fix(security): reset session on login to prevent fixation

Both password and WebAuthn login wrote the authenticated identity onto the
existing pre-auth session, so a fixed/planted session could be elevated to an
authenticated one. Add _establish_authenticated_session() which clears the
session (preserving only a pending OIDC authorization request) before setting
the identity, used by both login paths.

Tests that reused a pre-login CSRF token now re-fetch it after login, matching
real client behavior.

Refs: porchlight-vxr

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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
+    _establish_authenticated_session(request, user)
-    request.session["username"] = user.username
 
     response = Response()
     response.headers["HX-Redirect"] = _login_redirect_target(request)
@@ -167,7 +181,6 @@ 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
+    _establish_authenticated_session(request, user)
-    request.session["username"] = user.username
 
     return JSONResponse({"redirect": _login_redirect_target(request)})

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
 
 

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

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

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: