diff --git a/backend/main.py b/backend/main.py
index a915209..293520e 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -1,6 +1,5 @@
 import logging
 import os
-import stat
 import tempfile
 import uuid
 from contextlib import asynccontextmanager
@@ -24,27 +23,23 @@ async def lifespan(app: FastAPI):
     logger.info("ScriptCut backend shutting down")
 
 
-app = FastAPI(
-    title="ScriptCut Backend",
-    version="0.1.0",
-    lifespan=lifespan,
-)
-
+app = FastAPI(title="ScriptCut Backend", version="0.1.0", lifespan=lifespan)
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["*"],
-    allow_credentials=True,
-    allow_methods=["*"],
-    allow_headers=["*"],
+    allow_origins=["http://localhost:5173", "http://127.0.0.1:5173"],
+    allow_credentials=False,
+    allow_methods=["GET", "POST", "OPTIONS"],
+    allow_headers=["Content-Type", "Range", "X-ScriptCut-Token"],
     expose_headers=["Content-Range", "Accept-Ranges", "Content-Length"],
 )
 
 LOCAL_API_TOKEN = os.getenv("SCRIPTCUT_API_TOKEN", "")
+MAX_UPLOAD_BYTES = int(os.getenv("SCRIPTCUT_MAX_UPLOAD_BYTES", str(10 * 1024 * 1024 * 1024)))
 
 
 @app.middleware("http")
 async def require_local_api_token(request: Request, call_next):
-    """Protect packaged local APIs from other processes on the same machine."""
+    """Protect local APIs from other processes and browser origins."""
     if (
         LOCAL_API_TOKEN
         and request.method != "OPTIONS"
@@ -54,6 +49,7 @@ async def require_local_api_token(request: Request, call_next):
         return JSONResponse(status_code=401, content={"detail": "Unauthorized local API request"})
     return await call_next(request)
 
+
 app.include_router(transcribe.router)
 app.include_router(export.router)
 app.include_router(ai.router)
@@ -63,7 +59,6 @@ async def require_local_api_token(request: Request, call_next):
 app.include_router(background.router)
 app.include_router(system.router)
 
-
 MIME_MAP = {
     ".mp4": "video/mp4",
     ".mkv": "video/x-matroska",
@@ -75,27 +70,36 @@ async def require_local_api_token(request: Request, call_next):
     ".mp3": "audio/mpeg",
     ".flac": "audio/flac",
 }
-
 UPLOAD_DIR = Path(tempfile.gettempdir()) / "scriptcut_uploads"
 SUPPORTED_UPLOAD_EXTENSIONS = set(MIME_MAP)
 
 
 @app.post("/media/upload")
-async def upload_media(file: UploadFile = File(...)):
-    """Accept browser-selected media and return a local backend path."""
+async def upload_media(request: Request, file: UploadFile = File(...)):
+    """Accept browser-selected media with a bounded disk footprint."""
+    content_length = request.headers.get("content-length")
+    if content_length:
+        try:
+            if int(content_length) > MAX_UPLOAD_BYTES:
+                raise HTTPException(status_code=413, detail="Upload exceeds the configured size limit")
+        except ValueError:
+            raise HTTPException(status_code=400, detail="Invalid Content-Length header")
+
     source_name = Path(file.filename or "upload").name
     suffix = Path(source_name).suffix.lower()
     if suffix not in SUPPORTED_UPLOAD_EXTENSIONS:
         raise HTTPException(status_code=400, detail=f"Unsupported media type: {suffix or 'unknown'}")
 
-    UPLOAD_DIR.mkdir(parents=True, exist_ok=True)
+    UPLOAD_DIR.mkdir(parents=True, exist_ok=True, mode=0o700)
     upload_path = UPLOAD_DIR / f"{uuid.uuid4().hex}{suffix}"
     size = 0
-
     try:
-        with open(upload_path, "wb") as output:
+        with open(upload_path, "xb") as output:
+            os.chmod(upload_path, 0o600)
             while chunk := await file.read(1024 * 1024):
                 size += len(chunk)
+                if size > MAX_UPLOAD_BYTES:
+                    raise HTTPException(status_code=413, detail="Upload exceeds the configured size limit")
                 output.write(chunk)
     except Exception:
         upload_path.unlink(missing_ok=True)
@@ -103,38 +107,39 @@ async def upload_media(file: UploadFile = File(...)):
     finally:
         await file.close()
 
