diff --git a/src/js/logic/digital-sign-pdf.ts b/src/js/logic/digital-sign-pdf.ts
index 59729b5a..9fabc75a 100644
--- a/src/js/logic/digital-sign-pdf.ts
+++ b/src/js/logic/digital-sign-pdf.ts
@@ -103,8 +103,34 @@ export function parseCombinedPem(
  * and set VITE_CORS_PROXY_URL environment variable.
  *
  * If not set, certificates requiring external chain fetching will fail.
+ *
+ * On the official BentoPDF domains we fall back to the project-operated proxy.
+ * The generated CSP already allows that origin by default (see
+ * scripts/generate-security-headers.mjs), but the runtime never used it, which
+ * left the timestamp tool broken on the official HTTPS site. The fallback is
+ * deliberately scoped to the official domains so self-hosted instances keep
+ * requiring their own proxy.
  */
-const CORS_PROXY_URL = import.meta.env.VITE_CORS_PROXY_URL || '';
+const DEFAULT_CORS_PROXY_URL =
+  'https://bentopdf-cors-proxy.bentopdf.workers.dev';
+
+const OFFICIAL_HOSTNAMES = new Set(['bentopdf.com', 'www.bentopdf.com']);
+
+function resolveCorsProxyUrl(): string {
+  const configured = import.meta.env.VITE_CORS_PROXY_URL || '';
+  if (configured) {
+    return configured;
+  }
+
+  if (typeof window === 'undefined') {
+    return '';
+  }
+
+  const hostname = window.location?.hostname?.toLowerCase() ?? '';
+  return OFFICIAL_HOSTNAMES.has(hostname) ? DEFAULT_CORS_PROXY_URL : '';
+}
+
+const CORS_PROXY_URL = resolveCorsProxyUrl();
 
 /**
  * Shared secret for signing proxy requests (HMAC-SHA256).
diff --git a/src/tests/digital-sign-pdf.test.ts b/src/tests/digital-sign-pdf.test.ts
index 799a28b0..d6819a14 100644
--- a/src/tests/digital-sign-pdf.test.ts
+++ b/src/tests/digital-sign-pdf.test.ts
@@ -143,7 +143,11 @@ describe('timestampPdf', () => {
     vi.stubEnv('VITE_CORS_PROXY_URL', '');
     Object.defineProperty(window, 'location', {
       configurable: true,
-      value: { protocol: 'https:', origin: 'https://www.bentopdf.com' },
+      value: {
+        protocol: 'https:',
+        origin: 'https://selfhost.example.com',
+        hostname: 'selfhost.example.com',
+      },
     });
     vi.resetModules();
     const { timestampPdf: freshTimestamp } =
@@ -153,4 +157,32 @@ describe('timestampPdf', () => {
       freshTimestamp(samplePdfBytes, 'http://timestamp.digicert.com')
     ).rejects.toThrow(/HTTPS page|VITE_CORS_PROXY_URL/);
   });
+
+  it('should fall back to the default proxy on official domains', async () => {
+    vi.stubEnv('VITE_CORS_PROXY_URL', '');
+    Object.defineProperty(window, 'location', {
+      configurable: true,
+      value: {
+        protocol: 'https:',
+        origin: 'https://www.bentopdf.com',
+        hostname: 'www.bentopdf.com',
+      },
+    });
+    vi.resetModules();
+    const { timestampPdf: freshTimestamp } =
+      await import('@/js/logic/digital-sign-pdf');
+
+    mockSign.mockResolvedValueOnce(new Uint8Array([1]));
+    await freshTimestamp(samplePdfBytes, 'http://timestamp.digicert.com');
+
+    const callArg = vi.mocked(PdfSigner).mock.calls[0][0] as {
+      signdate: { url: string };
+    };
+    expect(callArg.signdate.url).toMatch(
+      /^https:\/\/bentopdf-cors-proxy\.bentopdf\.workers\.dev\?url=/
+    );
+    expect(callArg.signdate.url).toContain(
+      encodeURIComponent('http://timestamp.digicert.com')
+    );
+  });
 });