server.
• Choosing between assembling and configuring a framework? See OpenIddict vs. Duende IdentityServer.

Further Reading

• Duende IdentityServer product page
• The big picture (docs)
• Quickstarts
• OpenIddict vs. Duende IdentityServer