The best APIs for secure user authentication

This is Part 1 of a two-part series on the best APIs for secure user authentication. This part covers the foundational concepts of secure auth APIs, token architectures, and a detailed evaluation of six leading providers. Part 2 focuses on implementation code walkthroughs and advanced security topics like account takeover prevention and TCO.

Credential abuse shows up somewhere in the attack chain of 39% of all data breaches — more than any other single technique (Verizon DBIR, 2026⁠). Even after software-vulnerability exploitation overtook it as the most common initial entry point (31%), stolen and abused credentials remain the connective tissue of modern breaches — and when compromised credentials are the way in, the average breach runs $4.67 million (IBM Cost of a Data Breach, 2025⁠). The best APIs for secure user authentication are Clerk, Auth0, Firebase Auth, Supabase Auth, WorkOS, and AWS Cognito — each suited to different use cases. A provider that supports passkeys eliminates an entire class of phishing attacks that passwords can't survive, and a provider that validates sessions on every request aligns with zero-trust principles.

This guide evaluates all six through a security-first lens: the protocols behind secure auth, zero-trust principles applied to API selection, and implementation code across Next.js, React, and Express for each provider.

What makes an auth API "secure"?

Before comparing specific providers, it helps to nail down what "secure" actually means when applied to an authentication API. Marketing pages will claim security across the board. The criteria below strip that back to measurable, verifiable properties.

These criteria are drawn from the OWASP Authentication Cheat Sheet (OWASP, 2024⁠), NIST SP 800-207, and the practical realities of shipping auth in production. Each provider section later in this guide maps directly back to this table.

No single criterion is enough on its own. A provider with SOC 2 Type II but 24-hour token lifetimes has a different risk profile than one with 60-second tokens and no SAML support. The right choice depends on your threat model.

Understanding the auth protocol stack

One of the most common points of confusion in authentication: there's no such thing as "logging in with OAuth." OAuth 2.0 is an authorization framework. It handles permissions, not identity. When you click "Sign in with Google," you're actually using OpenID Connect (OIDC), which is an identity layer built on top of OAuth 2.0.

That distinction matters when you're evaluating auth APIs, because it determines what your tokens contain, how they're validated, and what guarantees you get about the person on the other end of the request.

OAuth 2.0, OIDC, and OAuth 2.1

OAuth 2.0 was published as RFC 6749 in 2012. It defined the Authorization Code, Implicit, Resource Owner Password Credentials (ROPC), and Client Credentials flows. Since then, security research has shown that Implicit and ROPC are unsafe for most use cases. RFC 9700, published in January 2025, formalized these findings as OAuth 2.0 Security Best Current Practice (IETF, 2025⁠).

PKCE (RFC 7636) was originally designed to protect mobile apps from authorization code interception. It works by generating a random code verifier, hashing it with S256, and sending the hash in the authorization request. The token exchange then requires the original verifier. RFC 9700 mandates PKCE for all public clients and recommends it for confidential clients (IETF, 2025⁠). The OAuth 2.1 draft goes further, requiring PKCE for all clients using the authorization code flow, with a narrow exception for confidential clients that demonstrate alternative injection mitigation.

DPoP (RFC 9449) goes a step further. It sender-constrains access tokens by binding them to a cryptographic key held by the client. If a token is stolen in transit, it can't be replayed from a different device. Adoption is still early, but it's worth checking whether your auth provider supports it.

OpenID Connect sits on top of OAuth 2.0 and adds the piece OAuth deliberately left out: identity. When a user authenticates through OIDC, the authorization server returns an ID token, a signed JWT containing claims like sub (subject identifier), email, name, and aud (audience). The discovery endpoint at /.well-known/openid-configuration publishes the issuer's public keys, token endpoints, and supported scopes, which lets clients verify tokens without pre-shared secrets.

OAuth 2.1 consolidates the best practices from RFC 9700 into a single specification. It removes Implicit and ROPC entirely and requires PKCE for all flows. As of its 15th draft (March 2026), it remains an Internet-Draft, not a finalized RFC. OAuth 3.0 doesn't exist.

