diff --git a/backend/app/main.py b/backend/app/main.py
index f80275e..4435697 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -576,9 +576,22 @@ app.add_middleware(
     expose_headers=["X-New-Token"],  # Allow frontend to read this header
 )
 
-# Session middleware for OIDC state (authlib needs it)
+# Session middleware for OIDC state (authlib needs it).
+#
+# The OIDC state and nonce live in this cookie for the length of one redirect,
+# and they are what stops somebody replaying an authorization response at you.
+# `lax` because the provider sends the browser back with a top-level GET, which
+# lax allows and `strict` would drop — taking the state with it and failing
+# every sign-in. Secure whenever the site is served over https, which is what
+# APP_URL says; a plain-http development host keeps a cookie it can actually
+# set.
 from starlette.middleware.sessions import SessionMiddleware
-app.add_middleware(SessionMiddleware, secret_key=settings.SECRET_KEY)
+app.add_middleware(
+    SessionMiddleware,
+    secret_key=settings.SECRET_KEY,
+    same_site="lax",
+    https_only=str(getattr(settings, "APP_URL", "")).startswith("https://"),
+)
 
 # Add token refresh middleware AFTER CORS (middleware applies in reverse)
 from app.utils.auth import TokenRefreshMiddleware
diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index 0b1ce3b..b856287 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -1,3 +1,4 @@
+import logging
 import secrets
 from datetime import datetime, timedelta
 
@@ -11,7 +12,7 @@ from app.models.email_verification import EmailVerification
 from app.models.password_reset import PasswordReset
 from app.schemas.auth import (
     UserCreate, UserResponse, Token, LoginRequest, LogoutRequest, RefreshRequest,
-    UserUpdateMe, ForgotPasswordRequest, ResetPasswordRequest,
+    UserUpdateMe, ForgotPasswordRequest, ResetPasswordRequest, SsoExchangeRequest,
 )
 from app.services import email_service
 from app.utils.auth import (
@@ -19,6 +20,7 @@ from app.utils.auth import (
     get_current_user,
 )
 
+logger = logging.getLogger(__name__)
 router = APIRouter()
 
 
@@ -126,6 +128,7 @@ def signup_policy(db: Session = Depends(get_db)):
 
 @router.post("/register")
 async def register(user_data: UserCreate, background_tasks: BackgroundTasks, db: Session = Depends(get_db)):
+    _refuse_when_sso_only("Accounts are created through SSO on this site.")
     # Failing open: an hCaptcha outage should cost the site a little spam, not
     # every account that would have been created while it lasted.
     await captcha.require_human(user_data.captcha_token, fail_open=True)
@@ -322,6 +325,7 @@ def verify_email(token: str, db: Session = Depends(get_db)):
 
 @router.post("/resend-verification")
 async def resend_verification(data: ForgotPasswordRequest, background_tasks: BackgroundTasks, db: Session = Depends(get_db)):
+    _refuse_when_sso_only("Accounts are created through SSO on this site.")
     email_normalized = data.email.lower().strip()
     user = db.query(User).filter(User.email == email_normalized).first()
     if not user:
@@ -348,6 +352,7 @@ async def resend_verification(data: ForgotPasswordRequest, background_tasks: Bac
 
 @router.post("/forgot-password")
 async def forgot_password(data: ForgotPasswordRequest, background_tasks: BackgroundTasks, db: Session = Depends(get_db)):
+    _refuse_when_sso_only("There is no password to reset. Please use SSO.")
     email_normalized = data.email.lower().strip()
     _check_reset_rate_limit(db, email_normalized)
 
@@ -371,6 +376,7 @@ async def forgot_password(data: ForgotPasswordRequest, background_tasks: Backgro
 
 @router.post("/reset-password")
 def reset_password(data: ResetPasswordRequest, db: Session = Depends(get_db)):
+    _refuse_when_sso_only("There is no password to reset. Please use SSO.")
     if len(data.new_password) < 8:
         raise HTTPException(status_code=400, detail="Password must be at least 8 characters")
 
@@ -458,6 +464,10 @@ def get_me(current_user: User = Depends(get_current_user)):
 @router.put("/me")
 def update_me(data: UserUpdateMe, db: Session = Depends(get_db), current_user: User = Depends(get_current_user)):
     if data.new_password:
+        # Setting one is the same door. A password nobody can log in with is
+        # not harmless: it is a credential waiting for the flag to be turned
+        # off again.
+        _refuse_when_sso_only("Passwords are not used on this site. Please use SSO.")
         # A first password is not a change of one. Somebody who signed in with
         # a code or through single sign-on has none to confirm, and asking for
         # it locked them out of ever setting one — the only way through was
@@ -498,6 +508,69 @@ def _get_sso_settings():
         return {"sso_only": False}
 
