Fix Microsoft login with signing keys that omit alg

examples/better-auth/README.md

@@ -127,3 +127,10 @@ only after these Microsoft-specific verification checks pass.
 
 See [Microsoft's claim reference](https://learn.microsoft.com/en-us/entra/identity-platform/optional-claims-reference)
 and [Better Auth's provider setup](https://better-auth.com/docs/authentication/microsoft).
+
+Microsoft signing keys are loaded with JOSE's remote JWKS resolver because
+Microsoft's public RSA keys omit the optional `alg` member. Better Auth 1.7.3's
+Microsoft key importer requires that member and otherwise throws. pgstencil pins
+RS256 and the Microsoft key endpoint, retaining signature, nonce, audience, issuer,
+expiry and tenant/object identity checks. Local Microsoft fixtures also omit `alg`
+so this production key shape is exercised in Node and Workers tests.

packages/auth/package.json

@@ -24,11 +24,12 @@
     "./better-auth-workers": "./src/better-auth-workers.ts"
   },
   "dependencies": {
+    "better-auth": "1.7.3",
     "hono": "^4.13.7",
+    "jose": "6.2.12",
     "kysely": "0.29.5",
     "openid-client": "6.8.8",
-    "pgstencil": "workspace:*",
-    "better-auth": "1.7.3"
+    "pgstencil": "workspace:*"
   },
   "engines": {
     "node": ">=24"

packages/auth/src/better-auth-oauth.ts

@@ -6,6 +6,7 @@ import {
 } from 'pgstencil/diagnostics';
 import type { BetterAuthOptions, BetterAuthPlugin } from 'better-auth';
 import { verifyProviderIdToken } from 'better-auth/oauth2';
+import { createRemoteJWKSet } from 'jose';
 import type { GithubProfile } from 'better-auth/social-providers';
 import { makeSignature } from 'better-auth/crypto';
 import { sql } from 'kysely';
@@ -66,6 +67,16 @@ export const verifiedOidc: BetterAuthPlugin = {
             : 'https://appleid.apple.com';
       if (provider.idToken && 'jwks' in provider.idToken) {
         provider.idToken.algorithms = ['RS256'];
+        if (provider.id === 'microsoft') {
+          // Microsoft omits JWK.alg. Better Auth 1.7.3 passes that missing value
+          // to importJWK, which throws. JOSE selects/imports the key using the
+          // protected header; the verifier still permits only RS256 above.
+          provider.idToken.jwks = createRemoteJWKSet(
+            new URL(
+              'https://login.microsoftonline.com/common/discovery/v2.0/keys',
+            ),
+          );
+        }
         if (typeof provider.idToken.jwks === 'function') {
           const keys = provider.idToken.jwks;
           provider.idToken.jwks = async (...args) => {

tests/support/oauth-server.ts

@@ -48,7 +48,8 @@ const jwk = {
 export const endpointPaths: Record<string, string> = {
   'https://login.microsoftonline.com/common/oauth2/v2.0/token':
     '/microsoft/token',
-  'https://login.microsoftonline.com/common/discovery/v2.0/keys': '/keys',
+  'https://login.microsoftonline.com/common/discovery/v2.0/keys':
+    '/microsoft/keys',
   'https://appleid.apple.com/.well-known/openid-configuration':
     '/apple/discovery',
   'https://appleid.apple.com/auth/token': '/apple/token',
@@ -128,6 +129,11 @@ export async function mockOAuthServer(
         });
       }
       if (url.pathname === '/keys') return json({ keys: [jwk] });
+      if (url.pathname === '/microsoft/keys') {
+        // Microsoft's real RSA signing keys omit the optional alg member.
+        const { alg: _alg, ...microsoftKey } = jwk;
+        return json({ keys: [microsoftKey] });
+      }
       if (url.pathname.endsWith('/token')) {
         const form = new URLSearchParams(body);
         const code = form.get('code') ?? '';