-    return {
-        "path": str(upload_path),
-        "filename": source_name,
-        "size": size,
-    }
+    return {"path": str(upload_path), "filename": source_name, "size": size}
 
 
 @app.get("/file")
 async def serve_local_file(request: Request, path: str = Query(...)):
-    """Stream a local file with HTTP Range support (required for video seeking)."""
-    file_path = Path(path)
+    """Stream a local file with validated single-range seeking."""
+    file_path = Path(path).expanduser().resolve()
     if not file_path.is_file():
-        raise HTTPException(status_code=404, detail=f"File not found: {path}")
+        raise HTTPException(status_code=404, detail="File not found")
 
     file_size = file_path.stat().st_size
     content_type = MIME_MAP.get(file_path.suffix.lower(), "application/octet-stream")
-
     range_header = request.headers.get("range")
     if range_header:
-        range_spec = range_header.replace("bytes=", "")
-        range_start_str, range_end_str = range_spec.split("-")
-        range_start = int(range_start_str) if range_start_str else 0
-        range_end = int(range_end_str) if range_end_str else file_size - 1
+        if not range_header.startswith("bytes=") or "," in range_header:
+            raise HTTPException(status_code=416, detail="Unsupported byte range")
+        try:
+            range_start_str, range_end_str = range_header[6:].split("-", 1)
+            range_start = int(range_start_str) if range_start_str else 0
+            range_end = int(range_end_str) if range_end_str else file_size - 1
+        except (ValueError, TypeError):
+            raise HTTPException(status_code=416, detail="Invalid byte range")
+        if file_size <= 0 or range_start < 0 or range_start >= file_size or range_end < range_start:
+            raise HTTPException(status_code=416, detail="Byte range is outside the file")
         range_end = min(range_end, file_size - 1)
         content_length = range_end - range_start + 1
 
         def iter_range():
-            with open(file_path, "rb") as f:
-                f.seek(range_start)
+            with open(file_path, "rb") as media:
+                media.seek(range_start)
                 remaining = content_length
                 while remaining > 0:
-                    chunk = f.read(min(65536, remaining))
+                    chunk = media.read(min(65536, remaining))
                     if not chunk:
                         break
                     remaining -= len(chunk)
@@ -152,17 +157,14 @@ def iter_range():
         )
 
     def iter_file():
-        with open(file_path, "rb") as f:
-            while chunk := f.read(65536):
+        with open(file_path, "rb") as media:
+            while chunk := media.read(65536):
                 yield chunk
 
     return StreamingResponse(
         iter_file(),
         media_type=content_type,
-        headers={
-            "Accept-Ranges": "bytes",
-            "Content-Length": str(file_size),
-        },
+        headers={"Accept-Ranges": "bytes", "Content-Length": str(file_size)},
     )
 
 
diff --git a/backend/network_security.py b/backend/network_security.py
new file mode 100644
index 0000000..c604680
--- /dev/null
+++ b/backend/network_security.py
@@ -0,0 +1,33 @@
+"""Validation for user-configurable AI provider endpoints."""
+
+from __future__ import annotations
+
+import ipaddress
+import socket
+from urllib.parse import urlparse
+
+
+def validate_provider_url(value: str | None, *, allow_loopback: bool = True) -> str | None:
+    if value is None:
+        return None
+    url = value.strip().rstrip("/")
+    parsed = urlparse(url)
+    if parsed.scheme not in {"http", "https"} or not parsed.hostname or parsed.username or parsed.password:
+        raise ValueError("Provider URL must be a plain HTTP(S) origin without credentials")
+    if parsed.scheme == "http" and parsed.hostname not in {"localhost", "127.0.0.1", "::1"}:
+        raise ValueError("Non-local provider URLs must use HTTPS")
+
+    try:
+        addresses = {
+            ipaddress.ip_address(item[4][0])
+            for item in socket.getaddrinfo(parsed.hostname, parsed.port or (443 if parsed.scheme == "https" else 80), type=socket.SOCK_STREAM)
+        }
+    except socket.gaierror as exc:
+        raise ValueError("Provider hostname could not be resolved") from exc
+
+    for address in addresses:
+        if address.is_loopback and allow_loopback:
+            continue
+        if address.is_private or address.is_link_local or address.is_multicast or address.is_reserved or address.is_unspecified:
+            raise ValueError("Provider URL resolves to a blocked network address")
+    return url
diff --git a/backend/routers/ai.py b/backend/routers/ai.py
index 6e2b9af..5ca4aca 100644
--- a/backend/routers/ai.py
+++ b/backend/routers/ai.py
@@ -6,6 +6,7 @@
 from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel
 