Here's the practical takeaway: if your application has user login, you're using OIDC. All six APIs evaluated in this guide implement OIDC. The differences lie in how they handle token lifetimes, session management, and the security defaults they ship with.

Zero-trust authentication principles

Zero trust is an architecture model defined in NIST SP 800-207 (NIST, 2020⁠) that operates on a single premise: no request is inherently trusted, regardless of where it originates.

Four principles from the framework apply directly to authentication API selection:

1. "Never trust, always verify." Every request must carry proof of identity. A session cookie set at login and trusted for hours doesn't meet this bar. Per-request token validation does.

2. Per-session access with least privilege. Users and services should receive the minimum permissions needed for their current task. Tokens should encode scopes tightly, not grant blanket access.

3. Dynamic policy evaluation. Access decisions should factor in real-time signals: device posture, location anomalies, time since last authentication. Static role checks are necessary but not sufficient.

4. Continuous monitoring. Session validity should be re-evaluated throughout its lifetime, not just at the moment of login. Revocation must propagate quickly.

What does this mean for choosing an auth API? Token lifetime is the clearest differentiator. A provider that issues tokens valid for 24 hours creates a 24-hour window where a stolen token works. A provider with 60-second token lifetimes and automatic background refresh shrinks that window to under a minute. The architectural difference is significant.

Session state matters too. Some providers store session validity on the server and check it on every request. Others encode everything in the token itself and trust it until expiration. The first model lets you revoke access instantly. The second can't.

Passkeys and FIDO2/WebAuthn fit squarely into zero-trust thinking. Phishing-resistant authentication removes the most exploited attack vector (stolen credentials) from the equation entirely. No password means no password to steal.

Sixty-three percent of organizations worldwide have fully or partially implemented a zero-trust strategy (Gartner, 2024⁠). OMB Memorandum M-22-09, issued under Executive Order 14028, requires phishing-resistant MFA for federal agency staff, contractors, and partners accessing federal systems (OMB, 2022⁠).

The next section applies these principles directly, comparing how each of the six auth APIs handles token lifetimes, session validation, passkey support, and revocation.

The 6 best APIs for secure user authentication

How we selected these providers

These 6 APIs were chosen based on specific criteria: developer-first API design, modern protocol support (OAuth 2.0/OIDC, FIDO2), first-party framework SDKs for Next.js, React, and Express, active maintenance with 2025 and 2026 updates, and relevance across B2C, B2B, and enterprise use cases. They represent the most commonly evaluated options for teams building new applications. Other notable providers (Stytch, Descope, Kinde, FusionAuth, Microsoft Entra External ID) exist but fall outside this comparison's scope due to narrower adoption, self-hosted focus, or enterprise-only positioning.

A note on pricing models: MRU vs MAU

All prices in this article are in USD unless otherwise noted.

Before comparing free tiers, it's worth understanding how providers count users. Most competitors use Monthly Active Users (MAU), which counts anyone who authenticates during the month. Clerk uses Monthly Retained Users (MRU): "a user who visits your app in a given month at least one day after signing up" (Clerk Pricing). Users who sign up and never return don't count toward your MRU limit. This distinction makes Clerk's 50,000 free tier effectively more generous for apps with typical trial-and-bounce patterns.

Feature comparison

Sources: Official documentation for each provider, verified March 2026

Evaluation rating criteria

The token architecture comparison below uses Strong, Moderate, and Weak ratings for zero-trust alignment. Here's what each means:

These ratings reflect default configurations with the following assumptions: free-tier plan unless noted, authorization code flow with PKCE, latest stable SDK version as of March 2026, and Next.js App Router for framework comparisons. All providers can be hardened with custom configuration. Your priorities will differ based on stack, scale, and compliance requirements, so weight the dimensions that matter most to your use case.

Token architecture comparison

Token TTLs aren't directly comparable across providers because they use different token types and architectural models. This table normalizes the comparison by showing equivalent constructs.

Clerk's 60-second session token represents a fundamentally different architecture from Auth0's 24-hour access token. Clerk separates the session token (short-lived, used for per-request validation) from the client token (long-lived, HttpOnly, stored on Clerk's Frontend API domain as the source of truth). A stolen Clerk session token expires in under 60 seconds. A stolen Auth0 access token remains valid for up to 24 hours unless explicitly revoked (How Clerk Works; Auth0 Docs⁠). Auth0's token lifetimes are fully configurable and can be shortened to match stricter requirements.

