diff --git a/src/porchlight/admin/routes.py b/src/porchlight/admin/routes.py
index 4bcc0a8..fe7404f 100644
--- a/src/porchlight/admin/routes.py
+++ b/src/porchlight/admin/routes.py
@@ -11,6 +11,15 @@ from porchlight.validation import GroupListInput, ProfileUpdate, UsernameInput,
 
 router = APIRouter(prefix="/admin", tags=["admin"])
 
+ADMIN_GROUP = "admin"
+
+
+async def _is_last_active_admin(request: Request, user: User) -> bool:
+    """True if removing this user's admin access would leave zero active admins."""
+    if not user.active or ADMIN_GROUP not in user.groups:
+        return False
+    return await request.app.state.user_repo.count_active_admins() <= 1
+
 
 async def _get_admin_user(request: Request) -> User | None:
     """Return the current user if they are an admin, else None."""
@@ -198,6 +207,9 @@ async def update_user_groups(
     if user is None:
         return HTMLResponse("User not found", status_code=404)
 
+    if ADMIN_GROUP not in validated.group_list and await _is_last_active_admin(request, user):
+        return HTMLResponse('<div role="alert">Cannot remove the last active admin</div>')
+
     updated = user.model_copy(update={"groups": validated.group_list})
     await user_repo.update(updated)
     return HTMLResponse('<div role="status">Groups updated</div>')
@@ -241,6 +253,9 @@ async def deactivate_user(request: Request, userid: str) -> Response:
     if user is None:
         return HTMLResponse("User not found", status_code=404)
 
+    if await _is_last_active_admin(request, user):
+        return HTMLResponse('<div role="alert">Cannot deactivate the last active admin</div>')
+
     updated = user.model_copy(update={"active": False})
     await user_repo.update(updated)
     return HTMLResponse(
@@ -261,10 +276,12 @@ async def delete_user_password(request: Request, userid: str) -> Response:
         return HTMLResponse("Forbidden", status_code=403)
 
     cred_repo = request.app.state.credential_repo
-    await cred_repo.delete_password(userid)
+    # Refuse atomically if this is the user's last credential (avoids lockout).
+    deleted = await cred_repo.delete_password_if_not_last(userid)
 
     # Re-render credentials section
     webauthn_credentials = await cred_repo.get_webauthn_by_user(userid)
+    password = await cred_repo.get_password_by_user(userid)
     templates = request.app.state.templates
     return templates.TemplateResponse(
         request,
@@ -272,7 +289,8 @@ async def delete_user_password(request: Request, userid: str) -> Response:
         {
             "target_user": await request.app.state.user_repo.get_by_userid(userid),
             "webauthn_credentials": webauthn_credentials,
-            "has_password": False,
+            "has_password": password is not None,
+            "error": None if deleted else "Cannot remove the user's last credential",
         },
     )
 
@@ -289,7 +307,8 @@ async def delete_user_webauthn(request: Request, userid: str, credential_id_b64:
     padded = credential_id_b64 + "=" * (-len(credential_id_b64) % 4)
     credential_id = urlsafe_b64decode(padded)
     cred_repo = request.app.state.credential_repo
-    await cred_repo.delete_webauthn(userid, credential_id)
+    # Refuse atomically if this is the user's last credential (avoids lockout).
+    deleted = await cred_repo.delete_webauthn_if_not_last(userid, credential_id)
 
     # Re-render credentials section
     webauthn_credentials = await cred_repo.get_webauthn_by_user(userid)
@@ -302,6 +321,7 @@ async def delete_user_webauthn(request: Request, userid: str, credential_id_b64:
             "target_user": await request.app.state.user_repo.get_by_userid(userid),
             "webauthn_credentials": webauthn_credentials,
             "has_password": password is not None,
+            "error": None if deleted else "Cannot remove the user's last credential",
         },
     )
 
@@ -345,6 +365,10 @@ async def delete_user(request: Request, userid: str) -> Response:
         return HTMLResponse('<div role="alert">Cannot delete your own account</div>')
 
     user_repo = request.app.state.user_repo
+    target = await user_repo.get_by_userid(userid)
+    if target is not None and await _is_last_active_admin(request, target):
+        return HTMLResponse('<div role="alert">Cannot delete the last active admin</div>')
+
     deleted = await user_repo.delete(userid)
     if not deleted:
         return HTMLResponse("User not found", status_code=404)
diff --git a/src/porchlight/authn/routes.py b/src/porchlight/authn/routes.py
index c710f3f..606b315 100644
--- a/src/porchlight/authn/routes.py
+++ b/src/porchlight/authn/routes.py
@@ -23,6 +23,21 @@ def _login_redirect_target(request: Request) -> str:
     return "/manage/credentials"
 
 
+def _establish_authenticated_session(request: Request, user: User) -> None:
+    """Reset the session before recording the authenticated identity.
+
+    Clearing first defeats session fixation: any values an attacker planted in
+    the pre-auth session are dropped and the session cookie is reissued. A
+    pending OIDC authorization request is the only pre-auth state worth keeping.
+    """
+    pending_oidc = request.session.get("oidc_auth_request")
+    request.session.clear()
+    if pending_oidc is not None:
+        request.session["oidc_auth_request"] = pending_oidc
+    request.session["userid"] = user.userid
+    request.session["username"] = user.username
+
+
 @router.get("/login", response_class=HTMLResponse)
 async def login_page(request: Request) -> HTMLResponse:
     templates = request.app.state.templates
@@ -56,8 +71,7 @@ async def login_password(
     if not user.active:
         return HTMLResponse(error_html)
 
-    request.session["userid"] = user.userid
-    request.session["username"] = user.username
+    _establish_authenticated_session(request, user)
 
     response = Response()
     response.headers["HX-Redirect"] = _login_redirect_target(request)
@@ -72,12 +86,34 @@ async def logout(request: Request) -> Response:
     return response
 
 
-@router.get("/register/{token}")
+@router.get("/register/{token}", response_class=HTMLResponse)
+async def register_magic_link_page(request: Request, token: str) -> Response:
+    """Show the registration confirmation page.
+
+    GET is side-effect free: it only validates the token (without consuming it)
+    and renders a form. The token is consumed by the POST below, so that simply
+    visiting the link (email scanners, prefetchers) cannot create a session.
+    """
+    magic_link_service = request.app.state.magic_link_service
+    link = await magic_link_service.validate(token)
+    if link is None:
+        return HTMLResponse("<p>Invalid or expired registration link.</p>", status_code=400)
+
+    templates = request.app.state.templates
+    return templates.TemplateResponse(
+        request,
+        "register.html",
+        {"token": token, "username": link.username},
+    )
+
+
+@router.post("/register/{token}")
 async def register_magic_link(request: Request, token: str) -> Response:
     magic_link_service = request.app.state.magic_link_service
     user_repo = request.app.state.user_repo
 
-    link = await magic_link_service.validate(token)
+    # Atomically validate and consume the token (single-use, no replay race).
+    link = await magic_link_service.consume(token)
     if link is None:
         return HTMLResponse("<p>Invalid or expired registration link.</p>", status_code=400)
 
@@ -85,16 +121,25 @@ async def register_magic_link(request: Request, token: str) -> Response:
     if existing_user is not None:
         if not existing_user.active:
             return HTMLResponse("<p>This account has been deactivated.</p>", status_code=400)
+        # An invite link is for account setup, not authentication. If the
+        # account already has credentials, refuse to establish a session —
+        # otherwise a re-invite would be a passwordless login. Account
+        # recovery must go through a separate, explicitly authenticated flow.
+        cred_repo = request.app.state.credential_repo
+        has_password = await cred_repo.get_password_by_user(existing_user.userid) is not None
+        has_webauthn = bool(await cred_repo.get_webauthn_by_user(existing_user.userid))
+        if has_password or has_webauthn:
+            return HTMLResponse(
+                "<p>This account is already set up. Please sign in instead.</p>",
+                status_code=400,
+            )
         user = existing_user
     else:
         userid = await generate_unique_userid(user_repo)
         user = User(userid=userid, username=link.username, groups=["users"])
         await user_repo.create(user)
 
-    await magic_link_service.mark_used(token)
-
-    request.session["userid"] = user.userid
-    request.session["username"] = user.username
+    _establish_authenticated_session(request, user)
 
     return RedirectResponse("/manage/credentials?setup=1", status_code=303)
 
@@ -156,7 +201,6 @@ async def login_webauthn_complete(request: Request) -> Response:
     if user is None or not user.active:
         return JSONResponse({"error": "Authentication failed"}, status_code=400)
 
