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For redirect-based sign-in, use OpenID Connect (OIDC) authorization code flow with PKCE, protect each login transaction, and validate the ID Token before trusting its claims. OAuth 2.0 is for delegated access to protected resources; OIDC adds an authentication protocol that lets a client learn about an End-User’s authentication.
What is the difference between OAuth 2.0 and OpenID Connect?
OAuth 2.0 is an authorization framework: it lets a client obtain limited access to protected resources. OpenID Connect builds on OAuth 2.0 to authenticate an End-User and convey information about the authentication event and, potentially, other requested claims.
| Item | Purpose | How to use it |
|---|---|---|
| OAuth 2.0 | Delegated access to protected resources | Use its authorization flow to obtain an access token for the resource the client needs. |
| OpenID Connect | Authentication and End-User claims on top of OAuth 2.0 | Validate the ID Token before relying on its identity claims. |
| ID Token | Information about an authentication event | Validate it as an identity token; do not send it to an API as though it were an access token. |
| Access token | Access to a protected resource | Present it to the intended resource, including UserInfo when additional End-User claims are needed. |
These tokens are not interchangeable. An ID Token tells the relying party about authentication. An access token is for accessing protected resources. The OpenID Foundation’s OpenID Connect Core 1.0 incorporating errata set 2 defines UserInfo as a protected resource that returns End-User claims when called with an access token.
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Use an authorization-code transaction for redirect-based sign-in. RFC 9700, the IETF’s Best Current Practice for OAuth 2.0 Security published in January 2025, recommends code flow over implicit access-token issuance in the authorization response. It also states: “Authorization servers MUST support PKCE [RFC7636].”
-
Identify the client and its trust boundaries
Determine whether the client is public or confidential, and where credentials can safely be held. A public client cannot safely keep a client secret confidential. RFC 9700 requires public clients to use PKCE and recommends it for confidential clients too. Where feasible, authorization servers should authenticate clients; RFC 9700 recommends asymmetric methods such as mutual TLS or signed JWT client authentication because the server need not keep a sensitive shared secret.
-
Start a fresh authorization-code transaction
Generate a fresh, unpredictable PKCE verifier for each transaction. Send its
S256challenge in the authorization request and retain the verifier in a way bound to the initiating client and user agent. When exchanging the returned code at the token endpoint, send that verifier. The authorization server must verify a supplied challenge and must not let a verifier bypass a missing challenge. Do not reuse a constant challenge or nonce. -
Bind the transaction against CSRF
Choose a defense that fits the flow and the authorization server’s support. RFC 9700 allows a client to rely on PKCE for CSRF protection only when it has ensured that the server supports PKCE. In OIDC, nonce provides CSRF protection. Otherwise, use a one-time
statevalue securely bound to the user-agent session. Ifstatecontains application data whose integrity matters, protect it against tampering and swapping.Recommended Free Tools
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Exchange the code, then validate the ID Token
Keep access tokens out of the authorization response. Validate the OIDC ID Token before using identity claims, and keep its role separate from the access token’s role.
-
Fetch additional claims only when needed
If the application needs further End-User claims, call the UserInfo endpoint over HTTPS with an access token. UserInfo is a separate protected resource, not a substitute for validating the ID Token.
Do I need PKCE for a web app?
Use PKCE with authorization code flow. RFC 9700 requires it for public clients and recommends it for confidential clients. A server-side web application may be able to keep a credential confidential, but that does not make PKCE irrelevant: the RFC recommendation covers confidential clients too. Use the S256 challenge method, and create a separate unpredictable verifier for each login transaction.
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Avoid the implicit grant and other response types that issue access tokens in the authorization response unless the leakage and injection risks identified by RFC 9700 are mitigated. Code flow keeps access tokens out of that response.
What are state and nonce for?
