Feat: Rate-limit login endpoint to block brute-force attacks

After LOGIN_MAX_FAILURES consecutive failed attempts from the same source
IP within LOGIN_WINDOW_SECONDS, POST /api/v1/auth/token returns HTTP 429
with a Retry-After header for LOGIN_COOLDOWN_SECONDS. A successful login
resets the counter. Trusted upstream proxy IPs/CIDRs can be declared via
LOGIN_TRUSTED_PROXY_IPS so X-Forwarded-For is honoured correctly behind
nginx ingress or similar reverse proxies.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

specs/009-login-rate-limiting/tasks.md

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+# Tasks: Login Brute-Force Protection
+
+**Input**: Design documents from `specs/009-login-rate-limiting/`
+**Prerequisites**: plan.md ✅, spec.md ✅, research.md ✅, data-model.md ✅, contracts/auth.md ✅, quickstart.md ✅
+
+**Tests**: TDD is non-negotiable (§5.1). Every test task appears before the implementation task it covers. For each red step, run the test and confirm it fails before proceeding to the implementation.
+
+**Organization**: Phase 1 adds env vars; Phase 2 adds config fields (shared by both stories); Phase 3 implements the core blocking behaviour (US1 MVP); Phase 4 adds observability-specific test coverage (US2); Phase 5 is polish.
+
+## Format: `[ID] [P?] [Story] Description`
+
+- **[P]**: Can run in parallel with other [P] tasks in the same phase
+- **[Story]**: Which user story this task belongs to
+- Exact file paths included in every task description
+
+---
+
+## Phase 1: Setup
+
+- [X] T001 Add a `# Login brute-force protection` comment block with `LOGIN_MAX_FAILURES=5`, `LOGIN_WINDOW_SECONDS=300`, `LOGIN_COOLDOWN_SECONDS=900`, and `LOGIN_TRUSTED_PROXY_IPS=` (empty by default, with an inline comment explaining it accepts comma-separated IPs/CIDRs) to both `.env.example` and `.env.test.example` at the repo root
+
+---
+
+## Phase 2: Foundational
+
+**Purpose**: Add the three new settings fields — required before any story implementation.
+
+- [X] T002 Add `login_max_failures: int = 5`, `login_window_seconds: int = 300`, `login_cooldown_seconds: int = 900`, `login_trusted_proxy_ips: str = ""` to the `Settings` class in `api/app/config.py` (append after `owner_password`)
+
+**Checkpoint**: `api/app/config.py` accepts all three new env vars with defaults.
+
+---
+
+## Phase 3: User Story 1 — Repeated failed logins are blocked (Priority: P1) 🎯 MVP
+
+**Goal**: After `LOGIN_MAX_FAILURES` consecutive failed login attempts from the same source IP within `LOGIN_WINDOW_SECONDS`, `POST /api/v1/auth/token` returns HTTP 429 for `LOGIN_COOLDOWN_SECONDS`. A successful login resets the counter.
+
+**Independent Test**: `cd api && python -m pytest tests/unit/test_rate_limiter.py tests/integration/test_login_rate_limit.py::test_repeated_failures_trigger_429 tests/integration/test_login_rate_limit.py::test_success_resets_counter tests/integration/test_login_rate_limit.py::test_429_has_retry_after_header tests/integration/test_login_rate_limit.py::test_xff_header_ignored_when_no_trusted_networks -v` — all pass.