-    request.session["userid"] = user.userid
-    request.session["username"] = user.username
+    _establish_authenticated_session(request, user)
 
     return JSONResponse({"redirect": _login_redirect_target(request)})
diff --git a/src/porchlight/config.py b/src/porchlight/config.py
index 38fb597..dff55a5 100644
--- a/src/porchlight/config.py
+++ b/src/porchlight/config.py
@@ -54,6 +54,10 @@ class Settings(BaseSettings):
 
     # Rate limiting (disable for e2e/load tests that authenticate repeatedly)
     rate_limit_enabled: bool = True
+    # Number of trusted reverse proxies in front of the app. When > 0, the
+    # client IP for rate limiting is taken from X-Forwarded-For, counting this
+    # many hops from the right. Keep 0 unless deployed behind a known proxy.
+    trusted_proxy_count: int = 0
 
     # Signing keys
     signing_key_path: str = "data/keys"
diff --git a/src/porchlight/invite/service.py b/src/porchlight/invite/service.py
index fd5bebf..4766442 100644
--- a/src/porchlight/invite/service.py
+++ b/src/porchlight/invite/service.py
@@ -1,3 +1,5 @@
+import hashlib
+import hmac
 import secrets
 from datetime import UTC, datetime, timedelta
 
@@ -6,11 +8,23 @@ from porchlight.store.protocols import MagicLinkRepository
 
 
 class MagicLinkService:
-    """Magic link token lifecycle management."""
+    """Magic link token lifecycle management.
 
-    def __init__(self, repo: MagicLinkRepository, ttl: int = 86400) -> None:
+    The raw token is only ever exposed to the user (in the registration URL).
+    Only a hash of the token is persisted, so a database disclosure does not
+    yield usable tokens. A pepper, when configured, keys the hash (HMAC);
+    otherwise a plain SHA-256 of the high-entropy token is stored.
+    """
+
+    def __init__(self, repo: MagicLinkRepository, ttl: int = 86400, pepper: str | None = None) -> None:
         self._repo = repo
         self._ttl = ttl
+        self._pepper = pepper
+
+    def _hash_token(self, token: str) -> str:
+        if self._pepper:
+            return hmac.new(self._pepper.encode(), token.encode(), hashlib.sha256).hexdigest()
+        return hashlib.sha256(token.encode()).hexdigest()
 
     async def create(
         self,
@@ -18,34 +32,52 @@ class MagicLinkService:
         created_by: str | None = None,
         note: str | None = None,
     ) -> MagicLink:
-        """Generate and store a new magic link for the given username."""
+        """Generate and store a new magic link for the given username.
+
+        Persists only the hashed token; the returned MagicLink carries the raw
+        token so the caller can build the registration URL.
+        """
         token = secrets.token_urlsafe(32)
         expires_at = datetime.now(UTC) + timedelta(seconds=self._ttl)
-        link = MagicLink(
-            token=token,
+        stored = MagicLink(
+            token=self._hash_token(token),
             username=username,
             expires_at=expires_at,
             created_by=created_by,
             note=note,
         )
-        return await self._repo.create(link)
+        await self._repo.create(stored)
+        return stored.model_copy(update={"token": token})
 
     async def validate(self, token: str) -> MagicLink | None:
         """Validate a magic link token.
 
-        Returns the MagicLink if it exists, is not used, and has not expired.
-        Returns None otherwise.
+        Returns the MagicLink (with the raw token re-attached) if it exists, is
+        not used, and has not expired. Returns None otherwise.
         """
-        link = await self._repo.get_by_token(token)
+        link = await self._repo.get_by_token(self._hash_token(token))
         if link is None or link.used:
             return None
         if link.expires_at < datetime.now(UTC):
             return None
-        return link
+        return link.model_copy(update={"token": token})
 
     async def mark_used(self, token: str) -> bool:
         """Mark a magic link as used. Returns True if found and marked."""
-        return await self._repo.mark_used(token)
+        return await self._repo.mark_used(self._hash_token(token))
+
+    async def consume(self, token: str) -> MagicLink | None:
+        """Atomically validate and consume a token in one step.
+
+        Prefer this over validate()+mark_used(): it closes the replay race
+        where two concurrent requests could both pass validation before either
+        marks the token used. Returns the link (raw token re-attached) on
+        success, else None.
+        """
+        link = await self._repo.consume(self._hash_token(token))
+        if link is None:
+            return None
+        return link.model_copy(update={"token": token})
 
     async def cleanup_expired(self) -> int:
         """Delete expired unused links. Returns count deleted."""
diff --git a/src/porchlight/manage/routes.py b/src/porchlight/manage/routes.py
index 69afdff..92a7912 100644
--- a/src/porchlight/manage/routes.py
+++ b/src/porchlight/manage/routes.py
@@ -13,13 +13,6 @@ from porchlight.validation import PasswordChange, PasswordSet, ProfileUpdate, fo
 router = APIRouter(prefix="/manage", tags=["manage"])
 
 
-async def _count_credentials(cred_repo: object, userid: str) -> int:
-    """Count total credentials (password + webauthn) for a user."""
-    webauthn = await cred_repo.get_webauthn_by_user(userid)  # type: ignore[union-attr]
-    password = await cred_repo.get_password_by_user(userid)  # type: ignore[union-attr]
-    return len(webauthn) + (1 if password else 0)
-
-
 @router.get("/credentials", response_class=HTMLResponse)
 async def credentials_page(request: Request) -> Response:
     session_user = get_session_user(request)
@@ -107,11 +100,9 @@ async def delete_password(request: Request) -> Response:
     userid, _username = session_user
     cred_repo = request.app.state.credential_repo
 
-    count = await _count_credentials(cred_repo, userid)
-    if count <= 1:
+    # Atomic: refuses if this is the user's last credential (not raceable).
+    if not await cred_repo.delete_password_if_not_last(userid):
         return HTMLResponse('<div role="alert">Cannot remove your last credential</div>')
-
-    await cred_repo.delete_password(userid)
     return HTMLResponse('<div role="status">Password removed</div>')
 
 
@@ -182,11 +173,9 @@ async def delete_webauthn(request: Request, credential_id_b64: str) -> Response:
     padded = credential_id_b64 + "=" * (-len(credential_id_b64) % 4)
     credential_id = urlsafe_b64decode(padded)
 
-    count = await _count_credentials(cred_repo, userid)
-    if count <= 1:
+    # Atomic: refuses if this is the user's last credential (not raceable).
+    if not await cred_repo.delete_webauthn_if_not_last(userid, credential_id):
         return HTMLResponse('<div role="alert">Cannot remove your last credential</div>')
-
-    await cred_repo.delete_webauthn(userid, credential_id)
     return HTMLResponse('<div role="status">Security key removed</div>')
 
 
diff --git a/src/porchlight/oidc/endpoints.py b/src/porchlight/oidc/endpoints.py
index 67d4968..12af079 100644
--- a/src/porchlight/oidc/endpoints.py
+++ b/src/porchlight/oidc/endpoints.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import html
 import json
 from types import SimpleNamespace
 from urllib.parse import urlencode
@@ -14,6 +15,15 @@ from porchlight.oidc.claims import PorchlightUserInfo, user_to_claims
 
 router = APIRouter(tags=["oidc"])
 
+
+def _error_page(message: object, status_code: int = 400, title: str = "Error") -> HTMLResponse:
+    """Render an error page, escaping the (possibly request-derived) message."""
+    return HTMLResponse(
+        f"<h1>{html.escape(title)}</h1><p>{html.escape(str(message))}</p>",
+        status_code=status_code,
+    )
+
+
 SCOPE_DESCRIPTIONS: dict[str, str] = {
     "openid": "Sign you in (required)",
     "profile": "Your name and profile information",
@@ -59,11 +69,11 @@ async def authorization(request: Request) -> Response:
     try:
         parsed = endpoint.parse_request(query_params)
     except Exception as exc:
-        return HTMLResponse(f"<h1>Invalid Request</h1><p>{exc}</p>", status_code=400)
+        return _error_page(exc, title="Invalid Request")
 
     if "error" in parsed:
         error_desc = parsed.get("error_description", parsed["error"])
-        return HTMLResponse(f"<h1>Error</h1><p>{error_desc}</p>", status_code=400)
+        return _error_page(error_desc)
 