 
+#: How long a one-time code is worth anything. The browser is mid-redirect and
+#: spends it immediately; a minute is generous for that and short enough that a
+#: code seen in a log is already dead.
+SSO_CODE_TTL = 60
+
+
+def _sso_store():
+    import redis as redis_lib
+    from app.config import settings as cfg
+    return redis_lib.from_url(cfg.REDIS_URL, decode_responses=True, socket_connect_timeout=1)
+
+
+def _stash_sso_token(token: str) -> str | None:
+    """Park a freshly minted token behind a random code. None if Redis is away."""
+    code = secrets.token_urlsafe(32)
+    try:
+        _sso_store().setex(f"sso:exchange:{code}", SSO_CODE_TTL, token)
+        return code
+    except Exception:
+        return None
+
+
+@router.post("/sso/exchange", response_model=Token)
+def sso_exchange(data: SsoExchangeRequest):
+    """Trade a one-time code for the token it stands for.
+
+    Once. The key is deleted as it is read, so a code replayed from a log, a
+    history entry or a Referer header buys nothing.
+    """
+    from app.config import settings as cfg
+
+    key = f"sso:exchange:{data.code}"
+    try:
+        store = _sso_store()
+        # GETDEL where the server has it, which is atomic and settles the race
+        # between two tabs; the pipeline is the same thing for an older Redis.
+        try:
+            token = store.getdel(key)
+        except Exception:
+            pipe = store.pipeline()
+            pipe.get(key)
+            pipe.delete(key)
+            token = pipe.execute()[0]
+    except Exception:
+        raise HTTPException(status_code=503, detail="Could not complete sign-in. Please try again.")
+    if not token:
+        raise HTTPException(status_code=400, detail="That sign-in link has expired. Please sign in again.")
+    return Token(access_token=token, expires_in=cfg.ACCESS_TOKEN_EXPIRE_MINUTES * 60)
+
+
+def _refuse_when_sso_only(what: str = "Password login is disabled. Please use SSO.") -> None:
+    """No password door while the site is single sign-on.
+
+    Login and the login codes checked this; register, forgot-password and
+    reset-password did not. So with SSO-only on, somebody could still be
+    issued a password account they could not use — and if the flag were ever
+    turned off, that is a password account nobody vetted, sitting in the
+    users table waiting.
+    """
+    if _get_sso_settings()["sso_only"]:
+        raise HTTPException(status_code=403, detail=what)
+
+
 @router.get("/sso/config")
 def sso_config():
     """Public endpoint — tells frontend whether SSO is available and login mode."""
@@ -616,5 +689,16 @@ async def sso_callback(request: Request, db: Session = Depends(get_db)):
         db.commit()
         db.refresh(user)
 
-    access_token = create_access_token(data={"sub": user.email})
-    return RedirectResponse(url=f"{cfg.APP_URL}/sso-callback?token={access_token}")
+    # A code in the address bar, never the token itself.
+    #
+    # This redirect is a page load: the browser asks nginx for it, and nginx
+    # logs `"$request"` — so every sign-in wrote a live bearer token, good for
+    # a day, into the frontend container's access log, and into the browser's
+    # history, and into the Referer of whatever the page loaded next. The code
+    # that goes there instead is worth one exchange, within a minute, and is
+    # gone the moment it is spent.
+    code = _stash_sso_token(create_access_token(data={"sub": user.email}))
+    if code is None:
+        logger.error("SSO succeeded but the exchange store is unreachable")
+        return RedirectResponse(url=f"{cfg.APP_URL}/login?error=sso_failed")
+    return RedirectResponse(url=f"{cfg.APP_URL}/sso-callback?code={code}")
diff --git a/backend/app/schemas/auth.py b/backend/app/schemas/auth.py
index ea48d92..ce5cef9 100644
--- a/backend/app/schemas/auth.py
+++ b/backend/app/schemas/auth.py
@@ -95,6 +95,12 @@ class ResetPasswordRequest(BaseModel):
     new_password: str
 
