diff --git a/app/[locale]/auth/callback/page.tsx b/app/[locale]/auth/callback/page.tsx
index 3bc12379..bb8c2896 100644
--- a/app/[locale]/auth/callback/page.tsx
+++ b/app/[locale]/auth/callback/page.tsx
@@ -13,7 +13,7 @@ function OAuthCallbackInner() {
   const params = useParams();
   const searchParams = useSearchParams();
   const t = useTranslations("login");
-  const { loginWithOAuth } = useAuthStore();
+  const { loginWithOAuth, loginWithServerSso } = useAuthStore();
   const [error, setError] = useState<string | null>(null);
 
   useEffect(() => {
@@ -32,44 +32,71 @@ function OAuthCallbackInner() {
     }
 
     const savedState = sessionStorage.getItem("oauth_state");
-    if (!state || state !== savedState) {
-      setError("invalid_state");
-      return;
-    }
 
-    const codeVerifier = sessionStorage.getItem("oauth_code_verifier");
-    const serverUrl = sessionStorage.getItem("oauth_server_url");
+    if (savedState) {
+      // Classic flow — sessionStorage has the PKCE state (same-tab OAuth)
+      if (!state || state !== savedState) {
+        setError("invalid_state");
+        return;
+      }
 
-    if (!codeVerifier || !serverUrl) {
-      setError("missing_params");
-      return;
-    }
+      const codeVerifier = sessionStorage.getItem("oauth_code_verifier");
+      const serverUrl = sessionStorage.getItem("oauth_server_url");
 
-    const redirectUri = `${window.location.origin}/${params.locale}/auth/callback`;
+      if (!codeVerifier || !serverUrl) {
+        setError("missing_params");
+        return;
+      }
 
-    loginWithOAuth(serverUrl, code, codeVerifier, redirectUri)
-      .then((success) => {
-        if (success) {
-          sessionStorage.removeItem("oauth_state");
-          sessionStorage.removeItem("oauth_code_verifier");
-          sessionStorage.removeItem("oauth_server_url");
-          sessionStorage.removeItem("oauth_add_account_mode");
-          let redirectTo = `/${params.locale}`;
-          try {
-            const saved = sessionStorage.getItem('redirect_after_login');
-            if (saved) {
-              sessionStorage.removeItem('redirect_after_login');
-              redirectTo = saved;
-            }
-          } catch { /* sessionStorage may be unavailable */ }
-          router.push(redirectTo);
-        } else {
+      const redirectUri = `${window.location.origin}/${params.locale}/auth/callback`;
+
+      loginWithOAuth(serverUrl, code, codeVerifier, redirectUri)
+        .then((success) => {
+          if (success) {
+            sessionStorage.removeItem("oauth_state");
+            sessionStorage.removeItem("oauth_code_verifier");
+            sessionStorage.removeItem("oauth_server_url");
+            sessionStorage.removeItem("oauth_add_account_mode");
+            let redirectTo = `/${params.locale}`;
+            try {
+              const saved = sessionStorage.getItem('redirect_after_login');
+              if (saved) {
+                sessionStorage.removeItem('redirect_after_login');
+                redirectTo = saved;
+              }
+            } catch { /* sessionStorage may be unavailable */ }
+            router.push(redirectTo);
+          } else {
+            setError("token_exchange_failed");
+          }
+        })
+        .catch(() => {
           setError("token_exchange_failed");
-        }
-      })
-      .catch(() => {
-        setError("token_exchange_failed");
-      });
+        });
+    } else if (state) {
+      // Server-side SSO flow — state was stored in encrypted httpOnly cookie
+      loginWithServerSso(code, state)
+        .then((success) => {
+          if (success) {
+            let redirectTo = `/${params.locale}`;
+            try {
+              const saved = sessionStorage.getItem('redirect_after_login');
+              if (saved) {
+                sessionStorage.removeItem('redirect_after_login');
+                redirectTo = saved;
+              }
+            } catch { /* sessionStorage may be unavailable */ }
+            router.push(redirectTo);
+          } else {
+            setError("token_exchange_failed");
+          }
+        })
+        .catch(() => {
+          setError("token_exchange_failed");
+        });
+    } else {
+      setError("invalid_state");
+    }
   }, []); // eslint-disable-line react-hooks/exhaustive-deps
 
   if (error) {
diff --git a/app/[locale]/layout.tsx b/app/[locale]/layout.tsx
index b5578732..825ffdc2 100644
--- a/app/[locale]/layout.tsx
+++ b/app/[locale]/layout.tsx
@@ -2,6 +2,7 @@ import { notFound } from "next/navigation";
 import { IntlProvider } from "@/components/providers/intl-provider";
 import { ThemeProvider } from "@/components/providers/theme-provider";
 import { CalendarAlertProvider } from "@/components/providers/calendar-alert-provider";
+import { EmbeddedBridgeProvider } from "@/components/providers/embedded-bridge-provider";
 import { TourProvider } from "@/components/tour/tour-provider";
 import { locales } from "@/i18n/routing";
 
