The Tool Desk
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Choose the right OAuth architecture
Native apps must follow the public-client guidance in RFC 8252. The app can safely contain a client ID, but it cannot protect a client secret. The recommended sequence is:
- Start authorization in an external user agent: the system browser or a native authentication session.
- Generate a high-entropy PKCE verifier and derive an S256 challenge.
- Receive an authorization code through a registered redirect URI.
- Send the code and verifier to the token endpoint.
- Use the resulting tokens over HTTPS and keep them in secure platform storage.
| Approach | Use in a React Native app | Reason |
|---|---|---|
| Authorization Code + PKCE | Recommended | Binds the code to the app’s retained verifier and is required for public native clients. |
| Implicit flow | Avoid | It does not provide the code-exchange protection used by the recommended native-app pattern. |
| Embedded WebView | Avoid | RFC 8252 requires an external user agent for native authorization; AppAuth-based libraries do not support WebViews. |
| Client secret in the bundle | Never use | Anyone can extract a value shipped in the JavaScript bundle or binary. |
Register the mobile client with your identity provider
Create a native or public client in the provider’s console before writing the login screen. Record the authorization endpoint, token endpoint, client ID, supported scopes, and logout behavior. Do not create a mobile client as a confidential server application merely to obtain a secret.
Register exact redirect URIs
Redirect matching is normally exact. Register every URI that the iOS and Android builds will actually use, including its scheme, host, path, capitalization, and trailing slash behavior. Prefer a verified HTTPS universal link or Android App Link when both the platform and provider support it. A custom scheme such as com.acme.reader://oauthredirect is workable, but another installed application may claim the same scheme.
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Keep the initial scope small
Request only scopes needed for the first screen or feature. Add broader scopes later through incremental authorization when the user invokes functionality that needs them. This produces a clearer consent request and follows Google’s OAuth guidance on incremental authorization.
Add OAuth support to the React Native project
react-native-app-auth is a practical option when you want native AppAuth-iOS and AppAuth-Android integration. Its documented design follows native-app OAuth practices, supports PKCE, and deliberately does not use WebViews. PKCE support and the available grant settings still depend on the identity provider.
npm install react-native-app-auth
cd ios
pod installUse the provider’s discovery URL when it supports OpenID Connect discovery. Otherwise provide the authorization and token endpoints through the library’s service configuration. The following configuration is illustrative; replace the example values with the exact values registered for your app.
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import { authorize } from 'react-native-app-auth';
const oauthConfig = {
issuer: 'https://id.example.com',
clientId: 'rn-reader-mobile',
redirectUrl: 'com.acme.reader://oauthredirect',
scopes: ['openid', 'profile'],
usePKCE: true
};
export async function signIn() {
const authorization = await authorize(oauthConfig);
return authorization;
}Do not add a clientSecret property to this mobile configuration. If your provider requires one for a token request, verify that you selected its native/public-client application type instead of weakening the app by embedding a secret.
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The redirect is only a hand-off signal. It must not contain an access token, refresh token, ID token, password, or other sensitive data. React Native’s security guidance warns that deep links are not secure because URL schemes are not centrally registered.
iOS custom URL scheme
Add the scheme portion of the registered URI to the application’s URL Types in Xcode, or use an equivalent Info.plist entry:
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<key>CFBundleURLTypes</key>
<array>
<dict>
<key>CFBundleURLSchemes</key>
<array>
<string>com.acme.reader</string>
</array>
</dict>
</array>Android custom URL scheme
Add an intent filter to the activity that receives the link. The scheme and host must correspond to the URI you registered, and the activity must be able to resume the existing sign-in task.
<activity
android:name=".MainActivity"
android:launchMode="singleTask">
<intent-filter>
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.BROWSABLE" />
<data
android:scheme="com.acme.reader"
android:host="oauthredirect" />
</intent-filter>
</activity>For verified HTTPS links, configure the platform association files and provider-side redirect exactly as required by the platform. The security benefit comes from domain verification; simply changing a custom scheme to an unverified HTTPS URL does not provide that guarantee.
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- Create the verifier. Generate a cryptographically random, high-entropy code verifier and keep it locally for this authorization attempt.
- Derive the challenge. Hash the verifier with SHA-256 and send the base64url-encoded S256 challenge in the authorization request. Never send the verifier in that first request.
- Open the external user agent. The browser or native authentication session handles the provider’s sign-in, multifactor authentication, and consent UI.
- Receive the redirect. Accept only the redirect URI you registered. Validate the returned state value and ensure an authorization code is present before continuing.