+from network_security import validate_provider_url
 from services.ai_provider import AIProvider, detect_filler_words, create_clip_suggestion, create_clip_metadata, create_edit_plan
 
 logger = logging.getLogger(__name__)
@@ -69,10 +70,20 @@ class ModelListRequest(BaseModel):
     api_key: Optional[str] = None
 
 
+def _safe_base_url(provider: str, value: Optional[str]) -> Optional[str]:
+    if not value:
+        return None
+    if provider not in {"ollama", "9router"}:
+        raise ValueError(f"Custom base URLs are not supported for provider: {provider}")
+    return validate_provider_url(value, allow_loopback=True)
+
+
 @router.post("/ai/filler-removal")
 async def filler_removal(req: FillerRequest):
     try:
         return run_filler_removal(req)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
     except Exception as e:
         logger.error(f"Filler detection failed: {e}", exc_info=True)
         raise HTTPException(status_code=500, detail=str(e))
@@ -82,6 +93,8 @@ async def filler_removal(req: FillerRequest):
 async def create_clip(req: ClipRequest):
     try:
         return run_create_clip(req)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
     except Exception as e:
         logger.error(f"Clip creation failed: {e}", exc_info=True)
         raise HTTPException(status_code=500, detail=str(e))
@@ -91,6 +104,8 @@ async def create_clip(req: ClipRequest):
 async def clip_metadata(req: ClipMetadataRequest):
     try:
         return run_clip_metadata(req)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
     except Exception as e:
         logger.error(f"Clip metadata failed: {e}", exc_info=True)
         raise HTTPException(status_code=500, detail=str(e))
@@ -100,6 +115,8 @@ async def clip_metadata(req: ClipMetadataRequest):
 async def edit_plan(req: EditPlanRequest):
     try:
         return run_edit_plan(req)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
     except Exception as e:
         logger.error(f"Edit plan failed: {e}", exc_info=True)
         raise HTTPException(status_code=500, detail=str(e))
@@ -115,7 +132,7 @@ def run_filler_removal(req: FillerRequest, progress_callback=None):
         provider=req.provider,
         model=req.model,
         api_key=req.api_key,
-        base_url=req.base_url,
+        base_url=_safe_base_url(req.provider, req.base_url),
         custom_filler_words=req.custom_filler_words,
     )
     _progress(progress_callback, 100, "Filler detection complete")
@@ -137,7 +154,7 @@ def run_create_clip(req: ClipRequest, progress_callback=None):
         provider=req.provider,
         model=req.model,
         api_key=req.api_key,
-        base_url=req.base_url,
+        base_url=_safe_base_url(req.provider, req.base_url),
     )
     _progress(progress_callback, 100, "Clip discovery complete")
     return result
@@ -151,7 +168,7 @@ def run_clip_metadata(req: ClipMetadataRequest, progress_callback=None):
         provider=req.provider,
         model=req.model,
         api_key=req.api_key,
-        base_url=req.base_url,
+        base_url=_safe_base_url(req.provider, req.base_url),
     )
     _progress(progress_callback, 100, "Clip package complete")
     return result
@@ -168,7 +185,7 @@ def run_edit_plan(req: EditPlanRequest, progress_callback=None):
         provider=req.provider,
         model=req.model,
         api_key=req.api_key,
-        base_url=req.base_url,
+        base_url=_safe_base_url(req.provider, req.base_url),
         mode=req.mode,
         platform=req.platform,
         target_duration=req.target_duration,
@@ -184,16 +201,26 @@ def _progress(progress_callback, percent: int, message: str):
 