@@ -27,9 +28,11 @@ export default async function LocaleLayout({
     <IntlProvider locale={locale} messages={messages}>
       <ThemeProvider>
         <CalendarAlertProvider>
-          <TourProvider>
-            {children}
-          </TourProvider>
+          <EmbeddedBridgeProvider>
+            <TourProvider>
+              {children}
+            </TourProvider>
+          </EmbeddedBridgeProvider>
         </CalendarAlertProvider>
       </ThemeProvider>
     </IntlProvider>
diff --git a/app/[locale]/login/page.tsx b/app/[locale]/login/page.tsx
index f98a3331..ba076724 100644
--- a/app/[locale]/login/page.tsx
+++ b/app/[locale]/login/page.tsx
@@ -32,7 +32,7 @@ export default function LoginPage() {
   const isAddAccountMode = searchParams.get("mode") === "add-account";
   const { login, loginDemo, isLoading, error, clearError, isAuthenticated } = useAuthStore();
   const { theme, setTheme, initializeTheme } = useThemeStore(useShallow((s) => ({ theme: s.theme, setTheme: s.setTheme, initializeTheme: s.initializeTheme })));
-  const { appName, jmapServerUrl: serverUrl, oauthEnabled, oauthOnly, oauthClientId, oauthIssuerUrl, rememberMeEnabled, devMode, demoMode, loginLogoLightUrl, loginLogoDarkUrl, loginCompanyName, loginImprintUrl, loginPrivacyPolicyUrl, loginWebsiteUrl, isLoading: configLoading, error: configError } = useConfig();
+  const { appName, jmapServerUrl: serverUrl, oauthEnabled, oauthOnly, oauthClientId, oauthIssuerUrl, rememberMeEnabled, devMode, demoMode, loginLogoLightUrl, loginLogoDarkUrl, loginCompanyName, loginImprintUrl, loginPrivacyPolicyUrl, loginWebsiteUrl, isLoading: configLoading, error: configError, autoSsoEnabled, embeddedMode: _embeddedMode } = useConfig();
   const resolvedTheme = useThemeStore((s) => s.resolvedTheme);
 
   const [formData, setFormData] = useState({
@@ -173,6 +173,63 @@ export default function LoginPage() {
       });
   }, [oauthEnabled, serverUrl, oauthIssuerUrl]);
 
+  // Auto-SSO: when enabled with OAUTH_ONLY, skip the login page entirely
+  const ssoError = searchParams.get("sso_error");
+  const autoSsoTriggered = useRef(false);
+
+  const startServerSideSso = useCallback(async () => {
+    setOauthLoading(true);
+    try {
+      const redirectUri = `${window.location.origin}/${params.locale}/auth/callback`;
+      const res = await fetch('/api/auth/sso/start', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        credentials: 'include',
+        body: JSON.stringify({ redirect_uri: redirectUri, locale: params.locale }),
+      });
+
+      if (!res.ok) {
+        setOauthLoading(false);
+        return;
+      }
+
+      const { authorize_url } = await res.json();
+
+      // Navigate to the authorize URL
+      const isIframe = (() => { try { return window.self !== window.top; } catch { return true; } })();
+      if (isIframe) {
+        // In an iframe, try top-level navigation
+        try {
+          window.top!.location.href = authorize_url;
+        } catch {
+          // Cross-origin restriction — fall back to current frame
+          window.location.href = authorize_url;
+        }
+      } else {
+        window.location.href = authorize_url;
+      }
+    } catch {
+      setOauthLoading(false);
+    }
+  }, [params.locale]);
+
+  useEffect(() => {
+    if (!autoSsoEnabled || !oauthOnly || !oauthDiscoveryDone || !oauthMetadata) return;
+    if (ssoError || isAddAccountMode || isAuthenticated) return;
+    if (autoSsoTriggered.current) return;
+
+    // Guard against redirect loops
+    try {
+      if (sessionStorage.getItem("sso_attempted")) return;
+      sessionStorage.setItem("sso_attempted", "1");
+      // Clear the flag after 30 seconds so retries are possible
+      setTimeout(() => { try { sessionStorage.removeItem("sso_attempted"); } catch { /* ignore */ } }, 30000);
+    } catch { /* sessionStorage unavailable */ }
+
+    autoSsoTriggered.current = true;
+    startServerSideSso();
+  }, [autoSsoEnabled, oauthOnly, oauthDiscoveryDone, oauthMetadata, ssoError, isAddAccountMode, isAuthenticated, startServerSideSso]);
+
   const handleThemeSelect = useCallback((newTheme: "light" | "dark" | "system") => {
     setTheme(newTheme);
     setShowThemeMenu(false);
diff --git a/app/api/auth/session/route.ts b/app/api/auth/session/route.ts
index a205fa8f..f1733f50 100644
--- a/app/api/auth/session/route.ts
+++ b/app/api/auth/session/route.ts
@@ -3,12 +3,10 @@ import { cookies } from 'next/headers';
 import { logger } from '@/lib/logger';
 import { encryptSession, decryptSession } from '@/lib/auth/crypto';
 import { SESSION_COOKIE_MAX_AGE, sessionCookieName } from '@/lib/auth/session-cookie';
+import { getCookieOptions } from '@/lib/oauth/cookie-config';
 
 const COOKIE_OPTIONS = {
-  httpOnly: true,
-  secure: process.env.NODE_ENV === 'production',
-  sameSite: 'lax' as const,
-  path: '/',
+  ...getCookieOptions(),
   maxAge: SESSION_COOKIE_MAX_AGE,
 };
 
