7 Steps to Single Sign On for Custom Apps for Engineers and PMs

For new custom web or mobile apps, use OpenID Connect with the authorization code plus PKCE flow; reserve SAML for cases where you must interoperate with legacy enterprise identity providers. Before writing any code, have these ready: a registered redirect URI, a client ID, an issuer or metadata URL, and access to the provider’s JWKS endpoint. Confirm PKCE uses the S256 method, tokens are scoped to a single audience, and both nonce and state parameters are validated on callback.

TL;DR:

• Use OpenID Connect with authorization code and PKCE for new cloud-native apps, reserving SAML only for legacy enterprise integrations.
• Confirm full PKCE validation, token audience restriction, and validate nonce and state parameters on each callback to ensure security.
• Support both protocols when needed, but default to OIDC for modern apps, especially with API and mobile architectures, while supporting SAML for enterprise partners.
• Properly register applications, validate issuer and JWKS, and test token validity and clock sync before production deployment; keep separate staging and production environments.
• Implement careful logout validation, monitor for common errors like redirect URI mismatches, and prepare for certificate rotations with detailed operational runbooks.

Table of Contents

• Choosing between OIDC and SAML for custom applications
• Step-by-step OpenID Connect implementation for custom apps
• Integrating SAML 2.0 for enterprise identity providers
• Getting Authorization Code plus PKCE right
• Where tokens should live: BFF, SPA, and mobile patterns
• Handling logout and session termination correctly
• Testing checklist and common SSO deployment errors
• Security standards every SSO design review should reference
• An operational checklist from delivery work
• Get help implementing SSO for your custom application
• Sources
• FAQ

Choosing between OIDC and SAML for custom applications

The protocol decision usually comes down to what kind of app you’re building and who you need to talk to. OpenID Connect is recommended for new, cloud-native applications, built as it is on top of OAuth 2.0 and designed to fit naturally into web and API architectures. SAML 2.0 remains common in enterprise B2B integrations, particularly where the identity provider is decades old, and the integration team has no plan to modernize it.

The tradeoffs are practical, not philosophical. SAML relies on XML metadata exchange and certificate-based trust, which means certificate rotation and attribute mapping become recurring maintenance tasks. OIDC leans on JSON web tokens and a discovery document, which most current libraries handle with far less custom code.

A few things to weigh before committing to one protocol across your app:

• App architecture: Single-page apps, mobile apps, and API-first backends fit OIDC’s token model more naturally than SAML’s browser-redirect assumptions.
• Partner IdP landscape: If your enterprise customers only expose SAML endpoints, you’ll need to support it regardless of your own preference.
• Multi-tenant complexity: Plan for issuer lookup, domain verification, and home-realm discovery early if you’re supporting multiple customer IdPs, since retrofitting tenant routing after launch is painful.

Most teams end up supporting both, with OIDC as the default and SAML as an accommodation for specific enterprise accounts.

Step-by-step OpenID Connect implementation for custom apps

Once you’ve settled on OIDC, the implementation sequence is fairly consistent across providers, whether you’re using Okta, Azure AD, or another identity platform.

1. Register the application with your identity provider, specifying the client type (public or confidential), exact redirect URIs, and allowed origins for CORS.
2. Fetch the discovery document from the provider’s /.well-known/openid-configuration endpoint and cache the issuer and JWKS URLs.
3. Validate the issuer on every token you receive, matching it exactly against the value in the discovery document.
4. Request the openid scope at minimum, adding profile or email only if your app actually needs those claims.
5. Initiate the Authorization Code flow with PKCE, generating a code verifier and S256-hashed code challenge before redirecting the user.
6. Validate the returned id_token, checking iss, aud, exp, and the nonce you generated at the start of the flow, as OWASP’s OAuth2 guidance recommends.
7. Handle secrets appropriately: confidential clients store the client secret server-side only; public clients rely entirely on PKCE instead.

A few deployment details trip up teams that skip staging environments: redirect URI allowlists are exact-match in most providers, so a trailing slash mismatch will fail silently or throw a cryptic error. Clock skew between your server and the IdP can cause valid tokens to be rejected as expired. Test against a staging IdP tenant with its own client registration before touching production credentials.

Pro Tip: Keep separate client registrations for staging and production, even if it means double the setup work: sharing a redirect URI allowlist across environments is a common source of “it worked in staging” surprises.