- Exchange the code. Post the code, the original redirect URI, client ID, and retained verifier to the token endpoint over HTTPS. The provider must recompute the challenge before issuing tokens.
- Persist the result safely. Keep access and refresh tokens in Keychain, Android Keystore-backed storage, or an equivalent platform-secure store. Avoid AsyncStorage for long-lived credentials.
PKCE protects against an intercepted authorization code: an app that captures the code still lacks the verifier required by the token endpoint. RFC 9700 identifies S256 as the appropriate challenge method for current OAuth deployments.
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Why a WebView is the wrong login surface
An embedded WebView makes it harder for the identity provider to provide trusted browser security signals, single sign-on, and consistent cookie handling. It also puts credential entry inside an application-controlled surface. RFC 8252 requires native apps to use an external user agent such as the browser, and the AppAuth libraries used by react-native-app-auth exclude WebViews.
A system browser or native authentication session can still return to the app through the configured redirect. The user may see an account picker or a short browser transition, but credentials remain in the provider-controlled context.
Protect tokens and API requests
- Send API requests only over HTTPS.
- Keep access tokens out of logs, crash reports, analytics events, screenshots, and deep-link parameters.
- Use the access token only for the API and scopes that issued it.
- Handle expiration by using the provider’s refresh-token policy; do not assume every provider returns a refresh token or permits indefinite reuse.
- Clear stored credentials on sign-out and revoke refresh tokens when the provider supports revocation.
- If your product has a backend, consider keeping provider refresh tokens server-side and issuing the mobile app a session controlled by your backend.
Choose a library and provider deliberately
| Decision area | What to verify | Why it matters |
|---|---|---|
| PKCE | S256 support and whether it is enabled for the native client | The verifier must be bound to the authorization code. |
| Browser integration | System browser or native authentication-session support | External user agents are the required native-app pattern. |
| Redirects | Custom schemes, universal links, or Android App Links | Redirect registration and collision resistance differ by platform. |
| Native integration | Quality of iOS and Android SDK bridges | Redirect handling and lifecycle behavior are platform-specific. |
| Refresh tokens | Rotation, expiration, revocation, and offline-access rules | Refresh behavior is provider-specific and affects session design. |
| Consent and scopes | Incremental authorization and consent-screen controls | Users should not grant unrelated access during first sign-in. |
| Logout | Local token clearing, provider logout, and browser-session behavior | Signing out of the app may not sign out of the browser account. |
| Operations | Documentation, support, quotas, and recurring provider cost | A secure flow still needs maintainable production operations. |
react-native-app-auth fits teams that want a thin React Native interface over AppAuth-native components. A provider’s own React Native SDK may offer deeper account, logout, or push-based features, but confirm its current PKCE, external-browser, redirect, and refresh-token behavior before adopting it.
Troubleshoot the common failures
“Redirect URI mismatch”
Compare the runtime URI with the provider registration character by character. Check the scheme, host, path, slash, case, and build-specific value. Also confirm that iOS and Android are not using different URIs unintentionally.
The browser finishes but the app does not resume
Verify the iOS URL Type or Android intent filter, confirm that the activity launch mode can receive a new intent, and test the link on a signed build. A provider redirect registered in its console does not configure the operating system automatically.
The provider rejects the PKCE request
Confirm that the client is registered as a native/public application, that the authorization request sends an S256 challenge, and that the token request sends the unchanged verifier. Do not “fix” the error by adding a secret to the mobile binary.
Login succeeds but API calls return unauthorized
Inspect the audience, issuer, expiry, and scopes in the token, then confirm that the API expects that provider and token type. A successful identity-provider login does not guarantee authorization for every API.
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Refresh works on one provider but not another
Refresh-token issuance, rotation, expiration, and revocation are provider policies. Read the selected provider’s current documentation and implement its documented response and reauthentication paths rather than assuming a universal behavior.
Quick Recap
Production checklist
- The app is registered as a public native client with no embedded secret.
- Authorization Code flow and PKCE with S256 are enabled.
- Authorization opens in the system browser or a native authentication session, not a WebView.
- Redirect URIs are exact and configured in both the provider and each native platform.
- State and authorization-code responses are validated.
- No token or other sensitive value is placed in a deep-link URL.
- Tokens are stored in Keychain, Keystore-backed storage, or an equivalent protected store.
- API traffic uses HTTPS and scopes are requested incrementally.
- Expiration, refresh failure, revocation, logout, cancellation, and provider errors have explicit UI paths.
- Provider-specific limits and current SDK behavior are reviewed before release.
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