     # Check if user is authenticated
     userid = request.session.get("userid")
@@ -95,11 +105,11 @@ async def authorization_complete(request: Request) -> Response:
     try:
         parsed = endpoint.parse_request(auth_request_params)
     except Exception as exc:
-        return HTMLResponse(f"<h1>Error</h1><p>{exc}</p>", status_code=400)
+        return _error_page(exc)
 
     if "error" in parsed:
         error_desc = parsed.get("error_description", parsed["error"])
-        return HTMLResponse(f"<h1>Error</h1><p>{error_desc}</p>", status_code=400)
+        return _error_page(error_desc)
 
     return await _check_consent_or_complete(
         request, oidc_server, endpoint, parsed, userid, username, auth_request_params
@@ -172,7 +182,7 @@ async def _complete_authorization(  # noqa: PLR0913
     if "error" in result.get("response_args", {}):
         response_args = result["response_args"]
         error_desc = response_args.get("error_description", response_args["error"])
-        return HTMLResponse(f"<h1>Error</h1><p>{error_desc}</p>", status_code=400)
+        return _error_page(error_desc)
 
     # Build redirect URL
     response_args = result.get("response_args", {})
@@ -338,8 +348,10 @@ async def consent_submit(request: Request) -> Response:
             return RedirectResponse(f"{redirect_uri}?{params}", status_code=303)
         return HTMLResponse("<h1>Error</h1><p>Invalid action</p>", status_code=400)
 
-    # Allow — collect approved scopes
-    approved_scopes: list[str] = [str(s) for s in form.getlist("scope")]
+    # Allow — collect approved scopes, rejecting anything outside the
+    # originally requested set (a forged form must not escalate scope).
+    requested_scopes = set(auth_params.get("scope", "openid").split())
+    approved_scopes: list[str] = [str(s) for s in form.getlist("scope") if str(s) in requested_scopes]
     if "openid" not in approved_scopes:
         approved_scopes = ["openid", *list(approved_scopes)]
 
@@ -359,6 +371,6 @@ async def consent_submit(request: Request) -> Response:
         if "error" in parsed:
             raise ValueError(parsed.get("error_description", parsed["error"]))
     except Exception as exc:
-        return HTMLResponse(f"<h1>Error</h1><p>{exc}</p>", status_code=400)
+        return _error_page(exc)
 
     return await _complete_authorization(request, oidc_server, endpoint, parsed, userid, username)
diff --git a/src/porchlight/oidc/provider.py b/src/porchlight/oidc/provider.py
index d1ac38b..aae64d3 100644
--- a/src/porchlight/oidc/provider.py
+++ b/src/porchlight/oidc/provider.py
@@ -3,6 +3,7 @@
 from pathlib import Path
 
 from idpyoidc.server import Server
+from idpyoidc.server.oauth2.add_on.pkce import CC_METHOD
 
 from porchlight.config import Settings
 from porchlight.oidc.claims import PorchlightUserInfo
@@ -126,6 +127,20 @@ def _build_server_config(settings: Settings) -> dict:
             "phone": ["phone_number", "phone_number_verified"],
         },
         "authentication": {},
+        # PKCE: advertise and verify S256 (only). Not "essential", so existing
+        # relying parties that do not yet send a code_challenge keep working;
+        # but any RP that does use PKCE must use S256 and is verified at the
+        # token endpoint. Set essential=True (or per-client pkce_essential) to
+        # require it once all clients have migrated.
+        "add_on": {
+            "pkce": {
+                "function": "idpyoidc.server.oauth2.add_on.pkce.add_support",
+                "kwargs": {
+                    "essential": False,
+                    "code_challenge_methods": {"S256": CC_METHOD["S256"]},
+                },
+            },
+        },
     }
 
 
diff --git a/src/porchlight/rate_limit.py b/src/porchlight/rate_limit.py
index 38404a8..a0a6eff 100644
--- a/src/porchlight/rate_limit.py
+++ b/src/porchlight/rate_limit.py
@@ -1,4 +1,25 @@
 from slowapi import Limiter
 from slowapi.util import get_remote_address
+from starlette.requests import Request
 
-limiter = Limiter(key_func=get_remote_address)
+
+def _client_identifier(request: Request) -> str:
+    """Rate-limit key: the client IP, proxy-aware.
+
+    By default the direct peer address is used. When ``trusted_proxy_count`` is
+    configured (the number of trusted reverse proxies in front of the app), the
+    client address is taken from the ``X-Forwarded-For`` chain instead, counting
+    that many hops back from the right. The header is ignored unless proxies are
+    trusted, so it cannot be spoofed in a direct-connection deployment.
+    """
+    settings = getattr(getattr(request.app, "state", None), "settings", None)
+    hops: int = getattr(settings, "trusted_proxy_count", 0) or 0
+    if hops > 0:
+        forwarded = request.headers.get("x-forwarded-for", "")
+        parts = [p.strip() for p in forwarded.split(",") if p.strip()]
+        if len(parts) >= hops:
+            return parts[-hops]
+    return get_remote_address(request)
+
+
+limiter = Limiter(key_func=_client_identifier)
diff --git a/src/porchlight/static/style.css b/src/porchlight/static/style.css
index 17319ff..15dc79e 100644
--- a/src/porchlight/static/style.css
+++ b/src/porchlight/static/style.css
@@ -205,6 +205,10 @@ main {
     line-height: 1;
 }
 
+#user-table-container {
+    overflow-x: auto;
+}
+
 .admin-table {
     width: 100%;
     border-collapse: collapse;
@@ -214,7 +218,7 @@ main {
 .admin-table th,
 .admin-table td {
     text-align: left;
-    padding: var(--sp-2) var(--sp-3);
+    padding: var(--sp-2);
     border-bottom: 1px solid var(--border);
 }
 
diff --git a/src/porchlight/store/protocols.py b/src/porchlight/store/protocols.py
index ba1af34..53e7c37 100644
--- a/src/porchlight/store/protocols.py
+++ b/src/porchlight/store/protocols.py
@@ -25,6 +25,8 @@ class UserRepository(Protocol):
 
     async def count_users(self, query: str | None = None) -> int: ...
 
+    async def count_active_admins(self) -> int: ...
+
     async def delete(self, userid: str) -> bool: ...
 
 
@@ -46,6 +48,10 @@ class CredentialRepository(Protocol):
 
     async def delete_password(self, user_id: str) -> bool: ...
 
+    async def delete_password_if_not_last(self, user_id: str) -> bool: ...
+
+    async def delete_webauthn_if_not_last(self, user_id: str, credential_id: bytes) -> bool: ...
+
 
 @runtime_checkable
 class MagicLinkRepository(Protocol):
@@ -55,6 +61,8 @@ class MagicLinkRepository(Protocol):
 
     async def mark_used(self, token: str) -> bool: ...
 
+    async def consume(self, token: str) -> MagicLink | None: ...
+
     async def delete_expired(self) -> int: ...
 
 
diff --git a/src/porchlight/store/sqlite/repositories.py b/src/porchlight/store/sqlite/repositories.py
index 37d02bb..f9ebd81 100644
--- a/src/porchlight/store/sqlite/repositories.py
+++ b/src/porchlight/store/sqlite/repositories.py
@@ -163,6 +163,17 @@ class SQLiteUserRepository:
                 row = await cursor.fetchone()
         return row[0] if row else 0
 
+    async def count_active_admins(self) -> int:
+        async with self._db.execute(
+            """
+            SELECT COUNT(*) FROM users u
+            JOIN user_groups g ON g.userid = u.userid
+            WHERE u.active = 1 AND g.group_name = 'admin'
+            """
+        ) as cursor:
+            row = await cursor.fetchone()
+        return row[0] if row else 0
+
     async def delete(self, userid: str) -> bool:
         cursor = await self._db.execute("DELETE FROM users WHERE userid = ?", (userid,))
         await self._db.commit()
@@ -266,6 +277,41 @@ class SQLiteCredentialRepository:
         await self._db.commit()
         return cursor.rowcount > 0
 