diff --git a/app/api/auth/sso/complete/route.ts b/app/api/auth/sso/complete/route.ts
new file mode 100644
index 00000000..fcbf6d33
--- /dev/null
+++ b/app/api/auth/sso/complete/route.ts
@@ -0,0 +1,80 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { cookies } from 'next/headers';
+import { logger } from '@/lib/logger';
+import { decryptPayload } from '@/lib/auth/crypto';
+import { exchangeCodeForTokens } from '@/lib/oauth/token-exchange';
+import { refreshTokenCookieName } from '@/lib/oauth/tokens';
+import { getCookieOptions } from '@/lib/oauth/cookie-config';
+
+const SSO_PENDING_COOKIE = 'sso_pending';
+const SSO_PENDING_MAX_AGE_MS = 5 * 60 * 1000; // 5 minutes
+
+export async function POST(request: NextRequest) {
+  const cookieStore = await cookies();
+
+  try {
+    const { code, state } = await request.json();
+
+    if (!code || !state) {
+      return NextResponse.json({ error: 'Missing code or state' }, { status: 400 });
+    }
+
+    // Read and decrypt the pending SSO cookie
+    const pendingCookie = cookieStore.get(SSO_PENDING_COOKIE)?.value;
+    if (!pendingCookie) {
+      logger.warn('SSO complete: no pending cookie found');
+      return NextResponse.json({ error: 'No pending SSO session. Please start the login flow again.' }, { status: 400 });
+    }
+
+    const pending = decryptPayload(pendingCookie);
+    if (!pending) {
+      cookieStore.delete(SSO_PENDING_COOKIE);
+      return NextResponse.json({ error: 'Invalid SSO session' }, { status: 400 });
+    }
+
+    // Validate state
+    if (pending.state !== state) {
+      logger.warn('SSO complete: state mismatch');
+      cookieStore.delete(SSO_PENDING_COOKIE);
+      return NextResponse.json({ error: 'State mismatch' }, { status: 400 });
+    }
+
+    // Validate TTL
+    const createdAt = pending.created_at as number;
+    if (!createdAt || Date.now() - createdAt > SSO_PENDING_MAX_AGE_MS) {
+      logger.warn('SSO complete: pending session expired');
+      cookieStore.delete(SSO_PENDING_COOKIE);
+      return NextResponse.json({ error: 'SSO session expired. Please try again.' }, { status: 400 });
+    }
+
+    const codeVerifier = pending.code_verifier as string;
+    const redirectUri = pending.redirect_uri as string;
+
+    if (!codeVerifier || !redirectUri) {
+      cookieStore.delete(SSO_PENDING_COOKIE);
+      return NextResponse.json({ error: 'Invalid SSO session data' }, { status: 400 });
+    }
+
+    // Exchange code for tokens
+    const tokens = await exchangeCodeForTokens(code, codeVerifier, redirectUri);
+
+    // Store refresh token
+    if (tokens.refresh_token) {
+      const cookieName = refreshTokenCookieName(0);
+      cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
+    }
+
+    // Delete pending cookie
+    cookieStore.delete(SSO_PENDING_COOKIE);
+
+    return NextResponse.json({
+      access_token: tokens.access_token,
+      expires_in: tokens.expires_in,
+    });
+  } catch (error) {
+    // Clean up pending cookie on any error
+    cookieStore.delete(SSO_PENDING_COOKIE);
+    logger.error('SSO complete error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Token exchange failed' }, { status: 401 });
+  }
+}
diff --git a/app/api/auth/sso/start/route.ts b/app/api/auth/sso/start/route.ts
new file mode 100644
index 00000000..07240ebe
--- /dev/null
+++ b/app/api/auth/sso/start/route.ts
@@ -0,0 +1,88 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { cookies } from 'next/headers';
+import { logger } from '@/lib/logger';
+import { encryptPayload } from '@/lib/auth/crypto';
+import { generateCodeVerifierServer, generateCodeChallengeServer, generateStateServer } from '@/lib/oauth/pkce-server';
+import { getRequiredConfig } from '@/lib/oauth/token-exchange';
+import { discoverOAuth } from '@/lib/oauth/discovery';
+import { OAUTH_SCOPES } from '@/lib/oauth/tokens';
+import { getCookieOptions } from '@/lib/oauth/cookie-config';
+
+const SSO_PENDING_COOKIE = 'sso_pending';
+const SSO_PENDING_MAX_AGE = 300; // 5 minutes
+
+export async function POST(request: NextRequest) {
+  try {
+    if (!process.env.SESSION_SECRET) {
+      return NextResponse.json({ error: 'SESSION_SECRET is required for SSO' }, { status: 500 });
+    }
+
+    const { redirect_uri, locale } = await request.json();
+
+    if (!redirect_uri || typeof redirect_uri !== 'string') {
+      return NextResponse.json({ error: 'Missing redirect_uri' }, { status: 400 });
+    }
+
+    // Validate redirect_uri origin matches the request origin to prevent open redirects
+    const requestOrigin = request.headers.get('origin') || request.nextUrl.origin;
+    try {
+      const redirectOrigin = new URL(redirect_uri).origin;
+      if (redirectOrigin !== requestOrigin) {
+        logger.warn('SSO start: redirect_uri origin mismatch', { redirectOrigin, requestOrigin });
+        return NextResponse.json({ error: 'Invalid redirect_uri' }, { status: 400 });
+      }
+    } catch {
+      return NextResponse.json({ error: 'Invalid redirect_uri' }, { status: 400 });
+    }
+
+    const { clientId, discoveryUrl } = getRequiredConfig();
+    const metadata = await discoverOAuth(discoveryUrl);
+
+    if (!metadata?.authorization_endpoint) {
+      return NextResponse.json({ error: 'OAuth discovery failed' }, { status: 502 });
+    }
+
+    // Generate PKCE + state server-side
+    const codeVerifier = generateCodeVerifierServer();
+    const codeChallenge = generateCodeChallengeServer(codeVerifier);
+    const state = generateStateServer();
+
+    // Encrypt and store in httpOnly cookie
+    const pendingData = {
+      state,
+      code_verifier: codeVerifier,
+      redirect_uri,
+      created_at: Date.now(),
+    };
+
+    const encrypted = encryptPayload(pendingData);
+    const cookieStore = await cookies();
+    const baseCookieOpts = getCookieOptions();
+    cookieStore.set(SSO_PENDING_COOKIE, encrypted, {
+      ...baseCookieOpts,
+      maxAge: SSO_PENDING_MAX_AGE,
+    });
+
+    // Build authorize URL
+    const authUrl = new URL(metadata.authorization_endpoint);
+    authUrl.searchParams.set('response_type', 'code');
+    authUrl.searchParams.set('client_id', clientId);
+    authUrl.searchParams.set('redirect_uri', redirect_uri);
+    authUrl.searchParams.set('scope', OAUTH_SCOPES);
+    authUrl.searchParams.set('state', state);
+    authUrl.searchParams.set('code_challenge', codeChallenge);
+    authUrl.searchParams.set('code_challenge_method', 'S256');
+
+    if (locale) {
+      authUrl.searchParams.set('ui_locales', locale);
+    }
+
+    return NextResponse.json({
+      authorize_url: authUrl.toString(),
+      state,
+    });
+  } catch (error) {
+    logger.error('SSO start error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}
diff --git a/app/api/auth/token/route.ts b/app/api/auth/token/route.ts
index b89ece03..9f2b152e 100644
--- a/app/api/auth/token/route.ts
+++ b/app/api/auth/token/route.ts
@@ -1,18 +1,9 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { cookies } from 'next/headers';
 import { logger } from '@/lib/logger';
-import { discoverOAuth } from '@/lib/oauth/discovery';
 import { refreshTokenCookieName } from '@/lib/oauth/tokens';
-
-const CLIENT_SECRET = process.env.OAUTH_CLIENT_SECRET || '';
-
-const COOKIE_OPTIONS = {
-  httpOnly: true,
-  secure: process.env.NODE_ENV === 'production',
-  sameSite: 'lax' as const,
-  path: '/',
-  maxAge: 30 * 24 * 60 * 60,
-};
+import { exchangeCodeForTokens, buildOAuthParams, getMetadata, getTokenEndpoint } from '@/lib/oauth/token-exchange';
+import { getCookieOptions } from '@/lib/oauth/cookie-config';
 
 function getSlot(request: NextRequest): number {
   const raw = request.nextUrl.searchParams.get('slot');
@@ -22,44 +13,6 @@ function getSlot(request: NextRequest): number {
   return slot;
 }
 
-
-function getRequiredConfig() {
-  const clientId = process.env.OAUTH_CLIENT_ID;
-  const serverUrl = process.env.JMAP_SERVER_URL || process.env.NEXT_PUBLIC_JMAP_SERVER_URL;
-  const issuerUrl = process.env.OAUTH_ISSUER_URL;
-  if (!clientId || !serverUrl) {
-    throw new Error(`OAuth misconfigured: ${[!clientId && 'OAUTH_CLIENT_ID', !serverUrl && 'JMAP_SERVER_URL'].filter(Boolean).join(', ')} not set`);
-  }
-  const discoveryUrl = issuerUrl?.trim() || serverUrl;
-  if (issuerUrl !== undefined && !issuerUrl.trim()) {
-    logger.warn('OAUTH_ISSUER_URL is set but empty, falling back to JMAP_SERVER_URL for discovery');
-  }
-  return { clientId, serverUrl, discoveryUrl };
-}
-
-async function getTokenEndpoint(): Promise<string> {
-  const { discoveryUrl } = getRequiredConfig();
-  const metadata = await discoverOAuth(discoveryUrl);
-  if (!metadata?.token_endpoint) {
-    throw new Error('OAuth token endpoint not found');
-  }
-  return metadata.token_endpoint;
-}
-
-async function getMetadata(): Promise<import('@/lib/oauth/discovery').OAuthMetadata | null> {
-  const { discoveryUrl } = getRequiredConfig();
-  return discoverOAuth(discoveryUrl);
-}
-
-function buildOAuthParams(base: Record<string, string>): URLSearchParams {
-  const { clientId } = getRequiredConfig();
-  const params = new URLSearchParams({ ...base, client_id: clientId });
-  if (CLIENT_SECRET) {
-    params.set('client_secret', CLIENT_SECRET);
-  }
-  return params;
-}
-
 export async function POST(request: NextRequest) {
   try {
     const { code, code_verifier, redirect_uri, slot: bodySlot } = await request.json();
@@ -69,43 +22,18 @@ export async function POST(request: NextRequest) {
     }
 
     const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4 ? bodySlot : getSlot(request);
-    const tokenEndpoint = await getTokenEndpoint();
 
-    const params = buildOAuthParams({
-      grant_type: 'authorization_code',
-      code,
-      redirect_uri,
-      code_verifier,
-    });
-
-    const tokenResponse = await fetch(tokenEndpoint, {
-      method: 'POST',
-      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
-      body: params.toString(),
-    });
-
-    if (!tokenResponse.ok) {
-      const errorText = await tokenResponse.text();
-      logger.error('Token exchange failed', { status: tokenResponse.status, error: errorText });
-      return NextResponse.json({ error: 'Token exchange failed' }, { status: 401 });
-    }
-
-    const tokens = await tokenResponse.json();
-
-    if (!tokens.access_token) {
-      logger.error('Token response missing access_token', { response: JSON.stringify(tokens).substring(0, 500) });
-      return NextResponse.json({ error: 'Invalid token response' }, { status: 502 });
-    }
+    const tokens = await exchangeCodeForTokens(code, code_verifier, redirect_uri);
 
     const response = NextResponse.json({
       access_token: tokens.access_token,
-      expires_in: tokens.expires_in || 3600,
+      expires_in: tokens.expires_in,
     });
 