 
+class SsoExchangeRequest(BaseModel):
+    """The one-time code the SSO redirect leaves in the address bar."""
+
+    code: str = Field(min_length=8, max_length=200)
+
+
 class LoginCodeRequest(BaseModel):
     email: EmailStr
 
diff --git a/backend/tests/api-contract.json b/backend/tests/api-contract.json
index c5680d5..b8e28ab 100644
--- a/backend/tests/api-contract.json
+++ b/backend/tests/api-contract.json
@@ -2285,6 +2285,14 @@
         "422"
       ]
     },
+    "POST /api/v1/auth/sso/exchange": {
+      "body": true,
+      "params": [],
+      "responses": [
+        "200",
+        "422"
+      ]
+    },
     "POST /api/v1/categories/": {
       "body": true,
       "params": [],
diff --git a/backend/tests/test_sso_hardening.py b/backend/tests/test_sso_hardening.py
new file mode 100644
index 0000000..c8395f5
--- /dev/null
+++ b/backend/tests/test_sso_hardening.py
@@ -0,0 +1,121 @@
+"""The password doors while a site is single sign-on, and the token in the URL.
+
+Run: DATABASE_URL=sqlite:///:memory: PYTHONPATH=backend python -m unittest discover -s backend/tests
+"""
+import os
+os.environ.setdefault("DATABASE_URL", "sqlite:///:memory:")
+
+import unittest
+from unittest.mock import patch
+
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session
+from sqlalchemy.pool import StaticPool
+
+from app.database import Base, get_db
+from app.models.user import User
+from app.routers import auth
+from app.utils.auth import get_current_user
+
+
+class SsoOnlyClosesThePasswordDoors(unittest.TestCase):
+    """Login checked the flag. Everything else that makes a password did not.
+
+    With SSO-only on, an account could still be registered, and a reset link
+    still issued and spent — so the site could accumulate password accounts
+    nobody vetted, waiting for the flag to be turned off again.
+    """
+
+    def setUp(self):
+        self.engine = create_engine("sqlite://", connect_args={"check_same_thread": False},
+                                    poolclass=StaticPool)
+        Base.metadata.create_all(self.engine)
+        self.db = Session(self.engine)
+        # No password of their own, so the "set a first password" path in
+        # PUT /auth/me is the one under test rather than bcrypt's opinion of a
+        # fake hash.
+        self.user = User(id=1, name="Reader", email="reader@example.com",
+                         hashed_password=None)
+        self.db.add(self.user)
+        self.db.commit()
+        app = FastAPI()
+        app.include_router(auth.router, prefix="/auth")
+        app.dependency_overrides[get_db] = lambda: self.db
+        app.dependency_overrides[get_current_user] = lambda: self.user
+        self.client = TestClient(app)
+
+    def tearDown(self):
+        self.client.close()
+        self.db.close()
+        self.engine.dispose()
+
+    def calls(self):
+        return [
+            ("/auth/register", {"email": "new@example.com", "password": "abcdefgh",
+                                "name": "New"}),
+            ("/auth/forgot-password", {"email": "reader@example.com"}),
+            ("/auth/reset-password", {"token": "x" * 20, "new_password": "abcdefgh"}),
+            ("/auth/resend-verification", {"email": "reader@example.com"}),
+            ("/auth/me", {"new_password": "abcdefgh"}),
+        ]
+
+    def test_every_password_door_is_shut(self):
+        with patch.object(auth, "_get_sso_settings", return_value={"sso_only": True}):
+            for path, body in self.calls():
+                method = self.client.put if path == "/auth/me" else self.client.post
+                response = method(path, json=body)
+                self.assertEqual(response.status_code, 403, f"{path}: {response.text}")
+                self.assertIn("SSO", response.json()["detail"], path)
+
+    def test_and_open_again_when_the_site_is_not_sso_only(self):
+        # Not asserting success — these have their own rules — only that the
+        # flag is no longer the thing refusing them.
+        with patch.object(auth, "_get_sso_settings", return_value={"sso_only": False}):
+            for path, body in self.calls():
+                method = self.client.put if path == "/auth/me" else self.client.post
+                self.assertNotEqual(method(path, json=body).status_code, 403, path)
+
+
+class OneTimeCodeTests(unittest.TestCase):
+    """The redirect carries a code, and the code is worth exactly one exchange.
+
+    It used to carry the access token itself, in a query string that nginx
+    writes to its access log on every sign-in — a live bearer token, good for
+    a day, in a file, in the browser's history, and in the next Referer.
+    """
+
+    def setUp(self):
+        app = FastAPI()
+        app.include_router(auth.router, prefix="/auth")
+        self.client = TestClient(app)
+        self.store = {}
+
+    def tearDown(self):
+        self.client.close()
+
+    class FakeRedis:
+        def __init__(self, store): self.store = store
+        def setex(self, key, ttl, value): self.store[key] = value
+        def getdel(self, key): return self.store.pop(key, None)
+
+    def test_a_code_is_spent_once(self):
+        with patch.object(auth, "_sso_store", return_value=self.FakeRedis(self.store)):
+            code = auth._stash_sso_token("a.jwt.value")
+            self.assertTrue(code and "a.jwt.value" not in code)
+            first = self.client.post("/auth/sso/exchange", json={"code": code})
+            self.assertEqual(first.status_code, 200, first.text)
+            self.assertEqual(first.json()["access_token"], "a.jwt.value")
+            # Replayed from a log, a history entry or a Referer: worth nothing.
+            again = self.client.post("/auth/sso/exchange", json={"code": code})
+            self.assertEqual(again.status_code, 400)
+
+    def test_an_unknown_code_is_refused(self):
+        with patch.object(auth, "_sso_store", return_value=self.FakeRedis(self.store)):