 @router.get("/ai/ollama-models")
 async def ollama_models(base_url: str = "http://localhost:11434"):
-    models = AIProvider.list_ollama_models(base_url)
-    return {"models": models}
+    try:
+        models = AIProvider.list_ollama_models(validate_provider_url(base_url, allow_loopback=True) or base_url)
+        return {"models": models}
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
 
 
 @router.get("/ai/ollama-status")
 async def ollama_status(base_url: str = "http://localhost:11434"):
-    return AIProvider.check_ollama(base_url)
+    try:
+        return AIProvider.check_ollama(validate_provider_url(base_url, allow_loopback=True) or base_url)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
 
 
 @router.post("/ai/9router-models")
 async def nine_router_models(req: ModelListRequest):
-    models = AIProvider.list_9router_models(req.base_url or "http://localhost:20128/v1", req.api_key)
-    return {"models": models}
+    try:
+        base_url = validate_provider_url(req.base_url or "http://localhost:20128/v1", allow_loopback=True)
+        models = AIProvider.list_9router_models(base_url or "http://localhost:20128/v1", req.api_key)
+        return {"models": models}
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
diff --git a/backend/routers/jobs.py b/backend/routers/jobs.py
index 87483a5..132504a 100644
--- a/backend/routers/jobs.py
+++ b/backend/routers/jobs.py
@@ -19,47 +19,45 @@
 router = APIRouter()
 
 
+def _create_job(kind, target):
+    try:
+        return {"job_id": job_manager.create(kind, target)}
+    except RuntimeError as exc:
+        raise HTTPException(status_code=429, detail=str(exc))
+
+
 @router.post("/jobs/export")
 async def create_export_job(req: ExportRequest):
-    job_id = job_manager.create("export", lambda progress: run_export(req, progress))
-    return {"job_id": job_id}
+    return _create_job("export", lambda progress: run_export(req, progress))
 
 
 @router.post("/jobs/transcribe")
 async def create_transcription_job(req: TranscribeRequest):
-    job_id = job_manager.create("transcribe", lambda progress: run_transcription(req, progress))
-    return {"job_id": job_id}
+    return _create_job("transcribe", lambda progress: run_transcription(req, progress))
 
 
 @router.post("/jobs/ai/filler-removal")
 async def create_filler_removal_job(req: FillerRequest):
-    job_id = job_manager.create("ai:filler-removal", lambda progress: run_filler_removal(req, progress))
-    return {"job_id": job_id}
+    return _create_job("ai:filler-removal", lambda progress: run_filler_removal(req, progress))
 
 
 @router.post("/jobs/ai/create-clip")
 async def create_clip_job(req: ClipRequest):
-    job_id = job_manager.create("ai:create-clip", lambda progress: run_create_clip(req, progress))
-    return {"job_id": job_id}
+    return _create_job("ai:create-clip", lambda progress: run_create_clip(req, progress))
 
 
 @router.post("/jobs/ai/clip-metadata")
 async def create_clip_metadata_job(req: ClipMetadataRequest):
-    job_id = job_manager.create("ai:clip-metadata", lambda progress: run_clip_metadata(req, progress))
-    return {"job_id": job_id}
+    return _create_job("ai:clip-metadata", lambda progress: run_clip_metadata(req, progress))
 
 
 @router.post("/jobs/ai/edit-plan")
 async def create_edit_plan_job(req: EditPlanRequest):
-    job_id = job_manager.create("ai:edit-plan", lambda progress: run_edit_plan(req, progress))
-    return {"job_id": job_id}
+    return _create_job("ai:edit-plan", lambda progress: run_edit_plan(req, progress))
 
 
 @router.get("/jobs/recent")
-async def get_recent_jobs(
-    kind: str | None = None,
-    limit: int = Query(default=3, ge=1, le=10),
-):
+async def get_recent_jobs(kind: str | None = None, limit: int = Query(default=3, ge=1, le=10)):
     return {"jobs": job_manager.recent(kind=kind, limit=limit)}
 
