feat: add rate limiting middleware for authentication endpoints

Add slowapi-based rate limiting: 5/min on password login, 10/min on
WebAuthn login. Includes shared rate limiter reset fixture for tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/porchlight/app.py

@@ -8,8 +8,10 @@ from urllib.parse import urlparse
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
+from slowapi.errors import RateLimitExceeded
 from starlette.middleware.sessions import SessionMiddleware
 from starlette.requests import Request
+from starlette.responses import HTMLResponse as StarletteHTMLResponse
 
 from porchlight.admin.routes import router as admin_router
 from porchlight.authn.password import PasswordService
@@ -18,6 +20,7 @@ from porchlight.authn.webauthn import WebAuthnService
 from porchlight.config import Settings, StorageBackend
 from porchlight.csrf import CSRFMiddleware, generate_csrf_token
 from porchlight.invite.service import MagicLinkService
+from porchlight.rate_limit import limiter
 from porchlight.manage.routes import router as manage_router
 from porchlight.oidc.endpoints import router as oidc_router
 from porchlight.oidc.provider import create_oidc_server
@@ -123,6 +126,16 @@ def create_app(settings: Settings | None = None) -> FastAPI:
         https_only=settings.session_https_only,
     )
 
+    # Rate limiting
+    app.state.limiter = limiter
+
+    @app.exception_handler(RateLimitExceeded)
+    async def rate_limit_handler(request: Request, exc: RateLimitExceeded) -> StarletteHTMLResponse:
+        return StarletteHTMLResponse(
+            '<div role="alert">Too many attempts. Please try again later.</div>',
+            status_code=429,
+        )
+
     # Templates
     templates = Jinja2Templates(directory=str(PACKAGE_DIR / "templates"))
 

src/porchlight/authn/routes.py

@@ -5,6 +5,7 @@ from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse
 from fido2.webauthn import AttestedCredentialData, AuthenticationResponse
 
 from porchlight.models import User
+from porchlight.rate_limit import limiter
 from porchlight.userid import generate_unique_userid
 
 router = APIRouter(tags=["authn"])
@@ -28,6 +29,7 @@ async def login_page(request: Request) -> HTMLResponse:
 
 
 @router.post("/login/password", response_class=HTMLResponse)
+@limiter.limit("5/minute")
 async def login_password(
     request: Request,
     username: str = Form(),
@@ -107,6 +109,7 @@ async def login_webauthn_begin(request: Request) -> Response:
 
 
 @router.post("/login/webauthn/complete")
+@limiter.limit("10/minute")
 async def login_webauthn_complete(request: Request) -> Response:
     webauthn_service = request.app.state.webauthn_service
     user_repo = request.app.state.user_repo

src/porchlight/rate_limit.py

@@ -0,0 +1,4 @@
+from slowapi import Limiter
+from slowapi.util import get_remote_address
+
+limiter = Limiter(key_func=get_remote_address)

tests/conftest.py

@@ -6,6 +6,7 @@ from httpx import ASGITransport, AsyncClient
 
 from porchlight.app import create_app
 from porchlight.config import Settings
+from porchlight.rate_limit import limiter
 
 
 @pytest.fixture
@@ -21,6 +22,12 @@ async def client(settings: Settings) -> AsyncIterator[AsyncClient]:
         yield ac
 
 
+@pytest.fixture(autouse=True)
+def _reset_rate_limiter() -> None:
+    """Reset the rate limiter storage before each test."""
+    limiter.reset()
+
+
 async def get_csrf_token(client: AsyncClient) -> str:
     """Get a CSRF token by visiting the login page.
 

tests/test_rate_limit.py

@@ -0,0 +1,25 @@
+import pytest
+from httpx import AsyncClient
+
+from tests.conftest import get_csrf_token
+
+
+@pytest.mark.asyncio
+async def test_password_login_rate_limited(client: AsyncClient) -> None:
+    """After 5 failed attempts, the 6th should be rate-limited."""
+    token = await get_csrf_token(client)
+
+    for _ in range(5):
+        await client.post(
+            "/login/password",
+            data={"username": "nobody", "password": "wrong"},
+            headers={"X-CSRF-Token": token},
+        )
+
+    response = await client.post(
+        "/login/password",
+        data={"username": "nobody", "password": "wrong"},
+        headers={"X-CSRF-Token": token},
+    )
+
+    assert response.status_code == 429