+    async def delete_password_if_not_last(self, user_id: str) -> bool:
+        """Delete the password credential only if it is not the user's last
+        credential. The count and delete happen in one atomic statement, so it
+        is not raceable. Returns True if a row was deleted."""
+        cursor = await self._db.execute(
+            """
+            DELETE FROM password_credentials
+            WHERE user_id = ?
+              AND (
+                (SELECT COUNT(*) FROM password_credentials WHERE user_id = ?)
+                + (SELECT COUNT(*) FROM webauthn_credentials WHERE user_id = ?)
+              ) > 1
+            """,
+            (user_id, user_id, user_id),
+        )
+        await self._db.commit()
+        return cursor.rowcount > 0
+
+    async def delete_webauthn_if_not_last(self, user_id: str, credential_id: bytes) -> bool:
+        """Delete a WebAuthn credential only if it is not the user's last
+        credential, atomically. Returns True if a row was deleted."""
+        cursor = await self._db.execute(
+            """
+            DELETE FROM webauthn_credentials
+            WHERE user_id = ? AND credential_id = ?
+              AND (
+                (SELECT COUNT(*) FROM password_credentials WHERE user_id = ?)
+                + (SELECT COUNT(*) FROM webauthn_credentials WHERE user_id = ?)
+              ) > 1
+            """,
+            (user_id, credential_id, user_id, user_id),
+        )
+        await self._db.commit()
+        return cursor.rowcount > 0
+
 
 class SQLiteMagicLinkRepository:
     def __init__(self, db: aiosqlite.Connection) -> None:
@@ -311,6 +357,25 @@ class SQLiteMagicLinkRepository:
         await self._db.commit()
         return cursor.rowcount > 0
 
+    async def consume(self, token: str) -> MagicLink | None:
+        """Atomically validate-and-mark a token as used.
+
+        The conditional UPDATE is the single point of decision, so two
+        concurrent requests cannot both consume the same token. Returns the
+        link if this call won the race (unused and unexpired), else None.
+        """
+        now = datetime.now(UTC).isoformat()
+        cursor = await self._db.execute(
+            "UPDATE magic_links SET used = 1 WHERE token = ? AND used = 0 AND expires_at > ?",
+            (token, now),
+        )
+        await self._db.commit()
+        if cursor.rowcount == 0:
+            return None
+        async with self._db.execute("SELECT * FROM magic_links WHERE token = ?", (token,)) as c:
+            row = await c.fetchone()
+        return self._row_to_magic_link(row) if row is not None else None
+
     async def delete_expired(self) -> int:
         now = datetime.now(UTC).isoformat()
         cursor = await self._db.execute("DELETE FROM magic_links WHERE expires_at < ? AND used = 0", (now,))
diff --git a/src/porchlight/templates/admin/_credentials_section.html b/src/porchlight/templates/admin/_credentials_section.html
index b2351eb..fbfbe8e 100644
--- a/src/porchlight/templates/admin/_credentials_section.html
+++ b/src/porchlight/templates/admin/_credentials_section.html
@@ -1,3 +1,4 @@
+{% if error %}<div role="alert">{{ error }}</div>{% endif %}
 <h3>Password</h3>
 {% if has_password %}
 <p>Password is set.
diff --git a/src/porchlight/templates/admin/base.html b/src/porchlight/templates/admin/base.html
index 686a815..4a82870 100644
--- a/src/porchlight/templates/admin/base.html
+++ b/src/porchlight/templates/admin/base.html
@@ -4,6 +4,7 @@
 <nav class="admin-nav" aria-label="Administration">
     <span class="admin-badge">Admin</span>
     <a href="/admin/users" {% if active_page == "users" %}aria-current="page"{% endif %}>Users</a>
+    <a href="/manage/profile">Profile</a>
     <button class="nav-logout" hx-post="/logout">Log out</button>
 </nav>
 {% block admin_content %}{% endblock %}
diff --git a/src/porchlight/templates/register.html b/src/porchlight/templates/register.html
new file mode 100644
index 0000000..6b9a668
--- /dev/null
+++ b/src/porchlight/templates/register.html
@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+
+{% block title %}Set up your account — Porchlight{% endblock %}
+
+{% block content %}
+<h1>Set up your account</h1>
+
+<p>You're about to set up the account for <strong>{{ username }}</strong>.</p>
+
+<form method="post" action="/register/{{ token }}">
+    <input type="hidden" name="csrf_token" value="{{ csrf_token_processor(request) }}">
+    <button type="submit">Continue</button>
+</form>
+{% endblock %}
diff --git a/src/porchlight/validation.py b/src/porchlight/validation.py
index 5d3b48c..e675e0b 100644
--- a/src/porchlight/validation.py
+++ b/src/porchlight/validation.py
@@ -1,3 +1,4 @@
+import html
 import re
 from typing import Annotated
 from urllib.parse import urlparse
@@ -166,7 +167,9 @@ def format_validation_errors(exc: ValidationError) -> str:
                 display_msg = f"{label}: {raw}"
         else:
             display_msg = f"{label}: {msg}"
-        messages.append(display_msg)
+        # Escape: messages can echo user input (e.g. an invalid group name),
+        # and the result is interpolated into HTML below.
+        messages.append(html.escape(display_msg))
 
     if len(messages) == 1:
         return f'<div role="alert">{messages[0]}</div>'
diff --git a/tests/e2e/full-flow.spec.js b/tests/e2e/full-flow.spec.js
index be797d2..584078f 100644
--- a/tests/e2e/full-flow.spec.js
+++ b/tests/e2e/full-flow.spec.js
@@ -9,7 +9,9 @@ test.describe('Full user journey', () => {
     expect(fixtures.register_token).toBeTruthy();
 
     // ---- Step 1: Register via magic link ----
+    // GET shows a confirmation page; submitting it (POST) consumes the token.
     await page.goto(`/register/${fixtures.register_token}`);
+    await page.click('button[type="submit"]');
 
     // Should redirect to /manage/credentials?setup=1
     await page.waitForURL('**/manage/credentials?setup=1', { timeout: 5000 });
diff --git a/tests/test_admin/test_admin_routes.py b/tests/test_admin/test_admin_routes.py
index 603af60..ae87b93 100644
--- a/tests/test_admin/test_admin_routes.py
+++ b/tests/test_admin/test_admin_routes.py
@@ -328,11 +328,15 @@ async def test_delete_password_credential(client: AsyncClient) -> None:
     await _login(client)
     target = await _create_target_user(client, username="bob")
 
-    # Create a password credential for the target user
+    # Create a password credential for the target user, plus a second
+    # credential so the password is not the last one (last-credential guard).
     app = client._transport.app  # type: ignore[union-attr]
     cred_repo = app.state.credential_repo
     svc = PasswordService(hasher=PasswordHasher(time_cost=1, memory_cost=8192))
     await cred_repo.create_password(PasswordCredential(user_id=target.userid, password_hash=svc.hash("bobpass")))
+    await cred_repo.create_webauthn(
+        WebAuthnCredential(user_id=target.userid, credential_id=b"\xaa\xbb", public_key=b"\x00" * 32)
+    )
 
     token = await get_csrf_token(client)
     response = await client.request(
@@ -351,7 +355,8 @@ async def test_delete_webauthn_credential(client: AsyncClient) -> None:
     await _login(client)
     target = await _create_target_user(client, username="bob")
 
-    # Create a webauthn credential for the target user
+    # Create a webauthn credential for the target user, plus a second
+    # credential so the key is not the last one (last-credential guard).
     app = client._transport.app  # type: ignore[union-attr]
     cred_repo = app.state.credential_repo
     credential_id = b"\x01\x02\x03\x04\x05\x06\x07\x08"
@@ -364,6 +369,8 @@ async def test_delete_webauthn_credential(client: AsyncClient) -> None:
             device_name="test-key",
         )
     )
+    svc = PasswordService(hasher=PasswordHasher(time_cost=1, memory_cost=8192))
+    await cred_repo.create_password(PasswordCredential(user_id=target.userid, password_hash=svc.hash("bobpass")))
 
