diff --git a/.env.example b/.env.example
index 70d62ee2eb..05cee19c8f 100644
--- a/.env.example
+++ b/.env.example
@@ -6,10 +6,14 @@ NEXTAUTH_SECRET="secret"
 
 # [[CRYPTO]]
 # Application Key for symmetric encryption and decryption
-# REQUIRED: This should be a random string of at least 32 characters
-NEXT_PRIVATE_ENCRYPTION_KEY="CAFEBABE"
-# REQUIRED: This should be a random string of at least 32 characters
-NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY="DEADBEEF"
+# REQUIRED: a random string of at least 32 characters. The server refuses to
+# boot when these are missing, shorter than 32 characters, equal to each other,
+# or still set to the published placeholders CAFEBABE / DEADBEEF.
+# Generate one with: openssl rand -base64 32
+NEXT_PRIVATE_ENCRYPTION_KEY=""
+# REQUIRED: a random string of at least 32 characters, different from the above.
+# Generate one with: openssl rand -base64 32
+NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY=""
 
 # [[AUTH OPTIONAL]]
 # Find documentation on setting up Google OAuth here:
@@ -35,7 +39,10 @@ NEXT_PRIVATE_OIDC_PROMPT="login"
 NEXT_PUBLIC_WEBAPP_URL="http://localhost:3000"
 # URL used by the web app to request itself (e.g. local background jobs)
 NEXT_PRIVATE_INTERNAL_WEBAPP_URL="http://localhost:3000"
-# OPTIONAL: Comma-separated hostnames or IPs whose webhooks are allowed to resolve to private/loopback addresses. (e.g., internal.example.com,192.168.1.5).
+# OPTIONAL: Comma-separated hostnames or IPs allowed to resolve to private/loopback addresses. (e.g., internal.example.com,192.168.1.5).
+# Applies to outbound webhook calls AND to OpenID Connect discovery
+# (wellKnownUrl), so a self-hosted identity provider such as a Keycloak on
+# localhost must be listed here or SSO configuration will be rejected.
 NEXT_PRIVATE_WEBHOOK_SSRF_BYPASS_HOSTS=
 
 # [[SERVER]]
diff --git a/apps/remix/server/router.ts b/apps/remix/server/router.ts
index c6212455b6..14a06cf2ab 100644
--- a/apps/remix/server/router.ts
+++ b/apps/remix/server/router.ts
@@ -1,6 +1,7 @@
 import { tsRestHonoApp } from '@documenso/api/hono';
 import { auth } from '@documenso/auth/server';
 import { csc } from '@documenso/ee/server-only/signing/csc/hono';
+import { assertEncryptionKeysConfigured } from '@documenso/lib/constants/crypto';
 import { jobsClient } from '@documenso/lib/jobs/client';
 import { LicenseClient } from '@documenso/lib/server-only/license/license-client';
 import { createRateLimitMiddleware } from '@documenso/lib/server-only/rate-limit/rate-limit-middleware';
@@ -41,6 +42,13 @@ import { reactRouterTrpcServer } from './trpc/hono-trpc-remix';
 // output to wire it into the React Router adapter.
 export { getLoadContext } from './load-context';
 
+// Refuse to serve traffic with encryption keys that are missing, still set to a
+// published placeholder, or too short to survive an offline attack. Everything
+// this app describes as encrypted at rest - DKIM private keys, SSO client
+// secrets, account-link tokens - depends on these two values, and a weak key
+// cannot be detected after data has already been written with it.
+assertEncryptionKeysConfigured();
+
 export interface HonoEnv {
   Variables: RequestIdVariables & {
     context: AppContext;
diff --git a/docker/testing/compose.yml b/docker/testing/compose.yml
index 7251ee523c..e9a97161f8 100644
--- a/docker/testing/compose.yml
+++ b/docker/testing/compose.yml
@@ -35,8 +35,11 @@ services:
       - ../../.env.example
     environment:
       - NEXTAUTH_SECRET=secret
-      - NEXT_PRIVATE_ENCRYPTION_KEY=CAFEBABE
-      - NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY=DEADBEEF
+      # Test-only values. Must satisfy the boot guard in
+      # packages/lib/constants/crypto.ts: at least 32 characters, not equal to
+      # each other, and not one of the published placeholders.
+      - NEXT_PRIVATE_ENCRYPTION_KEY=testing-only-encryption-key-0123456789
+      - NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY=testing-only-secondary-key-0123456789
       - NEXT_PRIVATE_DATABASE_URL=postgres://documenso:password@database:5432/documenso
       - NEXT_PRIVATE_DIRECT_DATABASE_URL=postgres://documenso:password@database:5432/documenso
       - NEXT_PUBLIC_UPLOAD_TRANSPORT=database
diff --git a/packages/auth/server/lib/utils/open-id.ts b/packages/auth/server/lib/utils/open-id.ts
index cd89c241dd..ca45e830ee 100644
--- a/packages/auth/server/lib/utils/open-id.ts
+++ b/packages/auth/server/lib/utils/open-id.ts
@@ -1,3 +1,4 @@
+import { assertNotPrivateUrl } from '@documenso/lib/server-only/webhooks/assert-webhook-url';
 import { z } from 'zod';
 
