diff --git a/app/(main)/[locale]/login/page.tsx b/app/(main)/[locale]/login/page.tsx
index 953195f4..563fbefe 100644
--- a/app/(main)/[locale]/login/page.tsx
+++ b/app/(main)/[locale]/login/page.tsx
@@ -14,7 +14,7 @@ import { useConfig } from "@/hooks/use-config";
 import { apiFetch, getPathPrefix, withBasePath } from "@/lib/browser-navigation";
 import { cn } from "@/lib/utils";
 import { AlertCircle, Loader2, X, Info, Eye, EyeOff, LogIn, Sun, Moon, Monitor, Check, Shield, Play, Copy } from "lucide-react";
-import { discoverOAuth, type OAuthMetadata } from "@/lib/oauth/discovery";
+import { type OAuthMetadata } from "@/lib/oauth/discovery";
 import { generateCodeVerifier, generateCodeChallenge, generateState } from "@/lib/oauth/pkce";
 import { useUpdateStore, selectBanner } from "@/stores/update-store";
 import type { PublicJmapServerEntry } from "@/lib/admin/jmap-servers";
@@ -329,16 +329,27 @@ export default function LoginPage() {
     if (!oauthEnabled || !serverUrl) return;
     setOauthDiscoveryDone(false);
     setOauthMetadata(null);
-    discoverOAuth(effectiveOauthIssuerUrl || serverUrl)
+    const controller = new AbortController();
+    // Discover via our own origin rather than fetching the IdP's /.well-known/*
+    // documents directly from the browser. A direct cross-origin discovery
+    // fetch is subject to CORS, and providers like Authentik serve those
+    // documents without Access-Control-Allow-Origin, so the browser blocks the
+    // response and login breaks (issue #382). The proxy runs discovery server
+    // side where CORS does not apply.
+    const query = selectedServer?.id ? `?server_id=${encodeURIComponent(selectedServer.id)}` : "";
+    apiFetch(`/api/auth/oauth/metadata${query}`, { signal: controller.signal })
+      .then(async (res) => (res.ok ? ((await res.json()) as OAuthMetadata) : null))
       .then((metadata) => {
         setOauthMetadata(metadata);
         setOauthDiscoveryDone(true);
       })
-      .catch(() => {
+      .catch((err) => {
+        if (err?.name === "AbortError") return;
         setOauthMetadata(null);
         setOauthDiscoveryDone(true);
       });
-  }, [oauthEnabled, serverUrl, effectiveOauthIssuerUrl]);
+    return () => controller.abort();
+  }, [oauthEnabled, serverUrl, effectiveOauthIssuerUrl, selectedServer?.id]);
 
   // Auto-SSO: when enabled with OAUTH_ONLY, skip the login page entirely
   const ssoError = searchParams.get("sso_error");
diff --git a/app/api/auth/oauth/metadata/route.ts b/app/api/auth/oauth/metadata/route.ts
new file mode 100644
index 00000000..a720a18a
--- /dev/null
+++ b/app/api/auth/oauth/metadata/route.ts
@@ -0,0 +1,53 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { configManager } from '@/lib/admin/config-manager';
+import { getMetadata, getRequiredConfig } from '@/lib/oauth/token-exchange';
+
+/**
+ * Same-origin OAuth metadata (discovery) proxy.
+ *
+ * The login page needs the authorization_endpoint to build the PKCE authorize
+ * URL in the browser. Discovering it directly from the browser means a
+ * cross-origin fetch to the IdP's /.well-known/* documents, which is subject
+ * to CORS: providers like Authentik serve those documents without an
+ * Access-Control-Allow-Origin header, so the browser blocks the response and
+ * discovery fails (issue #382). Performing discovery here - server to server,
+ * where CORS does not apply - and handing the result back as a same-origin
+ * response sidesteps the problem entirely.
+ *
+ * The discovery URL is resolved from admin config (via server_id), never from
+ * client input, so this cannot be abused as an open SSRF proxy. Endpoint URLs
+ * in the discovered document are still gated by the SSRF validator inside
+ * discoverOAuth. The returned fields are public well-known metadata.
+ */
+export async function GET(request: NextRequest) {
+  await configManager.ensureLoaded();
+  const serverId = request.nextUrl.searchParams.get('server_id');
+
+  let discoveryUrl: string;
+  try {
+    ({ discoveryUrl } = getRequiredConfig(serverId));
+  } catch {
+    // OAuth not configured for this server - surface as "no metadata" rather
+    // than a 500 so the login page just hides the SSO button.