     if (tokens.refresh_token) {
       const cookieName = refreshTokenCookieName(slot);
       const cookieStore = await cookies();
-      cookieStore.set(cookieName, tokens.refresh_token, COOKIE_OPTIONS);
+      cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
     }
 
     return response;
@@ -154,7 +82,7 @@ export async function PUT(request: NextRequest) {
     }
 
     if (tokens.refresh_token) {
-      cookieStore.set(cookieName, tokens.refresh_token, COOKIE_OPTIONS);
+      cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
     }
 
     return NextResponse.json({
diff --git a/app/api/config/route.ts b/app/api/config/route.ts
index 5558f4ba..e27294b6 100644
--- a/app/api/config/route.ts
+++ b/app/api/config/route.ts
@@ -36,5 +36,8 @@ export async function GET() {
     loginPrivacyPolicyUrl: process.env.LOGIN_PRIVACY_POLICY_URL || '',
     loginWebsiteUrl: process.env.LOGIN_WEBSITE_URL || '',
     demoMode: process.env.DEMO_MODE === 'true',
+    autoSsoEnabled: process.env.AUTO_SSO_ENABLED === 'true',
+    embeddedMode: !!process.env.ALLOWED_FRAME_ANCESTORS && process.env.ALLOWED_FRAME_ANCESTORS !== "'none'",
+    parentOrigin: process.env.NEXT_PUBLIC_PARENT_ORIGIN || '',
   });
 }
diff --git a/app/layout.tsx b/app/layout.tsx
index 5ab67392..2a1c0212 100644
--- a/app/layout.tsx
+++ b/app/layout.tsx
@@ -31,10 +31,14 @@ export default async function RootLayout({
 }) {
   const locale = await getLocale();
   const nonce = (await headers()).get("x-nonce") ?? "";
+  const parentOrigin = process.env.NEXT_PUBLIC_PARENT_ORIGIN || "";
 
   return (
     <html lang={locale} suppressHydrationWarning>
       <head>
+        {parentOrigin && (
+          <meta name="parent-origin" content={parentOrigin} />
+        )}
         <script
           nonce={nonce}
           suppressHydrationWarning
diff --git a/components/providers/embedded-bridge-provider.tsx b/components/providers/embedded-bridge-provider.tsx
new file mode 100644
index 00000000..4f4cc0f1
--- /dev/null
+++ b/components/providers/embedded-bridge-provider.tsx
@@ -0,0 +1,34 @@
+"use client";
+
+import { useEffect } from "react";
+import { isEmbedded, listenFromParent } from "@/lib/iframe-bridge";
+import { useAuthStore } from "@/stores/auth-store";
+import { useConfig } from "@/hooks/use-config";
+
+export function EmbeddedBridgeProvider({ children }: { children: React.ReactNode }) {
+  const { parentOrigin, embeddedMode } = useConfig();
+  const logout = useAuthStore((s) => s.logout);
+
+  useEffect(() => {
+    if (!embeddedMode || !isEmbedded()) return;
+
+    const unsubscribe = listenFromParent((msg) => {
+      switch (msg.type) {
+        case "sso:trigger-login": {
+          // Navigate to login page to start SSO flow
+          const segments = window.location.pathname.split("/").filter(Boolean);
+          const locale = segments[0] || "en";
+          window.location.href = `/${locale}/login`;
+          break;
+        }
+        case "sso:trigger-logout":
+          logout();
+          break;
+      }
+    }, parentOrigin || undefined);
+
+    return unsubscribe;
+  }, [embeddedMode, parentOrigin, logout]);
+
+  return <>{children}</>;
+}
diff --git a/hooks/use-config.ts b/hooks/use-config.ts
index 3a346b81..a3376f34 100644
--- a/hooks/use-config.ts
+++ b/hooks/use-config.ts
@@ -23,6 +23,9 @@ interface ConfigData {
   loginPrivacyPolicyUrl: string;
   loginWebsiteUrl: string;
   demoMode: boolean;
+  autoSsoEnabled: boolean;
+  embeddedMode: boolean;
+  parentOrigin: string;
 }
 