 const ZOpenIdConfigurationSchema = z.object({
@@ -17,6 +18,21 @@ export const getOpenIdConfiguration = async (
   wellKnownUrl: string,
   _options: GetOpenIdConfigurationOptions = {},
 ): Promise<OpenIdConfiguration> => {
+  // The discovery URL is operator-supplied — it comes from an organisation's
+  // authentication portal row or from NEXT_PRIVATE_OIDC_WELL_KNOWN — so it is
+  // treated like any other outbound target. A self-hosted identity provider on a
+  // private address must be listed in NEXT_PRIVATE_WEBHOOK_SSRF_BYPASS_HOSTS.
+  //
+  // Best-effort, like every caller of this helper: it resolves DNS separately
+  // from the fetch below so it does not defeat rebinding, and it fails open on
+  // lookup errors or timeouts. Egress filtering at the deployment level remains
+  // the real control.
+  try {
+    await assertNotPrivateUrl(wellKnownUrl);
+  } catch (error) {
+    throw new Error('OIDC discovery URL resolves to a private or loopback address', { cause: error });
+  }
+
   const response = await fetch(wellKnownUrl);
 
   if (!response.ok) {
diff --git a/packages/lib/constants/crypto.test.ts b/packages/lib/constants/crypto.test.ts
new file mode 100644
index 0000000000..9d3ba32258
--- /dev/null
+++ b/packages/lib/constants/crypto.test.ts
@@ -0,0 +1,82 @@
+import { describe, expect, it, vi } from 'vitest';
+
+import { describeEncryptionKeyProblem, MINIMUM_ENCRYPTION_KEY_LENGTH } from './crypto';
+
+const VALID_KEY = 'a'.repeat(MINIMUM_ENCRYPTION_KEY_LENGTH);
+const VALID_SECONDARY_KEY = 'b'.repeat(MINIMUM_ENCRYPTION_KEY_LENGTH);
+
+describe('describeEncryptionKeyProblem', () => {
+  it('accepts two distinct keys that meet the minimum length', () => {
+    expect(describeEncryptionKeyProblem(VALID_KEY, VALID_SECONDARY_KEY)).toBeNull();
+  });
+
+  it('accepts keys longer than the minimum', () => {
+    expect(describeEncryptionKeyProblem(`${VALID_KEY}extra`, `${VALID_SECONDARY_KEY}extra`)).toBeNull();
+  });
+
+  it('rejects a missing primary key', () => {
+    expect(describeEncryptionKeyProblem(undefined, VALID_SECONDARY_KEY)).toContain('both required');
+  });
+
+  it('rejects an empty secondary key', () => {
+    expect(describeEncryptionKeyProblem(VALID_KEY, '')).toContain('both required');
+  });
+
+  it('rejects the published placeholder values in either slot', () => {
+    expect(describeEncryptionKeyProblem('CAFEBABE', 'DEADBEEF')).toContain('placeholder');
+    expect(describeEncryptionKeyProblem(VALID_KEY, 'DEADBEEF')).toContain('placeholder');
+    expect(describeEncryptionKeyProblem('CAFEBABE', VALID_SECONDARY_KEY)).toContain('placeholder');
+  });
+
+  it('rejects a key one character below the minimum', () => {
+    const tooShort = 'a'.repeat(MINIMUM_ENCRYPTION_KEY_LENGTH - 1);
+
+    expect(describeEncryptionKeyProblem(tooShort, VALID_SECONDARY_KEY)).toContain(
+      `at least ${MINIMUM_ENCRYPTION_KEY_LENGTH} characters`,
+    );
+  });
+
+  it('rejects identical keys so that rotation can tell old ciphertext from new', () => {
+    expect(describeEncryptionKeyProblem(VALID_KEY, VALID_KEY)).toContain('must differ');
+  });
+
+  it('reports the placeholder problem before the length problem, since it is the more specific one', () => {
+    // CAFEBABE is also shorter than the minimum; naming it as a placeholder is
+    // what tells the operator they copied a shipped default rather than typed a
+    // short secret.
+    expect(describeEncryptionKeyProblem('CAFEBABE', VALID_SECONDARY_KEY)).not.toContain('at least');
+  });
+});
+
+describe('assertEncryptionKeysConfigured', () => {
+  it('refuses to start when the process environment still holds the shipped defaults', async () => {
+    vi.stubEnv('NEXT_PRIVATE_ENCRYPTION_KEY', 'CAFEBABE');
+    vi.stubEnv('NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY', 'DEADBEEF');
+    vi.resetModules();
+
+    try {
+      const reloaded = await import('./crypto');
+
+      expect(() => reloaded.assertEncryptionKeysConfigured()).toThrow(/Refusing to start/);
+      expect(() => reloaded.assertEncryptionKeysConfigured()).toThrow(/placeholder/);
+    } finally {
+      vi.unstubAllEnvs();
+      vi.resetModules();
+    }
+  });
+
+  it('starts when the process environment holds usable keys', async () => {
+    vi.stubEnv('NEXT_PRIVATE_ENCRYPTION_KEY', VALID_KEY);
+    vi.stubEnv('NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY', VALID_SECONDARY_KEY);
+    vi.resetModules();
+
+    try {
+      const reloaded = await import('./crypto');
+
+      expect(() => reloaded.assertEncryptionKeysConfigured()).not.toThrow();
+    } finally {
+      vi.unstubAllEnvs();
+      vi.resetModules();
+    }
+  });
+});
diff --git a/packages/lib/constants/crypto.ts b/packages/lib/constants/crypto.ts
index 8bf232632f..cff1ae608c 100644
--- a/packages/lib/constants/crypto.ts
+++ b/packages/lib/constants/crypto.ts
@@ -4,24 +4,64 @@ export const DOCUMENSO_ENCRYPTION_KEY = env('NEXT_PRIVATE_ENCRYPTION_KEY');
 