Integrating SAML 2.0 for enterprise identity providers

SAML integrations follow a different rhythm, built around metadata exchange rather than discovery endpoints. Setting up SAML SSO requires registering redirect and ACS URLs, exchanging metadata or certificates, and aligning issuer values between service provider and identity provider, and mismatches here are the most frequent cause of failed logins.

Practical steps for a clean integration:

• Exchange SP and IdP metadata early, including the Assertion Consumer Service (ACS) URL and your Entity ID, so both sides agree on where assertions get sent.
• Manage certificates deliberately: pull them from the IdP’s metadata endpoint when available rather than hardcoding a certificate that will eventually expire.
• Map SAML attributes into your app’s authorization model using an explicit allowlist, and test the mapping with a dedicated per-tenant test user before rolling out to real accounts.

SAML-specific failures tend to cluster around a handful of causes: unsigned or improperly signed assertions, clock skew between the IdP and your server, and audience restriction mismatches where the assertion’s intended recipient doesn’t match your Entity ID.

Pro Tip: Log the raw SAML response (minus sensitive attribute values) during integration testing. XML namespace issues are far easier to spot in a raw payload than in a stack trace three layers removed from the actual assertion.

Getting Authorization Code plus PKCE right

PKCE exists to close a specific gap: without it, an intercepted authorization code can be redeemed by whoever captured it. PKCE binds the initial authorization request to the token exchange using a verifier, so even a stolen code is useless without the matching verifier. The Authorization Code flow with PKCE is now the baseline security practice for public clients, with S256 as the recommended code challenge method over the weaker plain method.

The mechanics are straightforward once you’ve implemented them once: generate a random code verifier, hash it with SHA-256 to produce the code challenge, send the challenge in the authorization request, and present the original verifier when exchanging the code for tokens. The parts teams get wrong:

• Skipping nonce validation, which defeats one of the main protections against replay attacks in the OIDC layer.
• Allowing code reuse, when a code should be single-use and immediately invalidated after the first exchange attempt.
• Storing the verifier insecurely, such as in a place accessible to any script running on the page for a browser-based app.

Confidential clients, meaning server-side apps that can safely hold a secret, still combine PKCE with a client secret in many implementations. Public clients such as SPAs and native mobile apps rely on PKCE alone, since they can’t keep a secret confidential.

Where tokens should live: BFF, SPA, and mobile patterns

Token placement determines most of your app’s exposure to token theft. The backend-for-frontend pattern keeps tokens server-side and issues the browser a short-lived, HttpOnly cookie instead, a setup that’s become the standard recommendation for single-page apps that call private APIs.

RFC 9700 describes sender-constrained, cryptographically bound tokens as an emerging direction to reduce replay risk, indicating that token handling design decisions today should consider potential stricter binding requirements.

Practical rules for token topology:

• Confidential clients hold secrets server-side; public clients rely on PKCE and short-lived access tokens instead of a secret.
• Scope access tokens to a single audience for each API, and never accept an ID token as if it were an API access token.
• Rotate refresh tokens on each use and detect reuse of a retired token as a signal of possible theft.

Mobile apps generally follow the public client model, storing tokens in platform-provided secure storage (Keychain on iOS, Keystore on Android) rather than plain files.

Handling logout and session termination correctly

Single logout sounds simple until you consider who initiated it and how far it should propagate. In SP-initiated logout, your app tells the IdP the user is done; in IdP-initiated logout, the IdP tells every connected app to end the session. Both directions require validating the incoming message.

SAML Single Logout requires careful validation of logout requests, since naive implementations expose denial-of-service or forced-logout risks where an attacker sends an unauthenticated logout message to boot a legitimate user.

Build logout with these guardrails:

• Validate every logout request for signature and id_token_hint before acting on it.
• Treat global logout as best-effort: clean up the local session immediately, then attempt downstream notifications rather than blocking on them.
• For SAML specifically, require signed logout requests and configure the SLO endpoint explicitly rather than assuming a default.

Testing checklist and common SSO deployment errors

Before launch, run through a short preflight list rather than discovering issues in production.

1. Confirm redirect URIs match exactly, including trailing slashes and protocol.
2. Verify issuer and metadata align between your app’s configuration and what the IdP actually publishes.
3. Check JWKS availability and confirm your app can fetch and cache signing keys.
4. Sync server clocks, since even a few minutes of drift causes token validation failures.
5. Test with a staging IdP tenant using dedicated accounts before touching production credentials.