 interface AppConfig extends ConfigData {
@@ -93,6 +96,9 @@ export function useConfig(): AppConfig {
     loginPrivacyPolicyUrl: configCache?.loginPrivacyPolicyUrl || '',
     loginWebsiteUrl: configCache?.loginWebsiteUrl || '',
     demoMode: configCache?.demoMode || false,
+    autoSsoEnabled: configCache?.autoSsoEnabled || false,
+    embeddedMode: configCache?.embeddedMode || false,
+    parentOrigin: configCache?.parentOrigin || '',
     isLoading: !configCache,
     error: null,
   });
@@ -121,6 +127,9 @@ export function useConfig(): AppConfig {
         loginPrivacyPolicyUrl: configCache.loginPrivacyPolicyUrl,
         loginWebsiteUrl: configCache.loginWebsiteUrl,
         demoMode: configCache.demoMode,
+        autoSsoEnabled: configCache.autoSsoEnabled,
+        embeddedMode: configCache.embeddedMode,
+        parentOrigin: configCache.parentOrigin,
         isLoading: false,
         error: null,
       });
@@ -150,6 +159,9 @@ export function useConfig(): AppConfig {
           loginPrivacyPolicyUrl: data.loginPrivacyPolicyUrl,
           loginWebsiteUrl: data.loginWebsiteUrl,
           demoMode: data.demoMode,
+          autoSsoEnabled: data.autoSsoEnabled,
+          embeddedMode: data.embeddedMode,
+          parentOrigin: data.parentOrigin,
           isLoading: false,
           error: null,
         });
diff --git a/lib/auth/crypto.ts b/lib/auth/crypto.ts
index 9061a8e9..6a3d689a 100644
--- a/lib/auth/crypto.ts
+++ b/lib/auth/crypto.ts
@@ -48,3 +48,38 @@ export function decryptSession(token: string): { serverUrl: string; username: st
     return null;
   }
 }
+
+export function encryptPayload(payload: Record<string, unknown>): string {
+  const key = getKey();
+  const iv = randomBytes(IV_LENGTH);
+  const cipher = createCipheriv(ALGORITHM, key, iv);
+
+  const json = JSON.stringify(payload);
+  const encrypted = Buffer.concat([cipher.update(json, 'utf8'), cipher.final()]);
+  const tag = cipher.getAuthTag();
+
+  return Buffer.concat([iv, tag, encrypted]).toString('base64');
+}
+
+export function decryptPayload(token: string): Record<string, unknown> | null {
+  try {
+    const key = getKey();
+    const data = Buffer.from(token, 'base64');
+    if (data.length < IV_LENGTH + TAG_LENGTH) return null;
+
+    const iv = data.subarray(0, IV_LENGTH);
+    const tag = data.subarray(IV_LENGTH, IV_LENGTH + TAG_LENGTH);
+    const encrypted = data.subarray(IV_LENGTH + TAG_LENGTH);
+
+    const decipher = createDecipheriv(ALGORITHM, key, iv);
+    decipher.setAuthTag(tag);
+
+    const decrypted = Buffer.concat([decipher.update(encrypted), decipher.final()]);
+    return JSON.parse(decrypted.toString('utf8'));
+  } catch (error) {
+    logger.warn('Payload decryption failed', {
+      error: error instanceof Error ? error.message : 'Unknown error',
+    });
+    return null;
+  }
+}
diff --git a/lib/iframe-bridge.ts b/lib/iframe-bridge.ts
new file mode 100644
index 00000000..fbe28070
--- /dev/null
+++ b/lib/iframe-bridge.ts
@@ -0,0 +1,40 @@
+const PARENT_ORIGIN = typeof window !== 'undefined'
+  ? (document.querySelector('meta[name="parent-origin"]')?.getAttribute('content') || '')
+  : '';
+
+export function isEmbedded(): boolean {
+  try {
+    return window.self !== window.top;
+  } catch {
+    return true;
+  }
+}
+
+export function notifyParent(type: string, payload: Record<string, unknown> = {}) {
+  if (!isEmbedded()) return;
+
+  const targetOrigin = PARENT_ORIGIN || '*';
+  try {
+    window.parent.postMessage({ source: 'bulwark', type, ...payload }, targetOrigin);
+  } catch {
+    // Cross-origin postMessage may fail in restricted contexts
+  }
+}
+
+export function listenFromParent(
+  handler: (msg: { type: string; [k: string]: unknown }) => void,
+  allowedOrigin?: string,
+): () => void {
+  const listener = (event: MessageEvent) => {
+    // Validate origin if configured
+    if (allowedOrigin && event.origin !== allowedOrigin) return;
+
+    // Only accept messages from the portal
+    if (!event.data || event.data.source !== 'portal') return;
+
+    handler(event.data);
+  };
+
+  window.addEventListener('message', listener);
+  return () => window.removeEventListener('message', listener);
+}
diff --git a/lib/oauth/cookie-config.ts b/lib/oauth/cookie-config.ts
new file mode 100644
index 00000000..51ee9a65
--- /dev/null
+++ b/lib/oauth/cookie-config.ts
@@ -0,0 +1,11 @@
+const COOKIE_SAME_SITE = (process.env.COOKIE_SAME_SITE || 'lax') as 'lax' | 'none' | 'strict';
+
+export function getCookieOptions() {
+  return {
+    httpOnly: true,
+    secure: COOKIE_SAME_SITE === 'none' ? true : process.env.NODE_ENV === 'production',
+    sameSite: COOKIE_SAME_SITE,
+    path: '/',
+    maxAge: 30 * 24 * 60 * 60,
+  };
+}
diff --git a/lib/oauth/pkce-server.ts b/lib/oauth/pkce-server.ts
new file mode 100644
index 00000000..b71e6c3c
--- /dev/null
+++ b/lib/oauth/pkce-server.ts
@@ -0,0 +1,18 @@
+import { randomBytes, createHash } from 'node:crypto';
+
+function base64urlEncode(buffer: Buffer): string {
+  return buffer.toString('base64').replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}
+
+export function generateCodeVerifierServer(): string {
+  return base64urlEncode(randomBytes(32));
+}
+
+export function generateCodeChallengeServer(verifier: string): string {
+  const hash = createHash('sha256').update(verifier).digest();
+  return base64urlEncode(hash);
+}
+
+export function generateStateServer(): string {
+  return base64urlEncode(randomBytes(32));
+}
diff --git a/lib/oauth/token-exchange.ts b/lib/oauth/token-exchange.ts
new file mode 100644
index 00000000..c1cbb443
--- /dev/null
+++ b/lib/oauth/token-exchange.ts
@@ -0,0 +1,88 @@
+import { logger } from '@/lib/logger';
+import { discoverOAuth } from '@/lib/oauth/discovery';
+import type { OAuthMetadata } from '@/lib/oauth/discovery';
+
+const CLIENT_SECRET = process.env.OAUTH_CLIENT_SECRET || '';
+
+export function getRequiredConfig() {
+  const clientId = process.env.OAUTH_CLIENT_ID;
+  const serverUrl = process.env.JMAP_SERVER_URL || process.env.NEXT_PUBLIC_JMAP_SERVER_URL;
+  const issuerUrl = process.env.OAUTH_ISSUER_URL;
+  if (!clientId || !serverUrl) {
+    throw new Error(`OAuth misconfigured: ${[!clientId && 'OAUTH_CLIENT_ID', !serverUrl && 'JMAP_SERVER_URL'].filter(Boolean).join(', ')} not set`);
+  }
+  const discoveryUrl = issuerUrl?.trim() || serverUrl;
+  if (issuerUrl !== undefined && !issuerUrl.trim()) {
+    logger.warn('OAUTH_ISSUER_URL is set but empty, falling back to JMAP_SERVER_URL for discovery');
+  }
+  return { clientId, serverUrl, discoveryUrl };
+}
+
+export async function getTokenEndpoint(): Promise<string> {
+  const { discoveryUrl } = getRequiredConfig();
+  const metadata = await discoverOAuth(discoveryUrl);
+  if (!metadata?.token_endpoint) {
+    throw new Error('OAuth token endpoint not found');
+  }
+  return metadata.token_endpoint;
+}
+
+export async function getMetadata(): Promise<OAuthMetadata | null> {
+  const { discoveryUrl } = getRequiredConfig();
+  return discoverOAuth(discoveryUrl);
+}
+
+export function buildOAuthParams(base: Record<string, string>): URLSearchParams {
+  const { clientId } = getRequiredConfig();
+  const params = new URLSearchParams({ ...base, client_id: clientId });
+  if (CLIENT_SECRET) {
+    params.set('client_secret', CLIENT_SECRET);
+  }
+  return params;
+}
+
+export interface TokenResult {
+  access_token: string;
+  expires_in: number;
+  refresh_token?: string;
+}
+
+export async function exchangeCodeForTokens(
+  code: string,
+  codeVerifier: string,
+  redirectUri: string,
+): Promise<TokenResult> {
+  const tokenEndpoint = await getTokenEndpoint();
+
+  const params = buildOAuthParams({
+    grant_type: 'authorization_code',
+    code,
+    redirect_uri: redirectUri,
+    code_verifier: codeVerifier,
+  });
+
+  const tokenResponse = await fetch(tokenEndpoint, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+    body: params.toString(),
+  });
+
+  if (!tokenResponse.ok) {
+    const errorText = await tokenResponse.text();
+    logger.error('Token exchange failed', { status: tokenResponse.status, error: errorText });
+    throw new Error('Token exchange failed');
+  }
+
+  const tokens = await tokenResponse.json();
+
+  if (!tokens.access_token) {
+    logger.error('Token response missing access_token', { response: JSON.stringify(tokens).substring(0, 500) });
+    throw new Error('Invalid token response');
+  }
+
+  return {
+    access_token: tokens.access_token,
+    expires_in: tokens.expires_in || 3600,
+    refresh_token: tokens.refresh_token,
+  };
+}
diff --git a/proxy.ts b/proxy.ts
index d68fd7be..dc44a1cc 100644
--- a/proxy.ts
+++ b/proxy.ts
@@ -14,6 +14,8 @@ export function proxy(request: NextRequest) {
 