 export const DOCUMENSO_ENCRYPTION_SECONDARY_KEY = env('NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY');
 
-// if (typeof window === 'undefined') {
-//   if (!DOCUMENSO_ENCRYPTION_KEY || !DOCUMENSO_ENCRYPTION_SECONDARY_KEY) {
-//     throw new Error('Missing DOCUMENSO_ENCRYPTION_KEY or DOCUMENSO_ENCRYPTION_SECONDARY_KEY keys');
-//   }
-
-//   if (DOCUMENSO_ENCRYPTION_KEY === DOCUMENSO_ENCRYPTION_SECONDARY_KEY) {
-//     throw new Error(
-//       'DOCUMENSO_ENCRYPTION_KEY and DOCUMENSO_ENCRYPTION_SECONDARY_KEY cannot be equal',
-//     );
-//   }
-// }
-
-// if (DOCUMENSO_ENCRYPTION_KEY === 'CAFEBABE') {
-//   console.warn('*********************************************************************');
-//   console.warn('*');
-//   console.warn('*');
-//   console.warn('Please change the encryption key from the default value of "CAFEBABE"');
-//   console.warn('*');
-//   console.warn('*');
-//   console.warn('*********************************************************************');
-// }
+/**
+ * Both values are used as symmetric AEAD keys (see `universal/crypto.ts`).
+ * A short key is not merely weak: anyone who can read a single ciphertext can
+ * brute-force the key offline and then decrypt everything else protected by it,
+ * which in this app includes DKIM private keys and SSO client secrets.
+ */
+export const MINIMUM_ENCRYPTION_KEY_LENGTH = 32;
+
+/**
+ * Placeholder values that `.env.example` used to ship and that
+ * `docker/Dockerfile` still bakes in as build-time ENV defaults. They are public
+ * knowledge, so an instance running on them has no encryption at all.
+ */
+const PLACEHOLDER_ENCRYPTION_KEYS = ['CAFEBABE', 'DEADBEEF'];
+
+const GENERATE_HINT = 'Generate one with: openssl rand -base64 32';
+
+/**
+ * Explain why the configured encryption keys are unusable, or return null when
+ * they are acceptable. Pure and parameterised so it can be tested without
+ * touching process.env or re-importing the module.
+ */
+export const describeEncryptionKeyProblem = (
+  key: string | undefined,
+  secondaryKey: string | undefined,
+): string | null => {
+  if (!key || !secondaryKey) {
+    return `NEXT_PRIVATE_ENCRYPTION_KEY and NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY are both required. ${GENERATE_HINT}`;
+  }
+
+  if (PLACEHOLDER_ENCRYPTION_KEYS.includes(key) || PLACEHOLDER_ENCRYPTION_KEYS.includes(secondaryKey)) {
+    return `An encryption key is still set to a published placeholder (${PLACEHOLDER_ENCRYPTION_KEYS.join(' / ')}), which is not a secret. ${GENERATE_HINT}`;
+  }
+
+  if (key.length < MINIMUM_ENCRYPTION_KEY_LENGTH || secondaryKey.length < MINIMUM_ENCRYPTION_KEY_LENGTH) {
+    return `Both encryption keys must be at least ${MINIMUM_ENCRYPTION_KEY_LENGTH} characters. ${GENERATE_HINT}`;
+  }
+
+  if (key === secondaryKey) {
+    return 'NEXT_PRIVATE_ENCRYPTION_KEY and NEXT_PRIVATE_ENCRYPTION_SECONDARY_KEY must differ, otherwise rotation cannot tell old ciphertext from new.';
+  }
+
+  return null;
+};
+
+/**
+ * Refuse to serve traffic with unusable encryption keys.
+ *
+ * Called from the server entry point rather than at module load on purpose: the
+ * Docker image carries the placeholder values as ENV defaults, so throwing while
+ * modules are being imported would fail the image build itself, and every test
+ * or one-off script that imports this module would need a full server
+ * environment to do so.
+ */
+export const assertEncryptionKeysConfigured = (): void => {
+  const problem = describeEncryptionKeyProblem(DOCUMENSO_ENCRYPTION_KEY, DOCUMENSO_ENCRYPTION_SECONDARY_KEY);
+
+  if (problem) {
+    throw new Error(`Refusing to start: ${problem}`);
+  }
+};