+    return NextResponse.json({ error: 'OAuth not configured' }, { status: 404 });
+  }
+
+  try {
+    const metadata = await getMetadata(serverId);
+    if (!metadata?.authorization_endpoint || !metadata.token_endpoint) {
+      logger.warn('OAuth metadata discovery returned no usable endpoints', { discoveryUrl });
+      return NextResponse.json({ error: 'OAuth discovery failed' }, { status: 502 });
+    }
+    return NextResponse.json(metadata, {
+      // Mirror the in-process discovery cache TTL so repeated login-page loads
+      // hit the CDN/browser cache instead of re-running discovery.
+      headers: { 'Cache-Control': 'private, max-age=600' },
+    });
+  } catch (error) {
+    logger.error('OAuth metadata discovery error', {
+      error: error instanceof Error ? error.message : 'Unknown error',
+    });
+    return NextResponse.json({ error: 'OAuth discovery failed' }, { status: 502 });
+  }
+}
diff --git a/lib/__tests__/oauth-metadata-route.test.ts b/lib/__tests__/oauth-metadata-route.test.ts
new file mode 100644
index 00000000..04540209
--- /dev/null
+++ b/lib/__tests__/oauth-metadata-route.test.ts
@@ -0,0 +1,101 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import type { OAuthMetadata } from '../oauth/discovery';
+
+// Capture status/headers from NextResponse.json (the shared config-route mock
+// drops them, but this route's behavior depends on the status code).
+vi.mock('next/server', () => ({
+  NextResponse: {
+    json: (data: unknown, init?: { status?: number; headers?: Record<string, string> }) => ({
+      json: async () => data,
+      status: init?.status ?? 200,
+      headers: init?.headers ?? {},
+    }),
+  },
+}));
+
+vi.mock('@/lib/logger', () => ({
+  logger: { warn: vi.fn(), error: vi.fn(), debug: vi.fn() },
+}));
+
+vi.mock('@/lib/admin/config-manager', () => ({
+  configManager: { ensureLoaded: vi.fn().mockResolvedValue(undefined) },
+}));
+
+const getMetadata = vi.fn();
+const getRequiredConfig = vi.fn();
+vi.mock('@/lib/oauth/token-exchange', () => ({
+  getMetadata: (...args: unknown[]) => getMetadata(...args),
+  getRequiredConfig: (...args: unknown[]) => getRequiredConfig(...args),
+}));
+
+const VALID_METADATA: OAuthMetadata = {
+  issuer: 'https://auth.example.com',
+  authorization_endpoint: 'https://auth.example.com/authorize',
+  token_endpoint: 'https://auth.example.com/token',
+};
+
+function mockRequest(serverId?: string): unknown {
+  return {
+    nextUrl: {
+      searchParams: { get: (k: string) => (k === 'server_id' ? serverId ?? null : null) },
+    },
+  };
+}
+
+async function callRoute(serverId?: string) {
+  const { GET } = await import('@/app/api/auth/oauth/metadata/route');
+  const res = (await GET(mockRequest(serverId) as Parameters<typeof GET>[0])) as unknown as {
+    status: number;
+    headers: Record<string, string>;
+    json: () => Promise<Record<string, unknown>>;
+  };
+  return { status: res.status, headers: res.headers, body: await res.json() };
+}
+
+describe('oauth metadata route', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    getRequiredConfig.mockReturnValue({ discoveryUrl: 'https://auth.example.com' });
+  });
+
+  it('returns discovered metadata for the resolved server', async () => {
+    getMetadata.mockResolvedValue(VALID_METADATA);
+
+    const { status, body, headers } = await callRoute('server-1');
+
+    expect(status).toBe(200);
+    expect(body).toEqual(VALID_METADATA);
+    expect(headers['Cache-Control']).toContain('max-age=600');
+    expect(getMetadata).toHaveBeenCalledWith('server-1');
+  });
+
+  it('returns 404 when OAuth is not configured', async () => {
+    getRequiredConfig.mockImplementation(() => {
+      throw new Error('OAuth misconfigured: OAUTH_CLIENT_ID not set');
+    });
+
+    const { status, body } = await callRoute();
+
+    expect(status).toBe(404);
+    expect(body).toEqual({ error: 'OAuth not configured' });
+    expect(getMetadata).not.toHaveBeenCalled();
+  });
+
+  it('returns 502 when discovery yields no usable endpoints', async () => {
+    getMetadata.mockResolvedValue(null);
+
+    const { status, body } = await callRoute();
+
+    expect(status).toBe(502);
+    expect(body).toEqual({ error: 'OAuth discovery failed' });
+  });
+
+  it('returns 502 when discovery throws', async () => {
+    getMetadata.mockRejectedValue(new Error('network down'));
+
+    const { status, body } = await callRoute();
+
+    expect(status).toBe(502);
+    expect(body).toEqual({ error: 'OAuth discovery failed' });
+  });
+});