     # URL uses base64url without padding
     credential_id_b64 = urlsafe_b64encode(credential_id).decode().rstrip("=")
@@ -434,3 +441,92 @@ async def test_reinvite_user_404_for_nonexistent(client: AsyncClient) -> None:
     response = await client.post("/admin/users/nonexistent-id/invite", headers={"X-CSRF-Token": token})
     assert response.status_code == 404
     assert "not found" in response.text.lower()
+
+
+@pytest.mark.asyncio
+async def test_admin_cannot_delete_users_last_credential(client: AsyncClient) -> None:
+    await _login(client)
+    target = await _create_target_user(client, userid="lonecred-01", username="lonecred")
+
+    app = client._transport.app  # type: ignore[union-attr]
+    cred_repo = app.state.credential_repo
+    svc = PasswordService(hasher=PasswordHasher(time_cost=1, memory_cost=8192))
+    await cred_repo.create_password(PasswordCredential(user_id=target.userid, password_hash=svc.hash("x")))
+
+    token = await get_csrf_token(client)
+    res = await client.request(
+        "DELETE",
+        f"/admin/users/{target.userid}/credentials/password",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+
+    # The last credential must survive, and the admin is told why.
+    assert await cred_repo.get_password_by_user(target.userid) is not None
+    assert "last credential" in res.text.lower()
+
+
+async def _make_sole_admin_target(client: AsyncClient) -> User:
+    """Create a second admin, then return it as the target. The logged-in
+    admin from _login plus this one means 2 admins; callers deactivate the
+    _login admin first if they need this to be the *last* admin."""
+    return await _create_target_user(client, userid="admin2-01", username="admin2", groups=["admin"])
+
+
+@pytest.mark.asyncio
+async def test_cannot_deactivate_last_active_admin(client: AsyncClient) -> None:
+    await _login(client)  # the only admin
+    app = client._transport.app  # type: ignore[union-attr]
+    admin = await app.state.user_repo.get_by_username("admin")
+
+    token = await get_csrf_token(client)
+    res = await client.post(
+        f"/admin/users/{admin.userid}/deactivate",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    still = await app.state.user_repo.get_by_userid(admin.userid)
+    assert still.active is True
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
+
+
+@pytest.mark.asyncio
+async def test_cannot_remove_admin_group_from_last_admin(client: AsyncClient) -> None:
+    await _login(client)
+    app = client._transport.app  # type: ignore[union-attr]
+    admin = await app.state.user_repo.get_by_username("admin")
+
+    token = await get_csrf_token(client)
+    res = await client.post(
+        f"/admin/users/{admin.userid}/groups",
+        data={"groups": "users"},  # drops "admin"
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    still = await app.state.user_repo.get_by_userid(admin.userid)
+    assert "admin" in still.groups
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
+
+
+@pytest.mark.asyncio
+async def test_cannot_delete_last_active_admin(client: AsyncClient) -> None:
+    await _login(client)
+    app = client._transport.app  # type: ignore[union-attr]
+    # A second, separate admin to delete (not self — self-delete is blocked separately).
+    target = await _create_target_user(client, userid="otheradmin-01", username="otheradmin", groups=["admin"])
+    # Deactivate the logged-in admin's peer? Instead make target the only ACTIVE admin
+    # by deactivating the _login admin via repo, so deleting target would orphan admins.
+    login_admin = await app.state.user_repo.get_by_username("admin")
+    await app.state.user_repo.update(login_admin.model_copy(update={"active": False}))
+
+    token = await get_csrf_token(client)
+    res = await client.request(
+        "DELETE",
+        f"/admin/users/{target.userid}",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    assert await app.state.user_repo.get_by_userid(target.userid) is not None
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
diff --git a/tests/test_admin_groups_validation.py b/tests/test_admin_groups_validation.py
index ff662e9..32b1ecc 100644
--- a/tests/test_admin_groups_validation.py
+++ b/tests/test_admin_groups_validation.py
@@ -41,6 +41,8 @@ async def _setup_admin_and_target(client: AsyncClient) -> tuple[str, str]:
         data={"username": "admin_g", "password": "AdminPass123!"},
         headers={"HX-Request": "true", "X-CSRF-Token": token},
     )
+    # Login resets the session (fixation defense); fetch a fresh CSRF token.
+    token = await get_csrf_token(client)
     return token, target.userid
 
 
diff --git a/tests/test_admin_invite_validation.py b/tests/test_admin_invite_validation.py
index 6bf1b68..73ccd73 100644
--- a/tests/test_admin_invite_validation.py
+++ b/tests/test_admin_invite_validation.py
@@ -32,7 +32,8 @@ async def _login_admin(client: AsyncClient) -> str:
         data={"username": "admin", "password": "AdminPass123!"},
         headers={"HX-Request": "true", "X-CSRF-Token": token},
     )
-    return token
+    # Login resets the session (fixation defense); fetch a fresh CSRF token.
+    return await get_csrf_token(client)
 
 
 @pytest.mark.asyncio
diff --git a/tests/test_auth_routes/test_login_session.py b/tests/test_auth_routes/test_login_session.py
new file mode 100644
index 0000000..8a7f0cc
--- /dev/null
+++ b/tests/test_auth_routes/test_login_session.py
@@ -0,0 +1,42 @@
+from datetime import UTC, datetime
+
+from httpx import AsyncClient
+
+from porchlight.authn.password import PasswordHasher, PasswordService
+from porchlight.models import PasswordCredential, User
+from tests.conftest import get_csrf_token
+
+
+async def _create_active_user(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    user = User(
+        userid="login-01",
+        username="loginuser",
+        created_at=datetime.now(UTC),
+        updated_at=datetime.now(UTC),
+    )
+    await app.state.user_repo.create(user)
+    svc = PasswordService(hasher=PasswordHasher(time_cost=1, memory_cost=8192))
+    await app.state.credential_repo.create_password(
+        PasswordCredential(user_id=user.userid, password_hash=svc.hash("password123!Secure"))
+    )
+
+
+async def test_login_resets_session_to_prevent_fixation(client: AsyncClient) -> None:
+    """Logging in must reset the pre-auth session so an attacker-fixed session
+    cannot become authenticated. We observe this via the CSRF token (stored in
+    the session) changing across login."""
+    await _create_active_user(client)
+
+    pre_login_token = await get_csrf_token(client)
+
+    res = await client.post(
+        "/login/password",
+        data={"username": "loginuser", "password": "password123!Secure"},
+        headers={"HX-Request": "true", "X-CSRF-Token": pre_login_token},
+    )
+    assert res.status_code == 200
+    assert res.headers.get("HX-Redirect")  # logged in
+
+    post_login_token = await get_csrf_token(client)
+    assert post_login_token != pre_login_token
diff --git a/tests/test_auth_routes/test_register_magic_link.py b/tests/test_auth_routes/test_register_magic_link.py
index 1a03fed..7ff47ba 100644
--- a/tests/test_auth_routes/test_register_magic_link.py
+++ b/tests/test_auth_routes/test_register_magic_link.py
@@ -1,8 +1,10 @@
-from datetime import UTC, datetime, timedelta
-
+from argon2 import PasswordHasher
 from httpx import AsyncClient
 
-from porchlight.models import MagicLink, User
+from porchlight.authn.password import PasswordService
+from porchlight.invite.service import MagicLinkService
+from porchlight.models import PasswordCredential, User
+from tests.conftest import get_csrf_token
 
 
 async def test_register_invalid_token_returns_error_page(client: AsyncClient) -> None:
@@ -13,42 +15,48 @@ async def test_register_invalid_token_returns_error_page(client: AsyncClient) ->
 
 async def test_register_expired_token_returns_error_page(client: AsyncClient) -> None:
     app = client._transport.app  # type: ignore[union-attr]
-    repo = app.state.magic_link_repo
-    await repo.create(
-        MagicLink(
-            token="expired",
-            username="newuser",
-            expires_at=datetime.now(UTC) - timedelta(hours=1),
-        )
-    )
+    # An already-expired link (negative TTL), stored hashed like the real service.
+    expired_service = MagicLinkService(repo=app.state.magic_link_repo, ttl=-3600)
+    link = await expired_service.create(username="newuser")
 
-    res = await client.get("/register/expired", follow_redirects=False)
+    res = await client.get(f"/register/{link.token}", follow_redirects=False)
     assert res.status_code == 400
     assert "Invalid or expired" in res.text
 
 
-async def test_register_valid_token_creates_user_and_redirects(client: AsyncClient) -> None:
+async def test_register_get_does_not_consume_token(client: AsyncClient) -> None:
+    """GET is side-effect free: it shows a confirmation form and must not
+    consume the token or create a session."""
     app = client._transport.app  # type: ignore[union-attr]
-    magic_link_repo = app.state.magic_link_repo
+    magic_link_service = app.state.magic_link_service
+
+    link = await magic_link_service.create(username="newuser")
+    res = await client.get(f"/register/{link.token}", follow_redirects=False)
+    assert res.status_code == 200
+    assert f'action="/register/{link.token}"' in res.text
+    # Token still valid (not consumed by the GET).
+    assert await magic_link_service.validate(link.token) is not None
+
+
+async def test_register_post_creates_user_and_redirects(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    magic_link_service = app.state.magic_link_service
     user_repo = app.state.user_repo
 
-    await magic_link_repo.create(
-        MagicLink(
-            token="t1",
-            username="newuser",
-            expires_at=datetime.now(UTC) + timedelta(hours=1),
-        )
-    )
+    link = await magic_link_service.create(username="newuser")
 
