.NET MAUI Authentication

.NET MAUI Authentication

Security: Never Embed Secrets

❌ Never embed client secrets, API keys, or signing keys in a mobile app binary. They can be extracted trivially via decompilation.

The correct pattern:

1. App calls WebAuthenticator pointing to your server endpoint
2. Server initiates the OAuth flow with the identity provider (holds the client secret)
3. Provider redirects back to your server with an auth code
4. Server exchanges the code for tokens and returns them to the app via the callback URI

WebAuthenticator Gotchas

⚠️ Windows WebAuthenticator is broken

Windows WebAuthenticator is currently broken. See dotnet/​maui#2702. Use MSAL or a WinUI-specific workaround for Windows auth flows.

⚠️ Apple Sign In returns name/​email only once

Apple only returns the user's name and email on the first sign-in. Cache them immediately — subsequent sign-ins won't include them.

⚠️ PrefersEphemeralWebBrowserSession

Set to true on iOS 13+ to force a fresh login prompt. When false (default), the auth session shares cookies with Safari — the user may be auto-logged in, which can confuse logout/​switch-account flows.

⚠️ Callback URI mismatches

The most common auth failure is a URI scheme mismatch. The CallbackUrl in code must exactly match:

• Android: DataScheme + DataHost in the IntentFilter
• iOS: CFBundleURLSchemes in Info.plist
• Windows: Protocol Name in Package.appxmanifest

WebAuthenticator Checklist

• Callback URI scheme matches across all platform configs and CallbackUrl
• Android has WebAuthenticatorCallbackActivity with correct IntentFilter
• Android 11+ has <queries> for Custom Tabs in the manifest
• iOS/​Mac Catalyst has CFBundleURLTypes in Info.plist
• Client secrets are on the server, not in the app
• Tokens stored with SecureStorage, cleared on logout
• TaskCanceledException handled gracefully in UI

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Choosing Between WebAuthenticator and MSAL.NET

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MSAL.NET Gotchas

⚠️ Android: OnActivityResult is required

Forgetting AuthenticationContinuationHelper.SetAuthenticationContinuationEventArgs in MainActivity.OnActivityResult causes auth to hang silently after the browser returns.

⚠️ iOS: Keychain sharing is required

Without Entitlements.plist containing keychain group com.microsoft.adalcache, token caching fails silently and users are prompted to sign in every time.

⚠️ Handle MsalUiRequiredException

When AcquireTokenSilent throws MsalUiRequiredException, the cached token is expired and interaction is needed. Always fall back to AcquireTokenInteractive.

// ❌ Ignoring MsalUiRequiredException — user gets a crash
var result = await _pca.AcquireTokenSilent(scopes, account).ExecuteAsync(ct);

// ✅ Fall back to interactive when silent fails
try
{
    result = await _pca.AcquireTokenSilent(scopes, account).ExecuteAsync(ct);
}
catch (MsalUiRequiredException)
{
    result = await _pca.AcquireTokenInteractive(scopes).ExecuteAsync(ct);
}

⚠️ Handle user cancellation gracefully

// ✅ Don't treat cancellation as an error
catch (MsalClientException ex) when (ex.ErrorCode == "authentication_canceled")
{
    return null;  // User cancelled — not an error
}

⚠️ Blazor Hybrid: Auth happens at the MAUI layer

In MAUI Blazor Hybrid apps, authentication must happen at the MAUI layer (MSAL.NET), not in the Blazor WebView. Don't use AddMicrosoftIdentityWebApp or server-side OIDC patterns. Instead:

1. MAUI handles sign-in via IAuthService (MSAL.NET)
2. A custom MsalAuthenticationStateProvider exposes auth state to Blazor
3. HttpClient with DelegatingHandler attaches bearer tokens automatically

MSAL.NET Checklist

• Microsoft.Identity.Client NuGet package added
• App registration created in Entra ID with correct redirect URIs
• AuthConfig / appsettings.json has ClientId, TenantId, Scopes
• Android: AndroidManifest.xml has <queries> for broker and browsers
• Android: MainActivity.OnActivityResult calls AuthenticationContinuationHelper
• iOS: Info.plist has CFBundleURLSchemes with msauth.{BundleId}
• iOS: Entitlements.plist has keychain group com.microsoft.adalcache
• iOS: AppDelegate.OpenUrl calls AuthenticationContinuationHelper
• IAuthService registered as singleton in DI
• DelegatingHandler attached to HttpClient for API calls
• Login/​logout UI wired up
• MsalUiRequiredException handled (triggers interactive sign-in)
• MsalClientException with authentication_canceled handled gracefully