 
@@ -86,8 +84,10 @@ async def retry_job(job_id: str):
         raise HTTPException(status_code=404, detail="Job not found")
     if job.get("status") not in {"failed", "canceled"}:
         raise HTTPException(status_code=409, detail="Only failed or canceled jobs can be retried")
-
-    retry_job_id = job_manager.retry(job_id)
+    try:
+        retry_job_id = job_manager.retry(job_id)
+    except RuntimeError as exc:
+        raise HTTPException(status_code=429, detail=str(exc))
     if not retry_job_id:
         raise HTTPException(status_code=409, detail="Job cannot be retried")
     return {"job_id": retry_job_id}
diff --git a/backend/scripts/smoke_security.py b/backend/scripts/smoke_security.py
new file mode 100644
index 0000000..0c18afd
--- /dev/null
+++ b/backend/scripts/smoke_security.py
@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+import ipaddress
+import socket
+import sys
+import threading
+import time
+import unittest
+from pathlib import Path
+from unittest.mock import patch
+
+BACKEND_ROOT = Path(__file__).resolve().parents[1]
+if str(BACKEND_ROOT) not in sys.path:
+    sys.path.insert(0, str(BACKEND_ROOT))
+
+from local_api_auth import is_authorized_local_api_request
+from network_security import validate_provider_url
+from services.job_manager import JobManager
+
+
+class SecuritySmokeTests(unittest.TestCase):
+    def test_local_api_token_rejects_missing_and_wrong_values(self):
+        self.assertFalse(is_authorized_local_api_request("session-secret", None))
+        self.assertFalse(is_authorized_local_api_request("session-secret", "wrong"))
+        self.assertTrue(is_authorized_local_api_request("session-secret", "session-secret"))
+
+    def test_provider_url_allows_loopback_http(self):
+        with patch("socket.getaddrinfo", return_value=[(socket.AF_INET, socket.SOCK_STREAM, 6, "", ("127.0.0.1", 11434))]):
+            self.assertEqual(validate_provider_url("http://localhost:11434"), "http://localhost:11434")
+
+    def test_provider_url_rejects_private_remote_target(self):
+        private = str(ipaddress.ip_address("10.0.0.8"))
+        with patch("socket.getaddrinfo", return_value=[(socket.AF_INET, socket.SOCK_STREAM, 6, "", (private, 443))]):
+            with self.assertRaisesRegex(ValueError, "blocked network"):
+                validate_provider_url("https://internal.example")
+
+    def test_provider_url_rejects_cleartext_remote_target(self):
+        with self.assertRaisesRegex(ValueError, "must use HTTPS"):
+            validate_provider_url("http://example.com")
+
+    def test_job_queue_applies_backpressure(self):
+        release = threading.Event()
+        manager = JobManager(max_workers=1, max_pending_jobs=1)
+
+        def blocked(progress):
+            progress(10, "blocked")
+            release.wait(timeout=1)
+
+        manager.create("first", blocked)
+        time.sleep(0.03)
+        with self.assertRaisesRegex(RuntimeError, "queue is full"):
+            manager.create("second", blocked)
+        release.set()
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)
diff --git a/backend/services/job_manager.py b/backend/services/job_manager.py
index f06b0ff..8783797 100644
--- a/backend/services/job_manager.py
+++ b/backend/services/job_manager.py
@@ -1,9 +1,11 @@
-"""Small in-memory job registry for long-running local backend tasks."""
+"""Bounded in-memory job registry for long-running local backend tasks."""
 
 from __future__ import annotations
 
+import os
 import threading
 import traceback
+from concurrent.futures import ThreadPoolExecutor
 from datetime import datetime, timedelta, timezone
 from typing import Any, Callable
 from uuid import uuid4
@@ -13,6 +15,8 @@
 MAX_RETAINED_JOBS = 100
 TERMINAL_JOB_TTL = timedelta(hours=6)
 MAX_JOB_LOGS = 100
+MAX_WORKERS = max(1, min(int(os.getenv("SCRIPTCUT_JOB_WORKERS", "2")), 4))
+MAX_PENDING_JOBS = max(MAX_WORKERS, min(int(os.getenv("SCRIPTCUT_MAX_PENDING_JOBS", "8")), 32))
 
 
 class JobCanceled(RuntimeError):
@@ -20,9 +24,11 @@ class JobCanceled(RuntimeError):
 
 
 class JobManager:
-    def __init__(self) -> None:
+    def __init__(self, *, max_workers: int = MAX_WORKERS, max_pending_jobs: int = MAX_PENDING_JOBS) -> None:
         self._jobs: dict[str, dict[str, Any]] = {}
         self._lock = threading.Lock()
+        self._max_pending_jobs = max(max_workers, max_pending_jobs)
+        self._executor = ThreadPoolExecutor(max_workers=max_workers, thread_name_prefix="scriptcut-job")
 