-    res = await client.get("/register/t1", follow_redirects=False)
+    csrf = await get_csrf_token(client)
+    res = await client.post(
+        f"/register/{link.token}",
+        headers={"X-CSRF-Token": csrf},
+        follow_redirects=False,
+    )
     assert res.status_code in (302, 303)
     assert "/manage/credentials" in res.headers["location"]
     assert "setup=1" in res.headers["location"]
 
-    # Token should be marked used
-    link = await magic_link_repo.get_by_token("t1")
-    assert link is not None
-    assert link.used is True
+    # Token should be consumed (no longer valid)
+    assert await magic_link_service.validate(link.token) is None
 
     # User should exist
     user = await user_repo.get_by_username("newuser")
@@ -56,53 +64,81 @@ async def test_register_valid_token_creates_user_and_redirects(client: AsyncClie
     assert "users" in user.groups
 
 
+async def test_register_post_requires_csrf(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    magic_link_service = app.state.magic_link_service
+    link = await magic_link_service.create(username="newuser")
+
+    res = await client.post(f"/register/{link.token}", follow_redirects=False)
+    assert res.status_code == 403
+    # Token must not have been consumed by the rejected request.
+    assert await magic_link_service.validate(link.token) is not None
+
+
 async def test_register_used_token_returns_error(client: AsyncClient) -> None:
     app = client._transport.app  # type: ignore[union-attr]
-    repo = app.state.magic_link_repo
-    await repo.create(
-        MagicLink(
-            token="used",
-            username="newuser",
-            expires_at=datetime.now(UTC) + timedelta(hours=1),
-            used=True,
-        )
-    )
+    magic_link_service = app.state.magic_link_service
+    link = await magic_link_service.create(username="newuser")
+    await magic_link_service.mark_used(link.token)
 
-    res = await client.get("/register/used", follow_redirects=False)
+    res = await client.get(f"/register/{link.token}", follow_redirects=False)
     assert res.status_code == 400
 
 
 async def test_register_existing_user_logs_in_and_redirects(client: AsyncClient) -> None:
-    """When initial-admin creates a user, the invite link should log them in."""
+    """When initial-admin creates a user (no credentials yet), the invite link
+    should let them set up their account."""
     app = client._transport.app  # type: ignore[union-attr]
-    magic_link_repo = app.state.magic_link_repo
+    magic_link_service = app.state.magic_link_service
     user_repo = app.state.user_repo
 
-    # Pre-create the user (as initial-admin would)
+    # Pre-create the user (as initial-admin would), with no credentials.
     user = User(userid="lusab-bansen", username="admin", groups=["admin", "users"])
     await user_repo.create(user)
 
-    # Create invite for the same username
-    await magic_link_repo.create(
-        MagicLink(
-            token="admin-setup",
-            username="admin",
-            expires_at=datetime.now(UTC) + timedelta(hours=1),
-        )
-    )
+    link = await magic_link_service.create(username="admin")
 
-    res = await client.get("/register/admin-setup", follow_redirects=False)
+    csrf = await get_csrf_token(client)
+    res = await client.post(
+        f"/register/{link.token}",
+        headers={"X-CSRF-Token": csrf},
+        follow_redirects=False,
+    )
     assert res.status_code in (302, 303)
     assert "/manage/credentials" in res.headers["location"]
     assert "setup=1" in res.headers["location"]
 
-    # Token should be marked used
-    link = await magic_link_repo.get_by_token("admin-setup")
-    assert link is not None
-    assert link.used is True
+    # Token should be consumed
+    assert await magic_link_service.validate(link.token) is None
 
     # Original user should still exist with original groups
     existing = await user_repo.get_by_username("admin")
     assert existing is not None
     assert existing.userid == "lusab-bansen"
     assert "admin" in existing.groups
+
+
+async def test_register_rejects_user_that_already_has_credentials(client: AsyncClient) -> None:
+    """An invite/re-invite link must not act as a passwordless login for an
+    account that already has credentials. Recovery is a separate flow."""
+    app = client._transport.app  # type: ignore[union-attr]
+    magic_link_service = app.state.magic_link_service
+    user_repo = app.state.user_repo
+    cred_repo = app.state.credential_repo
+
+    user = User(userid="lusab-hascreds", username="hascreds", groups=["users"])
+    await user_repo.create(user)
+    svc = PasswordService(hasher=PasswordHasher(time_cost=1, memory_cost=8192))
+    await cred_repo.create_password(PasswordCredential(user_id=user.userid, password_hash=svc.hash("existing-pass")))
+
+    link = await magic_link_service.create(username="hascreds")
+    csrf = await get_csrf_token(client)
+    res = await client.post(
+        f"/register/{link.token}",
+        headers={"X-CSRF-Token": csrf},
+        follow_redirects=False,
+    )
+
+    # No passwordless login: rejected, not redirected to setup.
+    assert res.status_code == 400
+    assert "setup=1" not in res.headers.get("location", "")
diff --git a/tests/test_authn_active.py b/tests/test_authn_active.py
index 3564bb5..6126343 100644
--- a/tests/test_authn_active.py
+++ b/tests/test_authn_active.py
@@ -61,6 +61,11 @@ async def test_inactive_user_cannot_register_magic_link(client: AsyncClient) ->
 
     link = await magic_link_service.create(username="deactivated", created_by="admin", note="test")
 
-    response = await client.get(f"/register/{link.token}", follow_redirects=False)
+    csrf = await get_csrf_token(client)
+    response = await client.post(
+        f"/register/{link.token}",
+        headers={"X-CSRF-Token": csrf},
+        follow_redirects=False,
+    )
 
     assert response.status_code == 400 or "deactivated" in response.text.lower() or "Invalid" in response.text
diff --git a/tests/test_invite/test_service.py b/tests/test_invite/test_service.py
index 6464fce..166e7cd 100644
--- a/tests/test_invite/test_service.py
+++ b/tests/test_invite/test_service.py
@@ -42,6 +42,18 @@ async def test_create_returns_magic_link(service: MagicLinkService) -> None:
     assert link.expires_at > datetime.now(UTC)
 
 
+async def test_token_stored_hashed_not_plaintext(service: MagicLinkService, repo: SQLiteMagicLinkRepository) -> None:
+    # The raw token handed to the user must never be the value persisted in
+    # the store; a DB read must not yield a usable token.
+    link = await service.create(username="alice")
+    raw = link.token
+    assert await repo.get_by_token(raw) is None
+    # ...but the raw token must still validate through the service.
+    validated = await service.validate(raw)
+    assert validated is not None
+    assert validated.username == "alice"
+
+
 async def test_create_generates_unique_tokens(service: MagicLinkService) -> None:
     link1 = await service.create(username="alice")
     link2 = await service.create(username="bob")
@@ -90,6 +102,22 @@ async def test_validate_expired_token(service: MagicLinkService, repo: SQLiteMag
     assert result is None
 
 
+async def test_consume_validates_and_is_single_use(service: MagicLinkService) -> None:
+    link = await service.create(username="alice")
+    consumed = await service.consume(link.token)
+    assert consumed is not None
+    assert consumed.username == "alice"
+    assert consumed.token == link.token  # raw token re-attached
+    # A second consume of the same token fails.
+    assert await service.consume(link.token) is None
+
+
+async def test_consume_expired_returns_none(service: MagicLinkService, repo: SQLiteMagicLinkRepository) -> None:
+    expired_service = MagicLinkService(repo=repo, ttl=-1)
+    link = await expired_service.create(username="alice")
+    assert await service.consume(link.token) is None
+
+
 async def test_mark_used_returns_true(service: MagicLinkService) -> None:
     link = await service.create(username="alice")
     result = await service.mark_used(link.token)
diff --git a/tests/test_oidc/test_authorization.py b/tests/test_oidc/test_authorization.py
index 5994134..de3c70b 100644
--- a/tests/test_oidc/test_authorization.py
+++ b/tests/test_oidc/test_authorization.py
@@ -2,6 +2,16 @@ import secrets
 
 from httpx import AsyncClient
 
+from porchlight.oidc.endpoints import _error_page
+
+
+def test_error_page_escapes_html() -> None:
+    # OIDC error pages must not interpolate request-derived text as raw HTML.
+    resp = _error_page("<script>alert(1)</script>")
+    body = resp.body.decode()
+    assert "<script>" not in body
+    assert "&lt;script&gt;alert(1)&lt;/script&gt;" in body
+
 
 def _register_test_client(
     client: AsyncClient,
diff --git a/tests/test_oidc/test_consent_flow.py b/tests/test_oidc/test_consent_flow.py
index 91d3d73..9b40006 100644
--- a/tests/test_oidc/test_consent_flow.py
+++ b/tests/test_oidc/test_consent_flow.py
@@ -223,6 +223,30 @@ async def test_partial_consent_filters_scopes(client: AsyncClient) -> None:
     assert set(consent.scopes) == {"openid", "profile"}
 
