diff --git a/app/Services/Auth/OauthLoginService.php b/app/Services/Auth/OauthLoginService.php
index 6b7acbc5b..c093e290f 100644
--- a/app/Services/Auth/OauthLoginService.php
+++ b/app/Services/Auth/OauthLoginService.php
@@ -36,7 +36,11 @@ public function login(string $provider, object $oauthUser, OauthSetting $oauthSe
     private function resolveOauthUser(object $oauthUser, OauthSetting $oauthSetting, string $email): User
     {
         $provider = $oauthSetting->provider;
-        $providerUserId = (string) $oauthUser->id;
+        $providerUserId = $oauthUser->id ?? null;
+        if (! is_scalar($providerUserId) || trim((string) $providerUserId) === '') {
+            throw new HttpException(403, 'OAuth provider did not return a valid user ID');
+        }
+        $providerUserId = (string) $providerUserId;
         $rawClaims = is_array($oauthUser->user ?? null) ? $oauthUser->user : [];
 
         return DB::transaction(function () use ($oauthUser, $oauthSetting, $email, $provider, $providerUserId, $rawClaims) {
diff --git a/tests/Feature/OauthControllerTest.php b/tests/Feature/OauthControllerTest.php
index 62940b0ef..f0225e335 100644
--- a/tests/Feature/OauthControllerTest.php
+++ b/tests/Feature/OauthControllerTest.php
@@ -4,9 +4,11 @@
 use App\Models\OauthIdentity;
 use App\Models\OauthSetting;
 use App\Models\User;
+use App\Services\Auth\OauthLoginService;
 use Illuminate\Foundation\Testing\RefreshDatabase;
 use Illuminate\Support\Once;
 use Laravel\Socialite\Facades\Socialite;
+use Symfony\Component\HttpKernel\Exception\HttpException;
 
 uses(RefreshDatabase::class);
 
@@ -121,3 +123,31 @@
     'malformed email' => ['not-an-email'],
     'missing domain' => ['user@'],
 ]);
+
+it('rejects oauth logins when the provider does not return a valid user id', function (mixed $invalidId) {
+    $oauthUser = (object) [
+        'email' => 'user@example.edu',
+        'name' => 'Example User',
+    ];
+
+    if ($invalidId !== 'missing') {
+        $oauthUser->id = $invalidId;
+    }
+
+    try {
+        app(OauthLoginService::class)->login('google', $oauthUser, OauthSetting::where('provider', 'google')->firstOrFail());
+    } catch (HttpException $exception) {
+        expect($exception->getStatusCode())->toBe(403)
+            ->and(OauthIdentity::count())->toBe(0)
+            ->and(User::count())->toBe(0);
+
+        return;
+    }
+
+    $this->fail('Expected an invalid OAuth provider user ID to be rejected.');
+})->with([
+    'null id' => [null],
+    'missing id' => ['missing'],
+    'blank id' => ['   '],
+    'non-scalar id' => [[]],
+]);