feat: per-IP rate limit and write bans

Apply sliding-window 2/s limits and BannedIP checks on write routes.
Also force SVG attachment disposition and claim burn-after-read in DB
before streaming, deleting the file via BackgroundTask after the response.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/api/public.py

@@ -1,13 +1,15 @@
+from pathlib import Path
+
 from fastapi import APIRouter, Depends, File, Form, HTTPException, Query, Request, UploadFile
 from fastapi.responses import FileResponse
 from sqlalchemy.orm import Session
 from starlette.background import BackgroundTask
 
-from app import db as db_module
 from app.db import get_db
-from app.models import Item
 from app.schemas import ItemCreate, ItemOut, TTLOption, WallResponse
+from app.services import bans as bans_service
 from app.services import items as items_service
+from app.services import rate_limit as rate_limit_service
 from app.services import storage as storage_service
 
 router = APIRouter(prefix="/api/items", tags=["items"])
@@ -16,21 +18,47 @@ _INLINE_IMAGE_PREFIXES = ("image/",)
 
 
 def _client_ip(request: Request) -> str:
+    """IP used for rate limits, bans, and created_ip.
+
+    Prefer ``request.client.host`` (TestClient yields ``testclient``; direct
+    connections use the peer address). ``X-Forwarded-For`` is only a fallback
+    when client is missing — trusting XFF without a trusted reverse proxy
+    allows clients to spoof their IP for ban/rate-limit evasion.
+    """
+    if request.client and request.client.host:
+        return request.client.host
     forwarded = request.headers.get("x-forwarded-for")
     if forwarded:
         return forwarded.split(",")[0].strip()
-    if request.client and request.client.host:
-        return request.client.host
     return ""
 
 
+def _enforce_write_guards(request: Request, db: Session) -> str:
+    ip = _client_ip(request)
+    if bans_service.is_banned(db, ip):
+        raise HTTPException(status_code=403, detail="Forbidden")
+    if not rate_limit_service.allow(ip):
+        raise HTTPException(status_code=429, detail="Too Many Requests")
+    return ip
+
+
+def _content_disposition_type(media_type: str, filename: str) -> str:
+    # SVG can carry script; never inline even when mime is image/svg+xml.
+    if media_type == "image/svg+xml" or filename.lower().endswith(".svg"):
+        return "attachment"
+    if media_type.startswith(_INLINE_IMAGE_PREFIXES):
+        return "inline"
+    return "attachment"
+
+
 @router.post("", response_model=ItemOut)
 def create_item(
     payload: ItemCreate,
     request: Request,
     db: Session = Depends(get_db),
 ):
-    item = items_service.create_text_item(db, payload, _client_ip(request))
+    ip = _enforce_write_guards(request, db)
+    item = items_service.create_text_item(db, payload, ip)
     return items_service.item_to_dict(item)
 
 
@@ -44,6 +72,7 @@ async def upload_item(
     ttl: TTLOption = Form("24h"),
     title: str | None = Form(None),
 ):
+    ip = _enforce_write_guards(request, db)
     rel_path, size, mime, safe_name = await storage_service.save_upload(file)
     item = items_service.create_file_item(
         db,
@@ -54,7 +83,7 @@ async def upload_item(
         is_public=is_public,
         burn_after_read=burn_after_read,
         ttl=ttl,
-        created_ip=_client_ip(request),
+        created_ip=ip,
         title=title,
     )
     return items_service.item_to_dict(item)
@@ -75,14 +104,11 @@ def wall(
     )
 
 
-def _burn_file_after_response(item_id: int) -> None:
-    db = db_module.SessionLocal()
+def _delete_file_path(path: str) -> None:
     try:
-        item = db.get(Item, item_id)
-        if item is not None and item.burn_after_read and not item.burned:
-            items_service.burn_file_item(db, item)
-    finally:
-        db.close()
+        Path(path).unlink(missing_ok=True)
+    except OSError:
+        pass
 
 
 @router.get("/{slug}/file")
@@ -96,15 +122,19 @@ def download_file(slug: str, db: Session = Depends(get_db)):
 
     media_type = item.mime or "application/octet-stream"
     filename = item.file_name or path.name
-    inline = media_type.startswith(_INLINE_IMAGE_PREFIXES)
+    disposition = _content_disposition_type(media_type, filename)
+
     background = None
     if item.burn_after_read:
-        background = BackgroundTask(_burn_file_after_response, item.id)
+        # Claim burn before streaming so concurrent fetches cannot re-read.
+        items_service.claim_burn(db, item)
+        background = BackgroundTask(_delete_file_path, str(path))
+
     return FileResponse(
         path,
         media_type=media_type,
         filename=filename,
-        content_disposition_type="inline" if inline else "attachment",
+        content_disposition_type=disposition,
         background=background,
     )
 

backend/app/services/bans.py

@@ -0,0 +1,14 @@
+"""IP ban checks for write endpoints."""
+
+from __future__ import annotations
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.models import BannedIP
+
+
+def is_banned(db: Session, ip: str) -> bool:
+    if not ip:
+        return False
+    return db.scalar(select(BannedIP).where(BannedIP.ip == ip)) is not None

backend/app/services/items.py

@@ -134,11 +134,15 @@ def get_file_item(db: Session, slug: str) -> Item | None:
     return item
 
 
-def burn_file_item(db: Session, item: Item) -> None:
-    """Mark a file item burned and remove its bytes from disk."""
+def claim_burn(db: Session, item: Item) -> None:
+    """Mark burn consumed in DB before streaming bytes."""
     item.burned = True
-    path = resolve_file_path(item)
     db.commit()
+
+
+def delete_item_file(item: Item) -> None:
+    """Remove file bytes from disk (safe after response has finished streaming)."""
+    path = resolve_file_path(item)
     if path is not None:
         try:
             path.unlink(missing_ok=True)
@@ -146,6 +150,12 @@ def burn_file_item(db: Session, item: Item) -> None:
             pass
 
 
+def burn_file_item(db: Session, item: Item) -> None:
+    """Mark a file item burned and remove its bytes from disk."""
+    claim_burn(db, item)
+    delete_item_file(item)
+
+
 def get_item(db: Session, slug: str) -> Item | None:
     item = db.scalar(select(Item).where(Item.slug == slug))
     if item is None or _is_unavailable(item):

backend/app/services/rate_limit.py

@@ -0,0 +1,40 @@
+"""In-memory sliding-window rate limit (per IP)."""
+
+from __future__ import annotations
+
+import threading
+import time
+from collections import defaultdict, deque
+
+from app import config
+
+_lock = threading.Lock()
+_hits: dict[str, deque[float]] = defaultdict(deque)
+
+
+def reset() -> None:
+    """Clear all tracked hits (tests)."""
+    with _lock:
+        _hits.clear()
+
+
+def allow(ip: str, *, now: float | None = None) -> bool:
+    """Return True if ``ip`` may proceed; record the hit when allowed.
+
+    Allows when the number of hits in the last 1.0s is strictly less than
+    ``settings.rate_limit_per_second``.
+    """
+    if not ip:
+        ip = ""
+    limit = config.settings.rate_limit_per_second
+    ts = time.monotonic() if now is None else now
+    window_start = ts - 1.0
+
+    with _lock:
+        q = _hits[ip]
+        while q and q[0] <= window_start:
+            q.popleft()
+        if len(q) >= limit:
+            return False
+        q.append(ts)
+        return True