     def create(
         self,
@@ -36,6 +42,9 @@ def create(
         now = _now()
         with self._lock:
             self._prune_locked()
+            active = sum(1 for job in self._jobs.values() if job.get("status") not in TERMINAL_STATUSES)
+            if active >= self._max_pending_jobs:
+                raise RuntimeError("The local job queue is full. Wait for an active job to finish.")
             self._jobs[job_id] = {
                 "id": job_id,
                 "kind": kind,
@@ -52,9 +61,7 @@ def create(
                 "updatedAt": now,
                 "_target": target,
             }
-
-        thread = threading.Thread(target=self._run, args=(job_id, target), daemon=True)
-        thread.start()
+        self._executor.submit(self._run, job_id, target)
         return job_id
 
     def get(self, job_id: str) -> dict[str, Any] | None:
@@ -64,14 +71,9 @@ def get(self, job_id: str) -> dict[str, Any] | None:
             return self._public_job(job) if job else None
 
     def recent(self, *, kind: str | None = None, limit: int = 5) -> list[dict[str, Any]]:
-        """Return a small, support-safe summary of recent jobs."""
         with self._lock:
             self._prune_locked()
-            jobs = [
-                job
-                for job in self._jobs.values()
-                if kind is None or job.get("kind") == kind
-            ]
+            jobs = [job for job in self._jobs.values() if kind is None or job.get("kind") == kind]
             jobs.sort(key=lambda job: job.get("updatedAt") or "", reverse=True)
             return [
                 {
@@ -92,17 +94,15 @@ def retry(self, job_id: str) -> str | None:
         with self._lock:
             self._prune_locked()
             job = self._jobs.get(job_id)
-            if not job:
-                return None
-            if job.get("status") not in RETRYABLE_STATUSES:
+            if not job or job.get("status") not in RETRYABLE_STATUSES:
                 return None
             target = job.get("_target")
             if not target:
                 return None
             original_job_id = job.get("originalJobId") or job["id"]
             attempt = int(job.get("attempt") or 1) + 1
-
-        return self.create(job["kind"], target, original_job_id=original_job_id, attempt=attempt)
+            kind = job["kind"]
+        return self.create(kind, target, original_job_id=original_job_id, attempt=attempt)
 
     def cancel(self, job_id: str) -> dict[str, Any] | None:
         with self._lock:
@@ -114,13 +114,23 @@ def cancel(self, job_id: str) -> dict[str, Any] | None:
                 return self._public_job(job)
             job["cancelRequested"] = True
             now = _now()
-            job["status"] = "canceling"
-            job["message"] = "Cancel requested"
+            if job["status"] == "queued":
+                job["status"] = "canceled"
+                job["message"] = "Canceled"
+                job["completedAt"] = now
+                job["_target"] = None
+            else:
+                job["status"] = "canceling"
+                job["message"] = "Cancel requested"
             job["updatedAt"] = now
-            self._append_log_locked(job, now, "Cancel requested")
+            self._append_log_locked(job, now, job["message"])
             return self._public_job(job)
 
     def _run(self, job_id: str, target: Callable[[Callable[[int, str], None]], Any]) -> None:
+        with self._lock:
+            job = self._jobs.get(job_id)
+            if not job or job.get("status") == "canceled":
+                return
         self._update(job_id, status="running", progress=1, message="Started")
 