 
+async def test_consent_rejects_scopes_outside_request(client: AsyncClient) -> None:
+    """A forged consent POST cannot approve scopes that were not requested."""
+    app = client._transport.app  # type: ignore[union-attr]
+    _register_test_rp(app)
+    await _create_test_user(app)
+
+    # Only openid + profile were requested; "email" is forged into the POST.
+    await _login_and_start_auth(client, scope="openid profile")
+    token = await get_csrf_token(client)
+    res = await client.post(
+        "/consent",
+        data={"action": "allow", "scope": ["openid", "profile", "email"]},
+        headers={"X-CSRF-Token": token},
+        follow_redirects=False,
+    )
+    assert res.status_code == 303
+
+    consent_repo = app.state.consent_repo
+    consent = await consent_repo.get_consent("lusab-consent", "consent-rp")
+    assert consent is not None
+    assert "email" not in consent.scopes
+    assert set(consent.scopes) == {"openid", "profile"}
+
+
 # -- Test helpers --
 
 
diff --git a/tests/test_oidc/test_e2e_flow.py b/tests/test_oidc/test_e2e_flow.py
index b1b9545..cd3f547 100644
--- a/tests/test_oidc/test_e2e_flow.py
+++ b/tests/test_oidc/test_e2e_flow.py
@@ -1,6 +1,7 @@
+import hashlib
 import json
 import secrets
-from base64 import b64encode
+from base64 import b64encode, urlsafe_b64encode
 from datetime import UTC, datetime
 from typing import Any
 from urllib.parse import parse_qs, urlparse
@@ -54,19 +55,36 @@ async def _setup_rp_and_user(app: FastAPI) -> None:
     await cred_repo.create_password(PasswordCredential(user_id=user.userid, password_hash=svc.hash("testpass")))
 
 
-async def _login_and_authorize(client: AsyncClient, state: str, nonce: str) -> str:
+def _pkce_pair() -> tuple[str, str]:
+    """Return a (code_verifier, S256 code_challenge) pair."""
+    verifier = secrets.token_urlsafe(64)
+    challenge = urlsafe_b64encode(hashlib.sha256(verifier.encode()).digest()).rstrip(b"=").decode()
+    return verifier, challenge
+
+
+async def _login_and_authorize(
+    client: AsyncClient,
+    state: str,
+    nonce: str,
+    code_challenge: str | None = None,
+    code_challenge_method: str | None = None,
+) -> str:
     """Perform authorization request, login, consent, and return the auth code."""
+    params = {
+        "response_type": "code",
+        "client_id": CLIENT_ID,
+        "redirect_uri": REDIRECT_URI,
+        "scope": "openid profile email",
+        "state": state,
+        "nonce": nonce,
+    }
+    if code_challenge is not None:
+        params["code_challenge"] = code_challenge
+        params["code_challenge_method"] = code_challenge_method or "S256"
     # Step 1: Authorization request (unauthenticated) -> redirect to /login
     auth_res = await client.get(
         "/authorization",
-        params={
-            "response_type": "code",
-            "client_id": CLIENT_ID,
-            "redirect_uri": REDIRECT_URI,
-            "scope": "openid profile email",
-            "state": state,
-            "nonce": nonce,
-        },
+        params=params,
         follow_redirects=False,
     )
     assert auth_res.status_code in (302, 303), f"Expected redirect to /login, got {auth_res.status_code}"
@@ -116,16 +134,19 @@ async def _login_and_authorize(client: AsyncClient, state: str, nonce: str) -> s
     return code
 
 
-async def _exchange_token(client: AsyncClient, code: str) -> dict[str, Any]:
+async def _exchange_token(client: AsyncClient, code: str, code_verifier: str | None = None) -> dict[str, Any]:
     """Exchange authorization code for tokens."""
     auth_header = b64encode(f"{CLIENT_ID}:{CLIENT_SECRET}".encode()).decode()
+    data = {
+        "grant_type": "authorization_code",
+        "code": code,
+        "redirect_uri": REDIRECT_URI,
+    }
+    if code_verifier is not None:
+        data["code_verifier"] = code_verifier
     token_res = await client.post(
         "/token",
-        data={
-            "grant_type": "authorization_code",
-            "code": code,
-            "redirect_uri": REDIRECT_URI,
-        },
+        data=data,
         headers={
             "Authorization": f"Basic {auth_header}",
             "Content-Type": "application/x-www-form-urlencoded",
@@ -199,3 +220,46 @@ async def test_full_authorization_code_flow(client: AsyncClient) -> None:
 
     # sub should match between id_token and userinfo
     assert userinfo_data["sub"] == id_token_sub
+
+
+async def test_discovery_advertises_s256_pkce(client: AsyncClient) -> None:
+    res = await client.get("/.well-known/openid-configuration")
+    assert res.status_code == 200
+    assert res.json().get("code_challenge_methods_supported") == ["S256"]
+
+
+async def test_pkce_s256_flow_succeeds(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    await _setup_rp_and_user(app)
+    state, nonce = secrets.token_urlsafe(16), secrets.token_urlsafe(16)
+    verifier, challenge = _pkce_pair()
+
+    code = await _login_and_authorize(client, state, nonce, code_challenge=challenge)
+    token_data = await _exchange_token(client, code, code_verifier=verifier)
+    assert "access_token" in token_data
+
+
+async def test_pkce_wrong_verifier_rejected(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    await _setup_rp_and_user(app)
+    state, nonce = secrets.token_urlsafe(16), secrets.token_urlsafe(16)
+    _verifier, challenge = _pkce_pair()
+
+    code = await _login_and_authorize(client, state, nonce, code_challenge=challenge)
+
+    # Exchange with a verifier that does not match the challenge -> rejected.
+    auth_header = b64encode(f"{CLIENT_ID}:{CLIENT_SECRET}".encode()).decode()
+    res = await client.post(
+        "/token",
+        data={
+            "grant_type": "authorization_code",
+            "code": code,
+            "redirect_uri": REDIRECT_URI,
+            "code_verifier": "wrong-" + secrets.token_urlsafe(48),
+        },
+        headers={
+            "Authorization": f"Basic {auth_header}",
+            "Content-Type": "application/x-www-form-urlencoded",
+        },
+    )
+    assert res.status_code != 200, f"PKCE verifier mismatch should be rejected, got {res.status_code}: {res.text}"
diff --git a/tests/test_password_change.py b/tests/test_password_change.py
index ea8b672..d9ffb5b 100644
--- a/tests/test_password_change.py
+++ b/tests/test_password_change.py
@@ -31,7 +31,8 @@ async def _login_user_with_password(client: AsyncClient) -> str:
         data={"username": "pwuser", "password": "OldPass123!ok"},
         headers={"HX-Request": "true", "X-CSRF-Token": token},
     )
-    return token
+    # Login resets the session (fixation defense); fetch a fresh CSRF token.
+    return await get_csrf_token(client)
 
 
 async def _login_user_without_password(client: AsyncClient) -> str:
@@ -47,11 +48,15 @@ async def _login_user_without_password(client: AsyncClient) -> str:
     )
     await user_repo.create(user)
 
-    # Simulate session login via magic link
+    # Simulate session login via magic link (GET shows the form, POST consumes)
     token = await get_csrf_token(client)
     magic_link_service = app.state.magic_link_service
     link = await magic_link_service.create(username="newuser", created_by="admin", note="test")
-    await client.get(f"/register/{link.token}", follow_redirects=False)
+    await client.post(
+        f"/register/{link.token}",
+        headers={"X-CSRF-Token": token},
+        follow_redirects=False,
+    )
     # Re-fetch CSRF token after session change
     token = await get_csrf_token(client)
     return token
diff --git a/tests/test_rate_limit.py b/tests/test_rate_limit.py
index b84ea14..b25d19f 100644
--- a/tests/test_rate_limit.py
+++ b/tests/test_rate_limit.py
@@ -1,6 +1,10 @@
+from types import SimpleNamespace
+
 import pytest
 from httpx import AsyncClient
+from starlette.requests import Request
 
