fix: harden MCP endpoint after code review

Critical: the MCP SDK auto-enables DNS-rebinding protection when its host parameter is left at the 127.0.0.1 default, hard-coding a loopback-only Host allow-list. Every remote client (the only deployment this feature targets) was refused with HTTP 421 before auth or the tool ran. Now driven by a new MCP_ALLOWED_HOSTS setting, with '*' as an explicit, logged opt-out. Also bounds query/top_k to match AskRequest (top_k is amplified 4x downstream, so an unbounded value was a resource-exhaustion vector), decodes the Authorization header as latin-1 per the ASGI spec instead of raising a 500 on malformed bytes, and returns WWW-Authenticate on 401 per RFC 7235. Moves the psycopg2 import guard into backend/tests/conftest.py: duplicated across four test modules, it only worked because of alphabetical collection order, and any earlier-sorting package would have reintroduced a live connection attempt against the production database. Registers the mcp module in the authoritative backend architecture doc. 84 backend tests pass. Verified against a live server: allowed remote Host returns a valid initialize result, unknown Host returns 421, missing token returns 401 with WWW-Authenticate. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
2026-07-29 17:11:54 +08:00
parent bd3dc38d1d
commit 49ee50c104
13 changed files with 330 additions and 51 deletions
@@ -469,3 +469,18 @@ Confirm `search_regulations` appears in the tool list and returns a real answer
 | 1 | `app/mcp/__init__.py`, `app/mcp/server.py`, `tests/mcp/__init__.py`, `tests/mcp/test_search_regulations_tool.py` | — | 3 |
 | 2 | `tests/mcp/test_mcp_auth_middleware.py` | `app/mcp/server.py` | 4 |
 | 3 | — | `requirements.txt`, `api/main.py` | 0 (full-suite regression check + manual e2e) |
+
+## Task 4 (post-review): code-review fixes
+
+Added after a code review of the three implementation commits. Findings and resolutions:
+
+- [x] **Critical — every remote client rejected with HTTP 421.** The MCP SDK auto-enables DNS-rebinding protection when its `host` parameter is left at the `127.0.0.1` default, hard-coding a loopback-only `Host` allow-list; a client at `http://6.86.80.9:8000/mcp/` was refused before auth or the tool ran, making the feature non-functional in the only deployment it targets. Fixed by passing an explicit `TransportSecuritySettings` built from a new `MCP_ALLOWED_HOSTS` setting (`app/config/settings.py`, documented in `.env.example`), with `*` as a logged, explicit opt-out. Deliberately *not* fixed by passing `host="0.0.0.0"`, which would silently disable the protection.
+- [x] **Important — `top_k` unbounded on the MCP path.** `AskRequest` constrains the same parameter to 1–20, but the tool accepted any integer and `KnowledgeRetrievalService` amplifies it (`top_k * 4`), so `top_k=100000` would request 400,000 Milvus candidates. Fixed with `Annotated[int, Field(ge=1, le=20)]` (and `query` bounded to 1–2000 chars), which also publishes the bounds in the advertised JSON schema.
+- [x] **Important — order-dependent `psycopg2` test guard.** The guard was duplicated across four test modules and only worked because of pytest's alphabetical collection order; any new test package sorting earlier would have reintroduced a multi-second TCP timeout against the production database. Moved into a single `backend/tests/conftest.py` (imported before any test module regardless of order) and the four in-file copies deleted. `bootstrap.py`'s eager imports were left alone — restructuring the composition root every route depends on is disproportionate to a test-harness ordering problem.
+- [x] **Minor — non-UTF-8 `Authorization` header caused a 500.** ASGI header values are latin-1; strict UTF-8 decoding let any remote client trigger an unhandled `UnicodeDecodeError`. Now decoded as latin-1, yielding a clean 401.
+- [x] **Minor — 401 missing `WWW-Authenticate`.** Added `WWW-Authenticate: Bearer`, matching `get_current_user` and RFC 7235.
+- [x] **Minor — missing `#` comment** in `tests/mcp/test_search_regulations_tool.py` (AGENTS.md requires at least one per file). Added.
+
+Reviewer-confirmed as correct, no change needed: the `AsyncExitStack` lifespan wiring (including its failure path), the absence of auth-bypass vectors, and the statelessness of `ask()` without a `session_id`.
+
+New tests: `tests/mcp/test_mcp_transport_security.py` (5, exercising the real MCP app end-to-end) plus 3 more across the existing two files — 84 backend tests pass. Verified against a live server: `Host: 6.86.80.9:8000` → 200 with a valid `initialize` result, `Host: evil.example.com` → 421, no token → 401 with `WWW-Authenticate: Bearer`.