         def progress(percent: int, message: str) -> None:
@@ -134,15 +144,15 @@ def progress(percent: int, message: str) -> None:
             result = target(progress)
             job = self.get(job_id)
             if job and job.get("cancelRequested"):
-                self._update(job_id, status="canceled", message="Canceled")
+                self._update(job_id, status="canceled", message="Canceled", _target=None)
                 return
-            self._update(job_id, status="succeeded", progress=100, message="Complete", result=result)
+            self._update(job_id, status="succeeded", progress=100, message="Complete", result=result, _target=None)
         except JobCanceled:
-            self._update(job_id, status="canceled", message="Canceled")
+            self._update(job_id, status="canceled", message="Canceled", _target=None)
         except Exception as exc:
             job = self.get(job_id)
             if job and job.get("cancelRequested"):
-                self._update(job_id, status="canceled", message="Canceled")
+                self._update(job_id, status="canceled", message="Canceled", _target=None)
                 return
             self._update(
                 job_id,
@@ -163,12 +173,10 @@ def _update(self, job_id: str, **patch: Any) -> None:
             job["updatedAt"] = now
             if previous_status not in TERMINAL_STATUSES and job.get("status") in TERMINAL_STATUSES:
                 job["completedAt"] = now
-            message = patch.get("message")
-            log = patch.get("log")
-            if message:
-                self._append_log_locked(job, now, message)
-            if log:
-                self._append_log_locked(job, now, log)
+            if patch.get("message"):
+                self._append_log_locked(job, now, patch["message"])
+            if patch.get("log"):
+                self._append_log_locked(job, now, patch["log"])
 
     @staticmethod
     def _public_job(job: dict[str, Any]) -> dict[str, Any]:
@@ -193,7 +201,8 @@ def _prune_locked(self) -> None:
         expired_ids = [
             job_id
             for job_id, job in self._jobs.items()
-            if job.get("status") in TERMINAL_STATUSES and _parse_time(job.get("completedAt") or job.get("updatedAt")) < now - TERMINAL_JOB_TTL
+            if job.get("status") in TERMINAL_STATUSES
+            and _parse_time(job.get("completedAt") or job.get("updatedAt")) < now - TERMINAL_JOB_TTL
         ]
         for job_id in expired_ids:
             self._jobs.pop(job_id, None)
@@ -201,7 +210,6 @@ def _prune_locked(self) -> None:
         overflow = len(self._jobs) - MAX_RETAINED_JOBS
         if overflow <= 0:
             return
-
         removable = sorted(
             (
                 (job_id, _parse_time(job.get("completedAt") or job.get("updatedAt")))
diff --git a/electron/main.js b/electron/main.js
index fbd8acf..b874e0a 100644
--- a/electron/main.js
+++ b/electron/main.js
@@ -9,6 +9,7 @@ let backendStartupError = '';
 
 const isDev = !app.isPackaged;
 const BACKEND_PORT = 8642;
+const BACKEND_ORIGIN = `http://127.0.0.1:${BACKEND_PORT}`;
 const MAX_PROJECT_FILE_BYTES = 50 * 1024 * 1024;
 const PROJECT_EXTENSIONS = new Set(['.scriptcut', '.aive', '.cutscript']);
 
@@ -48,16 +49,17 @@ function assertSafeFilePath(filePath) {
 }
 
 function isTrustedAppUrl(url) {
-  if (isDev) {
-    return url.startsWith('http://localhost:5173/');
-  }
+  if (isDev) return url.startsWith('http://localhost:5173/');
   return url.startsWith('file://');
 }
 
+function assertTrustedSender(event) {
+  const senderUrl = event.senderFrame?.url || event.sender?.getURL?.() || '';
+  if (!isTrustedAppUrl(senderUrl)) throw new Error('IPC request came from an untrusted frame.');
+}
+
 function openExternalUrl(url) {
-  if (url.startsWith('https://')) {
-    void shell.openExternal(url);
-  }
+  if (url.startsWith('https://')) void shell.openExternal(url);
 }
 
 function createWindow() {
@@ -71,81 +73,60 @@ function createWindow() {
       preload: path.join(__dirname, 'preload.js'),
       contextIsolation: true,
       nodeIntegration: false,
-      webSecurity: isDev ? false : true,
+      sandbox: true,
+      webSecurity: true,
     },
     show: false,
   });
 
   if (isDev) {
     mainWindow.loadURL('http://localhost:5173');
-    if (process.env.SCRIPTCUT_OPEN_DEVTOOLS === '1') {
-      mainWindow.webContents.openDevTools();
-    }
+    if (process.env.SCRIPTCUT_OPEN_DEVTOOLS === '1') mainWindow.webContents.openDevTools();
   } else {
     mainWindow.loadFile(path.join(__dirname, '..', 'frontend', 'dist', 'index.html'));
   }
 