   const connectSrc = isDev ? `'self' https: ws: wss:` : `'self' https:`;
 
+  const frameAncestors = process.env.ALLOWED_FRAME_ANCESTORS?.trim() || "'none'";
+
   const csp = [
     `default-src 'self'`,
     `script-src ${scriptSrc}`,
@@ -25,7 +27,7 @@ export function proxy(request: NextRequest) {
     `object-src 'none'`,
     `base-uri 'self'`,
     `form-action 'self'`,
-    `frame-ancestors 'none'`,
+    `frame-ancestors ${frameAncestors}`,
   ].join("; ");
 
   let intlResponse: ReturnType<typeof intlMiddleware> | null = null;
@@ -44,7 +46,13 @@ export function proxy(request: NextRequest) {
   response.headers.set("x-middleware-request-x-nonce", nonce);
 
   response.headers.set("X-Content-Type-Options", "nosniff");
-  response.headers.set("X-Frame-Options", "DENY");
+
+  // X-Frame-Options only supports DENY/SAMEORIGIN. When frame-ancestors
+  // specifies explicit origins, we rely solely on the CSP header.
+  if (frameAncestors === "'none'") {
+    response.headers.set("X-Frame-Options", "DENY");
+  }
+
   response.headers.set("Referrer-Policy", "strict-origin-when-cross-origin");
   response.headers.set("X-XSS-Protection", "0");
   response.headers.set(
diff --git a/stores/auth-store.ts b/stores/auth-store.ts
index 96d2a28d..f3874772 100644
--- a/stores/auth-store.ts
+++ b/stores/auth-store.ts
@@ -13,6 +13,7 @@ import { fetchConfig } from '@/hooks/use-config';
 import { debug } from '@/lib/debug';
 import { generateAccountId } from '@/lib/account-utils';
 import { replaceWindowLocation } from '@/lib/browser-navigation';
+import { notifyParent } from '@/lib/iframe-bridge';
 import { snapshotAccount, restoreAccount, clearAllStores, evictAccount, evictAll } from '@/lib/account-state-manager';
 import type { Identity } from '@/lib/jmap/types';
 