+            response = self.client.post("/auth/sso/exchange", json={"code": "y" * 40})
+            self.assertEqual(response.status_code, 400)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/frontend/src/components/ArticleReader.jsx b/frontend/src/components/ArticleReader.jsx
index a1c012e..e48af67 100644
--- a/frontend/src/components/ArticleReader.jsx
+++ b/frontend/src/components/ArticleReader.jsx
@@ -393,8 +393,10 @@ export default function ArticleReader({
             the prose, and the site menu is in the header where it belongs. */}
         {drawerOpen && (
           <div className="article-drawer-head">
-            <button type="button" className="article-drawer-close" aria-label="Close"
-              onClick={() => setDrawerOpen(false)}>✕</button>
+            {/* No close button of its own. The header's menu button is a
+                cross while this is open, an inch above — two crosses for one
+                job, and the one people reach for is the one that is in the
+                same place on every screen. */}
             <div className="article-drawer-tabs" role="tablist">
               <button type="button" role="tab" aria-selected={drawerTab === 'menu'}
                 onClick={() => setDrawerTab('menu')}>Main menu</button>
diff --git a/frontend/src/pages/ArticlesPage.css b/frontend/src/pages/ArticlesPage.css
index de4ba40..537dd5f 100644
--- a/frontend/src/pages/ArticlesPage.css
+++ b/frontend/src/pages/ArticlesPage.css
@@ -160,11 +160,6 @@
     display: flex; align-items: center; gap: 10px;
     padding: 10px 12px; border-bottom: 1px solid var(--border); flex: none;
   }
-  .article-drawer-close {
-    flex: none; width: 34px; height: 34px;
-    background: none; border: 0; border-radius: 8px;
-    font-size: 1rem; cursor: pointer; color: var(--text-muted);
-  }
   .article-drawer-tabs {
     display: flex; flex: 1; border: 1px solid var(--border);
     border-radius: 8px; overflow: hidden;
diff --git a/frontend/src/pages/QuizPage.jsx b/frontend/src/pages/QuizPage.jsx
index 6265a08..c8d97ae 100644
--- a/frontend/src/pages/QuizPage.jsx
+++ b/frontend/src/pages/QuizPage.jsx
@@ -1602,12 +1602,9 @@ const timerStarted = timeLeft !== null
             <div className="quiz-drawer-head">
               {/* The same burger that opened it closes it: on a phone this
                   drawer is what that button does while a session is open. */}
-              {/* A cross, because the menu is open. The button that opened
-                  it kept its ☰ while the drawer covered the screen, which
-                  reads as a second menu to open rather than the way out of
-                  the one you are looking at. */}
-              <button type="button" className="quiz-drawer-close" aria-label="Close"
-                onClick={() => setNavOpen(false)}>✕</button>
+              {/* No close button here either: the header's menu button is a
+                  cross while this is open, and it is the one that is in the
+                  same place on every screen. */}
               <div className="quiz-drawer-tabs" role="tablist">
                 <button type="button" role="tab" aria-selected={drawerTab === 'menu'}
                   onClick={() => setDrawerTab('menu')}>Main menu</button>
diff --git a/frontend/src/pages/QuizPlayer.css b/frontend/src/pages/QuizPlayer.css
index 358bfd8..a67899e 100644
--- a/frontend/src/pages/QuizPlayer.css
+++ b/frontend/src/pages/QuizPlayer.css
@@ -498,11 +498,6 @@ body:has(.quiz-player.is-boxed) .site-footer { display: none; }
   display: flex; align-items: center; gap: 10px;
   padding: 10px 12px; border-bottom: 1px solid var(--border); flex: none;
 }
-.quiz-drawer-close {
-  flex: none; width: 34px; height: 34px;
-  background: none; border: 0; border-radius: 8px;
-  font-size: 1rem; cursor: pointer; color: var(--text-muted);
-}
 .quiz-drawer-tabs { display: flex; flex: 1; border: 1px solid var(--border); border-radius: 8px; overflow: hidden; }
 .quiz-drawer-tabs button {
   flex: 1; padding: 9px 6px; min-height: 40px;
diff --git a/frontend/src/pages/ResultsPage.jsx b/frontend/src/pages/ResultsPage.jsx
index 5283abe..9f15934 100644
--- a/frontend/src/pages/ResultsPage.jsx
+++ b/frontend/src/pages/ResultsPage.jsx
@@ -121,12 +121,9 @@ export default function ResultsPage() {
         <div className="quiz-drawer" onClick={e => e.target === e.currentTarget && setNavOpen(false)}>
           <div className="quiz-drawer-panel" role="dialog" aria-modal="true" aria-label="Answer review">
             <div className="quiz-drawer-head">
-              {/* A cross, because the menu is open. The button that opened
-                  it kept its ☰ while the drawer covered the screen, which
-                  reads as a second menu to open rather than the way out of
-                  the one you are looking at. */}
-              <button type="button" className="quiz-drawer-close" aria-label="Close"
-                onClick={() => setNavOpen(false)}>✕</button>
+              {/* No close button here either: the header's menu button is a
+                  cross while this is open, and it is the one that is in the
+                  same place on every screen. */}
               <div className="quiz-drawer-tabs" role="tablist">
                 <button type="button" role="tab" aria-selected={drawerTab === 'menu'}
                   onClick={() => setDrawerTab('menu')}>Main menu</button>
diff --git a/frontend/src/pages/SsoCallbackPage.jsx b/frontend/src/pages/SsoCallbackPage.jsx
index 273d08b..29b52a4 100644
--- a/frontend/src/pages/SsoCallbackPage.jsx
+++ b/frontend/src/pages/SsoCallbackPage.jsx
@@ -1,19 +1,38 @@
-import { useEffect } from 'react'
+import { useEffect, useRef } from 'react'
 import { useSearchParams, useNavigate } from 'react-router-dom'
+import api from '../api/client'
 import { useAuth } from '../context/AuthContext'
 