Common errors map to specific fixes: invalid_redirect_uri almost always means an exact-match mismatch in the allowlist, and issuer mismatch usually means your app cached a stale discovery document. Log authentication events, including failures and reuse attempts, without ever storing raw tokens in logs.

Security standards every SSO design review should reference

A handful of documents should anchor any SSO design or security review. RFC 9700 sets the current OAuth 2.0 security baseline, and RFC 7636 defines the PKCE mechanism that baseline depends on. The OWASP ASVS OAuth and OIDC guidance treats the OAuth and OIDC layer as a security perimeter worth auditing on its own.

Controls worth enforcing in every review:

• Audience-restrict every token and reject tokens on the resource server side that weren’t issued for it.
• Prefer stronger client authentication such as private_key_jwt or mutual TLS over shared secrets where your provider supports it.
• Rotate certificates and refresh tokens on a schedule, and implement revocation and replay detection rather than relying on token expiry alone.

Pro Tip: Set a calendar alert 30 days before any SAML certificate expires. Certificate expiry is a preventable outage, not a surprise.

An operational checklist from delivery work

Getting SSO working in a demo is the easy part. Getting it to survive a production launch, a certificate rotation, and an IdP outage six months later takes a bit more discipline.

• Separate staging and production entirely, with distinct client registrations, test user accounts, and metadata endpoints.
• Build a certificate rotation runbook with overlap windows and alerts 30 days before expiry, automating ingestion from metadata endpoints where the provider supports it.
• Hand off real documentation, including a token revocation runbook, monitoring dashboards for auth failures, and acceptance test scripts the client’s team can rerun after any change.

Teams that skip the handoff docs tend to relearn these lessons during an incident instead of during a calm Tuesday afternoon.

Get help implementing SSO for your custom application

If your team has the design mapped out but needs hands to build it, Ridiculous Engineering handles the full arc: architecture review, OIDC or SAML implementation, testing against staging IdP tenants, and the runbook and monitoring setup that keeps it running after launch. We work through custom software development engagements sized to the problem, not a fixed template, and we’d rather explain a tradeoff honestly than oversell a shortcut. If you’re weighing whether to build this in-house or bring in a team that has already worked through the certificate rotation headaches and callback URL mismatches, reach out through our contact page and we’ll talk through what your app actually needs.

Sources

• RFC 9700: OAuth 2.0 Security Best Current Practice
• Microsoft Entra single sign-on (SSO)
• OAuth2 - OWASP Cheat Sheet Series

FAQ

How do I create my own SSO?

You implement SSO by integrating your app with an identity provider using OpenID Connect or SAML rather than building authentication from scratch. For new apps, OpenID Connect with the Authorization Code and PKCE flow is the standard starting point, handled through provider SDKs like those from Okta or Azure AD.

Is SSO risky?

SSO is not inherently riskier than app-specific logins, and it typically reduces risk by consolidating authentication behind a provider with stronger security controls than most individual apps could build alone. The real risk lies in implementation mistakes such as skipping PKCE, failing to validate token audience, or mishandling logout, all of which OWASP’s OAuth2 guidance addresses directly.

Can you SSO into a mobile app?

Yes, mobile apps commonly implement SSO as OAuth 2.0 public clients using the Authorization Code flow with PKCE, since they can’t securely hold a client secret. Tokens are typically stored in platform-native secure storage rather than plain app files.

Who are the top SSO providers?

Common identity providers used for SSO integrations include Okta and Microsoft’s Azure AD (now part of Microsoft Entra), among others. The right choice depends on which protocols your app needs to support and which providers your enterprise customers already use.

How does single logout work across apps?

Single logout can be initiated by your app (SP-initiated) or by the identity provider (IdP-initiated), and both require validating the logout message before acting on it. Naive SAML logout implementations can expose denial-of-service or forced-logout risks, so most teams treat global logout as best-effort rather than a guaranteed instant kill-switch across every connected app.

Recommended

• AI-Speed Engineering Frameworks
• What Makes an App Truly Engaging? Here’s Our Secret Recipe!
• Project Manager, Product Owner, and Business Analyst in E-commerce and Application Development: Which one do you need?