+from porchlight.rate_limit import _client_identifier
 from tests.conftest import get_csrf_token
 
 
@@ -23,3 +27,38 @@ async def test_password_login_rate_limited(client: AsyncClient) -> None:
     )
 
     assert response.status_code == 429
+
+
+def _make_request(headers: dict[str, str], client_host: str, trusted_proxy_count: int) -> Request:
+    scope = {
+        "type": "http",
+        "headers": [(k.lower().encode(), v.encode()) for k, v in headers.items()],
+        "client": (client_host, 12345),
+        "app": SimpleNamespace(
+            state=SimpleNamespace(settings=SimpleNamespace(trusted_proxy_count=trusted_proxy_count))
+        ),
+    }
+    return Request(scope)
+
+
+def test_client_identifier_ignores_forwarded_when_no_trusted_proxy() -> None:
+    req = _make_request({"x-forwarded-for": "203.0.113.9"}, "127.0.0.1", trusted_proxy_count=0)
+    # Spoofable header must be ignored when no proxy is trusted.
+    assert _client_identifier(req) == "127.0.0.1"
+
+
+def test_client_identifier_uses_forwarded_with_one_trusted_proxy() -> None:
+    req = _make_request({"x-forwarded-for": "203.0.113.9"}, "10.0.0.1", trusted_proxy_count=1)
+    assert _client_identifier(req) == "203.0.113.9"
+
+
+def test_client_identifier_picks_correct_hop_with_two_trusted_proxies() -> None:
+    req = _make_request({"x-forwarded-for": "client, proxyA, proxyB"}, "10.0.0.1", trusted_proxy_count=2)
+    # Two trusted proxies appended their peers; the client as seen by the
+    # outermost trusted proxy is the 2nd-from-right entry.
+    assert _client_identifier(req) == "proxyA"
+
+
+def test_client_identifier_falls_back_when_forwarded_missing() -> None:
+    req = _make_request({}, "10.0.0.1", trusted_proxy_count=1)
+    assert _client_identifier(req) == "10.0.0.1"
diff --git a/tests/test_store/test_sqlite_credential_repo.py b/tests/test_store/test_sqlite_credential_repo.py
index 2c5d33d..1c3396a 100644
--- a/tests/test_store/test_sqlite_credential_repo.py
+++ b/tests/test_store/test_sqlite_credential_repo.py
@@ -182,3 +182,48 @@ async def test_create_duplicate_webauthn(credential_repo: SQLiteCredentialReposi
 
     with pytest.raises(DuplicateError):
         await credential_repo.create_webauthn(cred)
+
+
+async def test_delete_password_if_not_last_refuses_last(
+    credential_repo: SQLiteCredentialRepository, alice: User
+) -> None:
+    await credential_repo.create_password(PasswordCredential(user_id=alice.userid, password_hash="h"))
+
+    # Only credential -> must refuse and leave it in place.
+    assert await credential_repo.delete_password_if_not_last(alice.userid) is False
+    assert await credential_repo.get_password_by_user(alice.userid) is not None
+
+
+async def test_delete_password_if_not_last_allows_when_others_exist(
+    credential_repo: SQLiteCredentialRepository, alice: User
+) -> None:
+    await credential_repo.create_password(PasswordCredential(user_id=alice.userid, password_hash="h"))
+    await credential_repo.create_webauthn(
+        WebAuthnCredential(user_id=alice.userid, credential_id=b"\x01", public_key=b"\x02")
+    )
+
+    assert await credential_repo.delete_password_if_not_last(alice.userid) is True
+    assert await credential_repo.get_password_by_user(alice.userid) is None
+
+
+async def test_delete_webauthn_if_not_last_refuses_last(
+    credential_repo: SQLiteCredentialRepository, alice: User
+) -> None:
+    await credential_repo.create_webauthn(
+        WebAuthnCredential(user_id=alice.userid, credential_id=b"\x01", public_key=b"\x02")
+    )
+
+    assert await credential_repo.delete_webauthn_if_not_last(alice.userid, b"\x01") is False
+    assert len(await credential_repo.get_webauthn_by_user(alice.userid)) == 1
+
+
+async def test_delete_webauthn_if_not_last_allows_when_others_exist(
+    credential_repo: SQLiteCredentialRepository, alice: User
+) -> None:
+    await credential_repo.create_webauthn(
+        WebAuthnCredential(user_id=alice.userid, credential_id=b"\x01", public_key=b"\x02")
+    )
+    await credential_repo.create_password(PasswordCredential(user_id=alice.userid, password_hash="h"))
+
+    assert await credential_repo.delete_webauthn_if_not_last(alice.userid, b"\x01") is True
+    assert len(await credential_repo.get_webauthn_by_user(alice.userid)) == 0
diff --git a/tests/test_store/test_sqlite_magic_link_repo.py b/tests/test_store/test_sqlite_magic_link_repo.py
index c2c9571..bb4f8b6 100644
--- a/tests/test_store/test_sqlite_magic_link_repo.py
+++ b/tests/test_store/test_sqlite_magic_link_repo.py
@@ -63,6 +63,34 @@ async def test_mark_used_not_found(magic_link_repo: SQLiteMagicLinkRepository) -
     assert marked is False
 
 
+async def test_consume_marks_used_and_returns_link(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    await magic_link_repo.create(_make_link())
+
+    consumed = await magic_link_repo.consume("abc123")
+    assert consumed is not None
+    assert consumed.used is True
+    assert consumed.username == "alice"
+
+
+async def test_consume_is_single_use(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    await magic_link_repo.create(_make_link())
+
+    first = await magic_link_repo.consume("abc123")
+    assert first is not None
+    # A second consume of the same token must fail (atomic single-use).
+    second = await magic_link_repo.consume("abc123")
+    assert second is None
+
+
+async def test_consume_expired_returns_none(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    await magic_link_repo.create(_make_link(token="exp", expires_at=datetime.now(UTC) - timedelta(hours=1)))
+    assert await magic_link_repo.consume("exp") is None
+
+
+async def test_consume_nonexistent_returns_none(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    assert await magic_link_repo.consume("nope") is None
+
+
 async def test_delete_expired(magic_link_repo: SQLiteMagicLinkRepository) -> None:
     expired = _make_link(token="expired", expires_at=datetime.now(UTC) - timedelta(hours=1))
     await magic_link_repo.create(expired)
diff --git a/tests/test_store/test_sqlite_user_repo.py b/tests/test_store/test_sqlite_user_repo.py
index bc6255e..aa2e555 100644
--- a/tests/test_store/test_sqlite_user_repo.py
+++ b/tests/test_store/test_sqlite_user_repo.py
@@ -237,3 +237,14 @@ async def test_roundtrip_preserves_all_fields(user_repo: SQLiteUserRepository) -
     assert sorted(fetched.groups) == ["admin", "users"]
     assert fetched.created_at == user.created_at
     assert fetched.updated_at == user.updated_at
+
+
+async def test_count_active_admins(user_repo: SQLiteUserRepository) -> None:
+    await user_repo.create(_make_user(userid="a1", username="admin1", groups=["admin", "users"]))
+    await user_repo.create(_make_user(userid="a2", username="admin2", groups=["admin"]))
+    # Inactive admin must not count.
+    await user_repo.create(_make_user(userid="a3", username="admin3", groups=["admin"], active=False))
+    # Non-admin must not count.
+    await user_repo.create(_make_user(userid="u1", username="user1", groups=["users"]))
+
+    assert await user_repo.count_active_admins() == 2
diff --git a/tests/test_validation.py b/tests/test_validation.py
index 2be1afc..93bf2ea 100644
--- a/tests/test_validation.py
+++ b/tests/test_validation.py
@@ -292,3 +292,15 @@ class TestFormatValidationErrors:
             result = format_validation_errors(exc)
             assert "Picture URL must be a valid HTTP or HTTPS URL" in result
             assert "Value error" not in result
+
+    def test_escapes_user_input_in_error_message(self) -> None:
+        # A validation error message that echoes user input (e.g. an invalid
+        # group name) must not emit raw HTML — otherwise it is reflected XSS.
+        payload = "<img src=x onerror=alert(1)>"
+        try:
+            GroupListInput(groups=payload)
+        except ValidationError as exc:
+            result = format_validation_errors(exc)
+            assert payload not in result
+            assert "&lt;img src=x onerror=alert(1)&gt;" in result
+            assert "<img" not in result