I'm implementing Apple Sign-In in my Next.js application with a NestJS backend. After the user authenticates with Apple, instead of redirecting to my configured callback URL, the browser makes a POST request to a mysterious endpoint /appleauth/auth/federate that doesn't exist in my codebase, resulting in a 404 error.
Tech Stack
Frontend: Next.js 16.0.10, React 19.2.0
Backend: NestJS with Passport (using @arendajaelu/nestjs-passport-apple)
Frontend URL: https://myapp.example.com
Backend URL: https://api.example.com
Apple Developer Configuration
Service ID: (configured correctly in Apple Developer Console)
Return URL (only one configured):
https://api.example.com/api/v1/auth/apple/callback
Domains verified in Apple Developer Console:
myapp.example.com
api.example.com
example.com
Backend Configuration
NestJS Controller (auth.controller.ts):
typescript
@Public()
@Get('apple')
@UseGuards(AuthGuard('apple'))
async appleAuth() {
// Initiates Apple OAuth flow
}
@Public()
@Post('apple/callback') // Changed from @Get to @Post for form_post
@UseGuards(AuthGuard('apple'))
async appleAuthCallback(@Req() req: any, @Res() res: any) {
const result = await this.authService.socialLogin(req.user, ipAddress, userAgent);
// Returns HTML with tokens that uses postMessage to send to opener window
}
Environment Variables:
typescript
APPLE_CLIENT_ID=<service_id>
APPLE_TEAM_ID=<team_id>
APPLE_KEY_ID=<key_id>
APPLE_PRIVATE_KEY_PATH=./certs/AuthKey_XXX.p8
APPLE_CALLBACK_URL=https://api.example.com/api/v1/auth/apple/callback
FRONTEND_URL=https://myapp.example.com
The passport-apple strategy uses response_mode: 'form_post', so Apple POSTs the authorization response to the callback URL.
Frontend Implementation
Next.js API Route (/src/app/api/auth/apple/route.js):
javascript
export async function GET(request) {
const backendUrl = new URL(`${API_URL}/auth/apple`);
const response = await fetch(backendUrl.toString(), {
method: "GET",
headers: {
"Content-Type": "application/json",
},
});
const responseText = await response.text();
return new NextResponse(responseText, {
status: response.status,
headers: { "Content-Type": contentType || "text/html" },
});
}
Frontend Auth Handler:
javascript
export const handleAppleLogin = (router, setApiError) => {
const frontendUrl = window?.location?.origin;
// Opens popup to /api/auth/apple
window.open(
`${frontendUrl}/api/auth/apple`,
"appleLogin",
"width=500,height=600"
);
};
The Problem
Expected Flow:
User clicks "Login with Apple"
Frontend opens popup → https://myapp.example.com/api/auth/apple
Frontend proxies to → https://api.example.com/api/v1/auth/apple
Backend redirects to Apple's authentication page
User authenticates with Apple ID
Apple POSTs back to → https://api.example.com/api/v1/auth/apple/callback
Backend processes and returns success HTML
Actual Behavior:
After step 5 (user authentication with Apple), instead of Apple redirecting to my callback URL, the browser makes this unexpected request:
POST https://myapp.example.com/appleauth/auth/federate?isRememberMeEnabled=false
Status: 404 Not Found
Request Payload:
json
{
"accountName": "user@example.com",
"rememberMe": false
}
Network Tab Analysis
From Chrome DevTools, the call stack shows:
send @ app.js:234
ajax @ app.js:234
(anonymous) @ app.js:10
Ee.isFederated @ app.js:666
_callAuthFederate @ app.js:666
The Ee.isFederated and _callAuthFederate functions appear to be minified library code, but I cannot identify which library.
What I've Verified
✅ The /appleauth/auth/federate endpoint does not exist anywhere in my codebase:
bash
grep -r "appleauth" src/ # No results
grep -r "federate" src/ # No results
✅ Apple Developer Console shows only ONE Return URL configured (verified multiple times)
✅ Changed callback route from @Get to @Post to handle form_post response mode
✅ Rebuilt frontend completely multiple times:
bash
rm -rf .next
npm run build
✅ Tested in:
Incognito/Private browsing mode
Different browsers (Chrome, Firefox, Safari)
Different devices
After clearing all cache and cookies
✅ No service workers registered in the application
✅ No external <script> tags or CDN libraries loaded
✅ package.json contains no AWS Amplify, Auth0, Cognito, or similar federated auth libraries
✅ Checked layout.js and all root-level files - no external scripts
Additional Context
Google Sign-In works perfectly fine using the same approach
The mysterious endpoint uses a different path structure (/appleauth/ vs /api/auth/)
The call appears to originate from client-side JavaScript (based on the call stack)
The app.js file with the mysterious functions is the built Next.js bundle
Questions
Where could this /appleauth/auth/federate endpoint be coming from?
Why is the browser making this POST request instead of following Apple's redirect to my configured callback URL?
Could this be related to the response_mode: 'form_post' in the Apple Passport strategy?
Is there something in the Apple Developer Primary App ID configuration that could trigger this behavior?
Could this be a Next.js build artifact or some hidden dependency?
The mysterious call stack references (Ee.isFederated, _callAuthFederate) suggest some library is intercepting the Apple authentication flow, but I cannot identify what library or where it's being loaded from. The minified function names suggest federated authentication, but I have no such libraries in my dependencies.
Has anyone encountered similar issues with Apple Sign-In where an unexpected endpoint is being called?