@@ -35,6 +36,7 @@ interface AuthState {
 
   login: (serverUrl: string, username: string, password: string, totp?: string, rememberMe?: boolean) => Promise<boolean>;
   loginWithOAuth: (serverUrl: string, code: string, codeVerifier: string, redirectUri: string) => Promise<boolean>;
+  loginWithServerSso: (code: string, state: string) => Promise<boolean>;
   loginDemo: () => Promise<boolean>;
   refreshAccessToken: () => Promise<string | null>;
   logout: () => void;
@@ -534,6 +536,8 @@ export const useAuthStore = create<AuthState>()(
 
           scheduleRefresh(expires_in, get().refreshAccessToken, accountId);
 
+          notifyParent('sso:auth-success', { username });
+
           // Sync settings from server (only if enabled)
           fetchConfig().then(config => {
             if (!config.settingsSyncEnabled) return;
@@ -550,9 +554,118 @@ export const useAuthStore = create<AuthState>()(
           return true;
         } catch (error) {
           debug.error('OAuth login error:', error);
+          const errorMsg = error instanceof Error ? error.message : 'generic';
+          notifyParent('sso:auth-failure', { error: errorMsg });
           set({
             isLoading: false,
-            error: error instanceof Error ? error.message : 'generic',
+            error: errorMsg,
+            isAuthenticated: false,
+            client: null,
+          });
+          return false;
+        }
+      },
+
+      loginWithServerSso: async (code, state) => {
+        set({ isLoading: true, error: null });
+
+        try {
+          // Server-side SSO: the server holds the PKCE verifier in an encrypted cookie
+          const ssoRes = await fetch('/api/auth/sso/complete', {
+            method: 'POST',
+            headers: { 'Content-Type': 'application/json' },
+            credentials: 'include',
+            body: JSON.stringify({ code, state }),
+          });
+
+          if (!ssoRes.ok) {
+            const errorData = await ssoRes.json().catch(() => ({ error: 'token_exchange_failed' }));
+            throw new Error(errorData.error || 'token_exchange_failed');
+          }
+
+          const { access_token, expires_in } = await ssoRes.json();
+
+          // We need the server URL from config
+          const config = await fetchConfig();
+          const ssoServerUrl = config.jmapServerUrl;
+
+          if (!ssoServerUrl) {
+            throw new Error('Server URL not configured');
+          }
+
+          const accountStore = useAccountStore.getState();
+
+          const refreshFn = get().refreshAccessToken;
+          const client = JMAPClient.withBearer(ssoServerUrl, access_token, '', () => refreshFn());
+          client.onConnectionChange((connected) => {
+            set({ connectionLost: !connected });
+          });
+          await client.connect();
+
+          const username = client.getUsername();
+          const { identities, primaryIdentity } = loadIdentities(await client.getIdentities(), username);
+          initializeFeatureStores(client);
+
+          const accountId = generateAccountId(username, ssoServerUrl);
+
+          const prevAccountId = get().activeAccountId;
+          if (prevAccountId && prevAccountId !== accountId) {
+            snapshotAccount(prevAccountId);
+            clearAllStores();
+          }
+
+          clients.set(accountId, client);
+
+          accountStore.addAccount({
+            label: primaryIdentity?.name || username,
+            serverUrl: ssoServerUrl,
+            username,
+            authMode: 'oauth',
+            rememberMe: true,
+            displayName: primaryIdentity?.name || username,
+            email: primaryIdentity?.email || username,
+            lastLoginAt: Date.now(),
+            isConnected: true,
+            hasError: false,
+            isDefault: accountStore.accounts.length === 0,
+          });
+          accountStore.setActiveAccount(accountId);
+
+          set({
+            isAuthenticated: true,
+            isLoading: false,
+            serverUrl: ssoServerUrl,
+            username,
+            client,
+            identities,
+            primaryIdentity,
+            authMode: 'oauth',
+            accessToken: access_token,
+            tokenExpiresAt: Date.now() + expires_in * 1000,
+            connectionLost: false,
+            error: null,
+            activeAccountId: accountId,
+          });
+
+          scheduleRefresh(expires_in, get().refreshAccessToken, accountId);
+
+          notifyParent('sso:auth-success', { username });
+
+          fetchConfig().then(cfg => {
+            if (!cfg.settingsSyncEnabled) return;
+            useSettingsStore.getState().loadFromServer(username, ssoServerUrl).finally(() => {
+              useSettingsStore.getState().enableSync(username, ssoServerUrl);
+            });
+          }).catch(() => {});
+
+          return true;
+        } catch (error) {
+          debug.error('Server SSO login error:', error);
+          const errorMsg = error instanceof Error ? error.message : 'generic';
+          notifyParent('sso:auth-failure', { error: errorMsg });
+          set({
+            isLoading: false,
+            error: errorMsg,
             isAuthenticated: false,
             client: null,
           });
@@ -576,6 +689,7 @@ export const useAuthStore = create<AuthState>()(
             const res = await fetch(`/api/auth/token?slot=${slot}`, { method: 'PUT' });
 
             if (!res.ok) {
+              notifyParent('sso:session-expired');
               markSessionExpired();
               get().logout();
               return null;
@@ -594,6 +708,7 @@ export const useAuthStore = create<AuthState>()(
             return access_token;
           } catch (error) {
             debug.error('Token refresh failed:', error);
+            notifyParent('sso:session-expired');
             markSessionExpired();
             get().logout();
             return null;
@@ -694,6 +809,8 @@ export const useAuthStore = create<AuthState>()(
         // No accounts remaining (or demo mode) — full logout + redirect
         performFullLogout(set);
 
+        notifyParent('sso:logout');
+
         // Background cookie/token cleanup — keepalive ensures completion during navigation
         if (!wasDemoMode) {
           fetch(`/api/auth/session?slot=${slot}`, { method: 'DELETE', keepalive: true }).catch(() => {});