Clerk

Clerk uses a hybrid auth model that separates the session token — a short-lived JWT (60s TTL, RS256-signed) sent with each request — from a long-lived client token stored in an HttpOnly __client cookie scoped to Clerk's Frontend API (FAPI) domain rather than your app's. Clerk's frontend SDK refreshes the session token in the background on a 50-second cycle, roughly 10 seconds before it expires: it calls the Frontend API with the __client cookie, which validates the active session and mints a fresh token — no redirect, re-login, or user interaction. Because that client token is the source of truth, session tokens can be verified from their signature without database calls, while revocation takes effect at the source — once a session is revoked, the next refresh fails and the user is signed out (How Clerk Works).

Security certifications include SOC 2 Type II, HIPAA (Business+ plan), and GDPR compliance via the EU-US Data Privacy Framework (DPF certification). Bot detection runs through Cloudflare's CDN with CAPTCHA challenges for suspected bots. Sign-in attempts are rate-limited to 3 per 10 seconds per IP (system limits), meaning brute-force attacks are throttled well before reaching the account lockout threshold of 100 failed attempts and a 1-hour cooldown (bot protection; user lockout).

The developer experience starts with a 4-step quickstart and keyless mode for instant setup. Clerk ships SDKs for 15+ platforms with prebuilt, a11y-optimized components like <SignIn /> and <UserButton /> (auth strategies).

For B2B teams, Clerk provides native organization management (free tier includes basic orgs with up to 20 members), RBAC with the has() helper, and enterprise SSO (SAML + OIDC, Pro+ with 1 connection included). SCIM provisioning is on Clerk's roadmap but not yet available. The platform now manages 200M+ users across 15,000+ apps, backed by a $50M Series C led by Menlo Ventures and Anthropic (Series C).

Considerations: Clerk is fully managed with no self-hosting option. Native passkeys and MFA require the Pro plan ($20/mo). Clerk's strongest framework support targets React and Next.js; teams using Vue, Angular, or non-JavaScript stacks will find fewer prebuilt components compared to Auth0's 45+ SDKs. At high scale, Clerk's per-MRU pricing can exceed Cognito's Lite tier, which drops to $0.0025/MAU above 10 million users.

Auth0 (by Okta)

Auth0 ships 45+ SDKs across 12 languages and provides the Actions framework for injecting custom logic at any point in the authentication pipeline. Compliance certifications cover SOC 2, ISO 27001/27017/27018, PCI DSS, and HIPAA (Auth0 Compliance⁠).

Native passkey and WebAuthn support runs through Universal Login. Token defaults ship with a 10-hour ID token lifetime and a 24-hour access token lifetime, both configurable to shorter values (Auth0 ID Token⁠; Auth0 Access Token⁠).

Attack Protection bundles bot detection, suspicious IP throttling, brute force protection, and breached password detection. These features are available on Professional+ or as an Enterprise add-on (Auth0 Attack Protection⁠).

Considerations: Auth0's pricing has drawn criticism for its "growth penalty," where costs scale steeply as user counts climb. The free tier lacks MFA, bot detection, and RBAC. MFA becomes available on Essentials+, but SMS and Push MFA require the Professional plan with the Enterprise MFA Lite add-on. Authentication is redirect-based: users leave your app to sign in.

Firebase Auth

Firebase Authentication offers 50,000 MAU free with tight Google ecosystem integration and strong mobile SDKs for Android and iOS. Auth state persists via IndexedDB (not localStorage) since SDK v4.12.0 (Firebase Auth Persistence⁠).

Compliance flows through Google Cloud: SOC 1/2/3, ISO 27001 (Firebase Privacy⁠). Official SSR support arrived via FirebaseServerApp in May 2024, giving server-rendered apps a first-party path to session handling (Firebase SSR Blog⁠). However, FirebaseServerApp doesn't work with the Next.js Edge Runtime (GitHub Issue #8299⁠). Community libraries like next-firebase-auth-edge fill that gap for App Router integration.