+
+### Tests for User Story 1 (TDD red — write first, confirm failure before T005)
+
+- [X] T003 [P] [US1] Create `api/tests/unit/test_rate_limiter.py` with ten failing unit tests — import `LoginRateLimiter` and `get_client_ip` from `app.auth.rate_limiter`; for `LoginRateLimiter` (instantiate with `max_failures=3, window_seconds=60, cooldown_seconds=300`): `test_not_blocked_initially`, `test_blocked_after_threshold`, `test_success_clears_failures`, `test_ips_are_isolated`, `test_window_resets_after_expiry`, `test_log_warning_on_lockout` (caplog at WARNING level: call `record_failure()` until threshold, assert `"Login blocked" in caplog.text` and IP in log output); for `get_client_ip` (construct a mock using `from unittest.mock import MagicMock` and `from starlette.requests import Request`: `req = MagicMock(spec=Request); req.client.host = "10.0.0.1"; req.headers = {"X-Forwarded-For": "203.0.113.5"}`): `test_get_client_ip_no_trusted_networks_returns_peer` (empty `trusted_networks=[]` → returns `req.client.host`), `test_get_client_ip_trusted_peer_uses_xff` (peer `"10.0.0.1"` in trusted CIDR `"10.0.0.0/8"` → returns `"203.0.113.5"`), `test_get_client_ip_untrusted_peer_ignores_xff` (peer `"8.8.8.8"` not in trusted CIDR `"10.0.0.0/8"` → returns `"8.8.8.8"` despite XFF), `test_get_client_ip_trusted_peer_falls_back_to_real_ip` (peer trusted, no XFF header, `X-Real-IP: "203.0.113.9"` → returns `"203.0.113.9"`); run `python -m pytest tests/unit/test_rate_limiter.py -v` and confirm `ImportError` or `ModuleNotFoundError` (red)
+- [X] T004 [P] [US1] Create `api/tests/integration/test_login_rate_limit.py` with four failing integration tests; each must override both `app.state.login_rate_limiter` (fresh `LoginRateLimiter(max_failures=3, window_seconds=60, cooldown_seconds=30)`) and `app.state.login_trusted_networks` (set to `[]` for all four tests — the `ASGITransport` peer is `"testclient"`, not a valid IP, so trusted-network matching can't be exercised here; proxy extraction is fully covered by T003 unit tests) via try/finally: (1) `test_repeated_failures_trigger_429` — POST three bad-credential requests then assert fourth returns 429 with `resp.json()["code"] == "login_rate_limited"`; (2) `test_success_resets_counter` — two failures → one valid login using `{"username": os.environ["OWNER_USERNAME"], "password": os.environ["OWNER_PASSWORD"]}` (matching conftest.py defaults: `testowner`/`testpassword`) → three more failures → assert all three return 401, not 429; (3) `test_429_has_retry_after_header` — trigger lockout (three failures), then assert `"Retry-After" in resp.headers` and `int(resp.headers["Retry-After"]) > 0`; (4) `test_xff_header_ignored_when_no_trusted_networks` — send three bad-cred requests with `headers={"X-Forwarded-For": "1.2.3.4"}` then a fourth with `headers={"X-Forwarded-For": "9.9.9.9"}` — assert the fourth returns 429 (not 401), proving the limiter tracked the real peer `"testclient"` for all requests and XFF was ignored; run `python -m pytest tests/integration/test_login_rate_limit.py -v` and confirm failure (red)
+
+### Implementation for User Story 1
+
+- [X] T005 [US1] Create `api/app/auth/rate_limiter.py` with two exports: (1) `get_client_ip(request: Request, trusted_networks: list[IPv4Network | IPv6Network]) -> str` — imports `ipaddress`, `from ipaddress import IPv4Network, IPv6Network`, `from starlette.requests import Request`; extracts `peer = request.client.host if request.client else "unknown"`; if `trusted_networks` is non-empty and peer is parseable as an IP address and falls within any trusted network, returns first `X-Forwarded-For` entry (strip whitespace) or `X-Real-IP` value, otherwise returns `peer`; wraps `ipaddress.ip_address(peer)` in `try/except ValueError` and falls back to `peer` on error; (2) `LoginRateLimiter` class: `__init__(self, max_failures: int = 5, window_seconds: int = 300, cooldown_seconds: int = 900)` storing params as `_max`, `_window`, `_cooldown`; `_store: dict[str, _Record]` and `_lock: threading.Lock`; `@dataclass _Record` with `failures: int = 0`, `window_start: float = field(default_factory=time.time)`, `blocked_until: float = 0.0`; `is_blocked(ip: str) -> bool`, `record_failure(ip: str) -> None` (logs WARNING on lockout), `record_success(ip: str) -> None`, `cooldown_seconds` property; stdlib imports: `import ipaddress, logging, time`, `from dataclasses import dataclass, field`, `from threading import Lock`
+- [X] T006 [US1] Update `api/app/main.py` lifespan: add `import ipaddress` at top; import `LoginRateLimiter` from `app.auth.rate_limiter`; inside `lifespan` before `engine = get_engine()`, consolidate to `settings = get_settings()` (remove the existing bare `get_settings()` call), then set `application.state.login_rate_limiter = LoginRateLimiter(max_failures=settings.login_max_failures, window_seconds=settings.login_window_seconds, cooldown_seconds=settings.login_cooldown_seconds)`; then parse `settings.login_trusted_proxy_ips` — split on `","`, strip each part, skip empty strings, call `ipaddress.ip_network(part, strict=False)` inside a `try/except ValueError` (skip invalid entries silently), collect results into `trusted_networks: list`; set `application.state.login_trusted_networks = trusted_networks`
+- [X] T007 [US1] Update `api/app/routers/auth.py` login endpoint: add `Request` to FastAPI imports and add `from fastapi.responses import JSONResponse`; add `from app.auth.rate_limiter import LoginRateLimiter, get_client_ip`; add `request: Request` as first parameter to `login()`; extract `limiter: LoginRateLimiter = request.app.state.login_rate_limiter` and `ip: str = get_client_ip(request, request.app.state.login_trusted_networks)`; add guard block — if `limiter.is_blocked(ip)`: return `JSONResponse(status_code=429, content={"detail": "Too many failed login attempts. Please try again later.", "code": "login_rate_limited"}, headers={"Retry-After": str(limiter.cooldown_seconds)})`; after `verify_credentials` returns False: call `limiter.record_failure(ip)` before the existing `HTTPException`; after `auth.create_token()`: call `limiter.record_success(ip)` before returning `TokenResponse`
+- [X] T008 [US1] Verify TDD green: run `cd api && python -m pytest tests/unit/test_rate_limiter.py -v` — all 10 pass; run `make test-integration` — all tests pass including `test_repeated_failures_trigger_429`, `test_success_resets_counter`, `test_429_has_retry_after_header`, and `test_xff_header_ignored_when_no_trusted_networks`
+
+**Checkpoint**: Brute-force blocking is live. Automated repeated failures are stopped after threshold; the owner can still log in after cooldown; unit and integration tests pass.