+/**
+ * Where the provider sends you back to, holding a code rather than a token.
+ *
+ * The token used to travel in this page's own query string, which meant the
+ * browser asked nginx for `/sso-callback?token=<a live bearer token>` — and
+ * nginx logs the request line. Every sign-in wrote a token good for a day into
+ * the frontend container's access log, the browser's history, and the Referer
+ * of whatever loaded next.
+ *
+ * So the redirect carries a one-time code instead, worth a single exchange
+ * inside a minute. It is spent here, over POST, where nothing writes it down.
+ */
 export default function SsoCallbackPage() {
   const [searchParams] = useSearchParams()
   const navigate = useNavigate()
   const { loginWithToken } = useAuth()
+  // React runs an effect twice in development's strict mode, and the code is
+  // deliberately good for one exchange — the second attempt would fail and
+  // bounce a perfectly good sign-in to the error page.
+  const spent = useRef(false)
 
   useEffect(() => {
-    const token = searchParams.get('token')
-    if (token) {
-      loginWithToken(token).then(() => navigate('/')).catch(() => navigate('/login?error=sso_failed'))
-    } else {
-      navigate('/login?error=sso_failed')
-    }
+    if (spent.current) return
+    spent.current = true
+    const code = searchParams.get('code')
+    if (!code) { navigate('/login?error=sso_failed'); return }
+    api.post('/auth/sso/exchange', { code })
+      .then(res => loginWithToken(res.data.access_token))
+      .then(() => navigate('/'))
+      .catch(() => navigate('/login?error=sso_failed'))
   }, [])
 
   return (