Both values help bind a response to a login transaction, but they belong to different layers and are not interchangeable. Use the option appropriate to the protocol and the guarantees your client can establish.
| Mechanism | Role in this guidance | Implementation condition |
|---|---|---|
| PKCE | Protects the authorization-code exchange and can provide CSRF protection. | Use a fresh verifier and its S256 challenge. Rely on it for CSRF only when PKCE support is assured. |
| OIDC nonce | Provides CSRF protection in an OIDC transaction and is checked in the ID Token. | If the request included a nonce, require the matching nonce claim. |
state |
Provides a CSRF defense when another suitable transaction-bound mechanism is not being relied on. | Make it one-time and securely bind it to the user-agent session; protect any integrity-sensitive application data it carries. |
How should I protect redirect URIs and callbacks?
Treat the redirect and callback as security boundaries: a loose redirect check or an unbound response can expose authorization data or let a response be misdirected.
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- Register and compare exact URIs. Authorization servers must use exact string matching between a registered redirect URI and the URI in the authorization request. The specified exception permits differing ports for localhost redirects in native apps.
- Do not add an open redirector. Avoid forwarding a browser to an arbitrary destination supplied in a query parameter; an open redirector can leak codes or tokens.
- Check which authorization server responded. If a client uses multiple authorization servers, defend against mix-up attacks by checking the authorization response
issparameter or a suitable issuer value in the response. Distinct redirect URIs are an alternative when issuer identification is unavailable.
How do I validate an OpenID Connect ID Token?
Do not trust claims just because a token can be decoded. Decoding shows a token’s contents; validation establishes whether it was issued by the expected authority, for this client, and is valid now. Follow OpenID Connect Core before relying on identity claims.
- Check the issuer. Confirm that the token’s issuer is the issuer the client expects.
- Check the audience. Confirm that the audience includes the client’s registered
client_id. Apply the specification’s checks for multiple audiences and authorized party where they apply. - Verify the signature. Validate the token’s signature rather than treating a decoded JWT as proof of authenticity.
- Check expiration. Confirm that the token has not expired.
- Check nonce when used. If the authorization request included a nonce, require the matching claim in the ID Token.
- Use claims for an appropriate purpose. OpenID Connect Core defines
subas a locally unique, never-reassigned identifier within its issuer. Do not assume it is globally unique across different issuers.
Which client-authentication and mix-up defenses should I choose?
The right implementation depends on what the client can protect and what the authorization server supports. Keep these choices tied to the relevant trust boundary rather than treating one mechanism as a universal replacement for the others.
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|---|---|---|
| Client authentication | Whether the client can keep a credential confidential, the work of issuing and rotating keys or certificates, and existing deployment support. | Where feasible, RFC 9700 recommends asymmetric client authentication such as mutual TLS or signed JWT-based authentication. |
| CSRF protection | Whether PKCE support is assured and whether the transaction is bound to the user agent. | Rely on PKCE only when support is assured; use OIDC nonce in OIDC transactions, or a one-time session-bound state value otherwise. |
| Multiple authorization servers | Whether the response provides issuer identification or the deployment can maintain separate redirect URIs. | Check the response issuer when available; use distinct redirect URIs as an alternative when issuer identification cannot be used. |
What should I verify before launch?
- The application uses authorization code flow for redirect-based sign-in and does not expose access tokens in the authorization response.
- PKCE uses a fresh, unpredictable verifier and an
S256challenge for every transaction. - Callback handling checks the registered redirect URI exactly and does not forward users to arbitrary query-provided destinations.
- The CSRF defense matches the flow: assured PKCE support, OIDC nonce, or one-time session-bound
state. - The client validates the ID Token’s issuer, audience, signature, expiration, and applicable nonce before using identity claims.
- When multiple authorization servers are in play, the client identifies the responding issuer or uses distinct redirect URIs.
- Any additional claims are requested from UserInfo with an access token over HTTPS, rather than by misusing the ID Token.
Provider-specific registration labels and interoperability details vary by deployment; confirm the exact callback URI and supported response behavior with the authorization server being integrated. The guidance above follows RFC 9700 and OpenID Connect Core 1.0 incorporating errata set 2.
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