H-01: Add global CSRF middleware for all mutating endpoints

Replace the admin-only verify_xhr dependency with a pure ASGI
CSRFHeaderMiddleware that validates X-Requested-With: XMLHttpRequest
on all POST/PUT/PATCH/DELETE requests globally. Pre-auth endpoints
(login, setup, register, totp-verify, enforce-setup/confirm) are
exempt. This closes the CSRF gap where non-admin routes accepted
requests without origin validation.

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

backend/app/main.py

@@ -19,6 +19,59 @@ from app.models import system_config as _system_config_model  # noqa: F401
 from app.models import audit_log as _audit_log_model  # noqa: F401
 
 
+# ---------------------------------------------------------------------------
+# Pure ASGI CSRF middleware — SEC-08 (global)
+# ---------------------------------------------------------------------------
+
+class CSRFHeaderMiddleware:
+    """
+    Require X-Requested-With: XMLHttpRequest on all state-mutating requests.
+
+    Browsers never send this header cross-origin without a CORS preflight,
+    which our CORS policy blocks. This prevents CSRF attacks from simple
+    form submissions and cross-origin fetches.
+
+    Uses pure ASGI (not BaseHTTPMiddleware) to avoid streaming/memory overhead.
+    """
+
+    _EXEMPT_PATHS = frozenset({
+        "/health",
+        "/",
+        "/api/auth/login",
+        "/api/auth/setup",
+        "/api/auth/register",
+        "/api/auth/totp-verify",
+        "/api/auth/totp/enforce-setup",
+        "/api/auth/totp/enforce-confirm",
+    })
+    _MUTATING_METHODS = frozenset({"POST", "PUT", "PATCH", "DELETE"})
+
+    def __init__(self, app):
+        self.app = app
+
+    async def __call__(self, scope, receive, send):
+        if scope["type"] == "http":
+            method = scope.get("method", "")
+            path = scope.get("path", "")
+
+            if method in self._MUTATING_METHODS and path not in self._EXEMPT_PATHS:
+                headers = dict(scope.get("headers", []))
+                if headers.get(b"x-requested-with") != b"XMLHttpRequest":
+                    body = b'{"detail":"Invalid request origin"}'
+                    await send({
+                        "type": "http.response.start",
+                        "status": 403,
+                        "headers": [
+                            [b"content-type", b"application/json"],
+                            [b"content-length", str(len(body)).encode()],
+                        ],
+                    })
+                    await send({"type": "http.response.body", "body": body})
+                    return
+
+        await self.app(scope, receive, send)
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     scheduler = AsyncIOScheduler()
@@ -47,7 +100,12 @@ app = FastAPI(
     openapi_url="/openapi.json" if _is_dev else None,
 )
 
-# CORS configuration
+# Middleware stack — added in reverse order (last added = outermost).
+# CSRF is added first (innermost), then CORS wraps it (outermost).
+# This ensures CORS headers appear on CSRF 403 responses.
+app.add_middleware(CSRFHeaderMiddleware)
+
+# CORS configuration — outermost layer
 app.add_middleware(
     CORSMiddleware,
     allow_origins=[o.strip() for o in settings.CORS_ORIGINS.split(",") if o.strip()],

backend/app/routers/admin.py

@@ -4,7 +4,7 @@ Admin router — full user management, system config, and audit log.
 Security measures implemented:
   SEC-02: Session revocation on role change
   SEC-05: Block admin self-actions (own role/password/MFA/active status)
-  SEC-08: X-Requested-With header check (verify_xhr) on all state-mutating requests
+  SEC-08: X-Requested-With validation (now handled globally by CSRFHeaderMiddleware)
   SEC-13: Session revocation + ntfy alert on MFA disable
 
 All routes require the `require_admin` dependency (which chains through
@@ -15,7 +15,7 @@ from datetime import datetime
 from typing import Optional
 
 import sqlalchemy as sa
-from fastapi import APIRouter, Depends, HTTPException, Query, Request
+from fastapi import APIRouter, Depends, HTTPException, Path, Query, Request
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
@@ -48,26 +48,12 @@ from app.services.audit import log_audit_event
 from app.services.auth import hash_password
 
 # ---------------------------------------------------------------------------
-# CSRF guard — SEC-08
+# Router — all endpoints inherit require_admin
-# ---------------------------------------------------------------------------
+# (SEC-08 CSRF validation is now handled globally by CSRFHeaderMiddleware)
-
-async def verify_xhr(request: Request) -> None:
-    """
-    Lightweight CSRF mitigation: require X-Requested-With on state-mutating
-    requests. Browsers never send this header cross-origin without CORS
-    pre-flight, which our CORS policy blocks.
-    """
-    if request.method not in ("GET", "HEAD", "OPTIONS"):
-        if request.headers.get("X-Requested-With") != "XMLHttpRequest":
-            raise HTTPException(status_code=403, detail="Invalid request origin")
-
-
-# ---------------------------------------------------------------------------
-# Router — all endpoints inherit require_admin + verify_xhr
 # ---------------------------------------------------------------------------
 
 router = APIRouter(
-    dependencies=[Depends(require_admin), Depends(verify_xhr)],
+    dependencies=[Depends(require_admin)],
 )