+
+---
+
+## Phase 4: User Story 2 — Operators can observe blocking activity (Priority: P2)
+
+**Goal**: The 429 response includes a `Retry-After` header with a positive integer; the response body `code` is `"login_rate_limited"` and contains no threshold numeric values; server logs a WARNING when blocking triggers.
+
+**Independent Test**: Trigger the rate limiter (already works from Phase 3) and assert `Retry-After` header is present in the response and `code` field is `"login_rate_limited"`.
+
+### Tests for User Story 2 (TDD red — extend existing file)
+
+- [X] T009 [US2] Add one test to `api/tests/integration/test_login_rate_limit.py` targeting observability properties not yet covered: `test_429_body_shape` — override `app.state.login_rate_limiter` with a fresh `LoginRateLimiter(max_failures=3, window_seconds=60, cooldown_seconds=30)` via try/finally (same isolation pattern as T004), trigger lockout (three failures), then assert `resp.json() == {"detail": "Too many failed login attempts. Please try again later.", "code": "login_rate_limited"}` (exact match — confirms no threshold values leak and shape is correct); confirm this test is green immediately against the US1 implementation (T007 already returns this exact body)
+
+**Checkpoint**: US2 observability properties are explicitly exercised by integration tests; a future regression in the Retry-After header or code field will be caught.
+
+---
+
+## Phase 5: Polish & Cross-Cutting Concerns
+
+- [X] T010 Run `cd api && ruff check app/auth/rate_limiter.py app/routers/auth.py app/config.py app/main.py tests/unit/test_rate_limiter.py tests/integration/test_login_rate_limit.py` — fix any violations
+
+---
+
+## Dependencies & Execution Order
+
+### Phase Dependencies
+
+- **Phase 1 (Setup)**: No external dependencies — can start immediately
+- **Phase 2 (Foundational)**: No external dependencies — can start immediately (parallel with Phase 1)
+- **Phase 3 (US1)**: Depends on Phase 2 (T002 must exist before T006 can use `settings.login_max_failures`)
+- **Phase 4 (US2)**: Depends on Phase 3 (tests verify behaviour implemented in T007)
+- **Phase 5 (Polish)**: Depends on all prior phases
+
+### Within Phase 3
+
+- T003 ∥ T004 (different files, no dependency — write tests in parallel)
+- T005 after T003, T004 (implement after tests confirm they fail)
+- T006 ∥ T007 after T005 (both import from `rate_limiter.py`; write to different files — `main.py` and `auth.py`; T006 sets `app.state.login_trusted_networks` which T007's router reads)
+- T008 after T005, T006, T007 (verify all pass)
+
+### Execution Order Summary
+
+```
+Step 1: T001 ∥ T002 (setup + foundational — parallel, different files)
+Step 2: T003 ∥ T004 (write failing tests — parallel)
+Step 3: T005 (implement LoginRateLimiter — after red tests confirmed)
+Step 4: T006 ∥ T007 (wire limiter into app — parallel, different files)
+Step 5: T008 (verify green)
+Step 6: T009 (US2 observability tests — verify green immediately)
+Step 7: T010 (ruff clean)
+```
+
+---
+
+## Implementation Strategy
+
+### MVP (US1 — the blocker)
+
+1. Complete T001–T002 (config setup)
+2. Complete T003–T008 (core blocking)
+3. **Validate**: Run `make test-integration` — all 88 existing tests still pass; 2 new rate-limit tests pass
+4. US2 adds verification coverage for already-implemented observability features
+
+### Incremental Delivery
+
+- After Phase 3: Brute-force attacks on the login endpoint are blocked — core security net is in place
+- After Phase 4: Observability properties are explicitly tested — regressions in headers/logs will be caught
+- After Phase 5: Lint clean, ready for merge