Considerations: Firebase has no native passkey support and no prebuilt React UI components. Data residency is limited to US servers, which raises GDPR concerns for EU-facing apps. ID tokens carry a fixed 1-hour TTL that can't be configured.

Supabase Auth

Supabase Auth is open-source, built on GoTrue, and offers 50,000 MAU free. It's PostgreSQL-native, meaning auth and data live in the same database. Row Level Security policies handle authorization at the database level, eliminating a whole category of access control bugs (Supabase Auth⁠).

The full BaaS platform bundles auth, database, storage, and edge functions. EU region hosting is available for teams with data residency requirements (Supabase SOC 2⁠).

Considerations: The original Auth UI library entered maintenance mode in February 2024 and was archived in October 2025 (GitHub: auth-ui⁠); Supabase now ships copy-in, shadcn-based auth blocks through its UI Library⁠ rather than maintained drop-in components. Native passkey (WebAuthn) support arrived in beta in May 2026 (Supabase docs⁠). Free tier projects pause after 7 days of inactivity, which can disrupt development workflows.

WorkOS

WorkOS offers 1,000,000 MAU free for full authentication through AuthKit. Every auth method ships on the free tier: email, social login, magic auth, MFA, and passkeys. No feature gates (WorkOS Pricing⁠).

Enterprise SSO is the platform's core strength. Connections start at $125 each, with SCIM directory sync and an Admin Portal that lets customers configure SSO themselves. The Next.js 16 integration is clean: a roughly 10-minute quickstart with explicit proxy.ts support (WorkOS Next.js Quickstart⁠).

Considerations: Authentication is redirect-based (users leave your app to sign in via hosted AuthKit). The platform leans enterprise and B2B. React SPA integration is less documented than the Next.js path.

AWS Cognito

Cognito uses a three-tier pricing model: Lite (10,000 MAU free), Essentials (10,000 MAU free), and Plus (no free tier). Each tier gates different feature sets (AWS Cognito Pricing⁠).

The deep AWS integration is Cognito's signature strength. Native connections to IAM, API Gateway, Lambda, and ALB mean auth decisions can feed directly into infrastructure policies. Identity Pools are a unique capability: they issue temporary AWS credentials to authenticated users, granting scoped access to S3, DynamoDB, and other AWS resources without managing IAM users.

Native passkey and WebAuthn support landed in November 2024, available on Essentials+ (AWS Security Blog⁠). Compliance inherits from AWS: SOC 1/2/3, ISO 27001, PCI DSS, FedRAMP, and HIPAA eligibility (Cognito Compliance⁠). Managed Login UI (redirect-based) is available on Essentials+, and the Amplify UI Authenticator provides an embedded React component.

Considerations: Amplify documentation still references middleware.ts, with no proxy.ts guidance for Next.js 16. SAML and OIDC federation is free only for 50 MAU. Advanced security features (bot detection, adaptive auth, compromised credential detection) require the Plus plan at $0.02/MAU with no free tier. Passkeys and required MFA are mutually exclusive in the same user pool. User data is region-locked.

What's next in Part 2

Evaluating the core architecture and features of these six providers is only the first step. In Part 2, we move from theory to practice with hands-on code walkthroughs for implementing these providers across Next.js, React, and Express. We also explore advanced security considerations, including account takeover prevention, passkey adoption, and how to choose the right provider based on a total cost of ownership (TCO) analysis.

FAQ

Why do token architectures matter for secure authentication? Token architecture defines the exposure window if a credential is intercepted. Short-lived tokens (e.g., 60 seconds) with automatic refresh mechanisms strictly limit how long a compromised session remains valid without requiring continuous backend verification for every local action.

What is the difference between MRU and MAU pricing for auth providers? Monthly Active Users (MAU) typically counts any user who interacts with the system in a billing cycle. Monthly Retained Users (MRU) excludes users who sign up but never return after their first day. For applications with high trial-and-bounce rates, MRU can run meaningfully lower than an equivalent MAU count, because users who sign up and never come back are never billed.

In this series

1. The best APIs for secure user authentication (you are here)
2. The best APIs for secure user authentication - Part 2