-  mainWindow.once('ready-to-show', () => {
-    mainWindow.show();
-  });
-
+  mainWindow.once('ready-to-show', () => mainWindow.show());
   mainWindow.webContents.setWindowOpenHandler(({ url }) => {
     openExternalUrl(url);
     return { action: 'deny' };
   });
-
   mainWindow.webContents.on('will-navigate', (event, url) => {
     if (isTrustedAppUrl(url)) return;
     event.preventDefault();
     openExternalUrl(url);
   });
-
   mainWindow.webContents.session.webRequest.onBeforeSendHeaders((details, callback) => {
     const token = pythonBackend?.apiToken;
-    if (token && details.url.startsWith(`http://127.0.0.1:${BACKEND_PORT}/`)) {
+    if (token && details.url.startsWith(`${BACKEND_ORIGIN}/`)) {
       details.requestHeaders['X-ScriptCut-Token'] = token;
     }
     callback({ requestHeaders: details.requestHeaders });
   });
-
-  mainWindow.on('closed', () => {
-    mainWindow = null;
-  });
+  mainWindow.on('closed', () => { mainWindow = null; });
 }
 
 app.whenReady().then(async () => {
-  pythonBackend = new PythonBackend(BACKEND_PORT, isDev);
+  pythonBackend = new PythonBackend(BACKEND_PORT, isDev, process.env.SCRIPTCUT_API_TOKEN || null);
   try {
     await pythonBackend.start();
   } catch (error) {
     backendStartupError = error instanceof Error ? error.message : String(error);
     console.error('[backend] Startup failed:', backendStartupError);
   }
-
   createWindow();
-
   app.on('activate', () => {
-    if (BrowserWindow.getAllWindows().length === 0) {
-      createWindow();
-    }
+    if (BrowserWindow.getAllWindows().length === 0) createWindow();
   });
 });
 
 app.on('window-all-closed', () => {
-  if (process.platform !== 'darwin') {
-    app.quit();
-  }
-});
-
-app.on('before-quit', () => {
-  if (pythonBackend) {
-    pythonBackend.stop();
-  }
+  if (process.platform !== 'darwin') app.quit();
 });
+app.on('before-quit', () => pythonBackend?.stop());
 
-// IPC Handlers
-
-ipcMain.handle('dialog:openFile', async (_event, options) => {
+ipcMain.handle('dialog:openFile', async (event, options) => {
+  assertTrustedSender(event);
   const result = await dialog.showOpenDialog(mainWindow, {
     properties: ['openFile'],
     filters: [
@@ -158,7 +139,8 @@ ipcMain.handle('dialog:openFile', async (_event, options) => {
   return result.canceled ? null : result.filePaths[0];
 });
 
-ipcMain.handle('dialog:openDirectory', async (_event, options) => {
+ipcMain.handle('dialog:openDirectory', async (event, options) => {
+  assertTrustedSender(event);
   const result = await dialog.showOpenDialog(mainWindow, {
     properties: ['openDirectory', 'createDirectory'],
     ...options,
@@ -166,7 +148,8 @@ ipcMain.handle('dialog:openDirectory', async (_event, options) => {
   return result.canceled ? null : result.filePaths[0];
 });
 
-ipcMain.handle('dialog:saveFile', async (_event, options) => {
+ipcMain.handle('dialog:saveFile', async (event, options) => {
+  assertTrustedSender(event);
   const result = await dialog.showSaveDialog(mainWindow, {
     filters: [
       { name: 'Video Files', extensions: ['mp4', 'mov', 'webm'] },
@@ -177,79 +160,60 @@ ipcMain.handle('dialog:saveFile', async (_event, options) => {
   return result.canceled ? null : result.filePath;
 });
 
-ipcMain.handle('dialog:openProject', async () => {
+ipcMain.handle('dialog:openProject', async (event) => {
+  assertTrustedSender(event);
   const result = await dialog.showOpenDialog(mainWindow, {
     properties: ['openFile'],
-    filters: [
-      { name: 'ScriptCut Project', extensions: ['scriptcut', 'aive', 'cutscript'] },
-    ],
+    filters: [{ name: 'ScriptCut Project', extensions: ['scriptcut', 'aive', 'cutscript'] }],
   });
   return result.canceled ? null : result.filePaths[0];
 });
 
-ipcMain.handle('safe-storage:encrypt', (_event, data) => {
-  if (safeStorage.isEncryptionAvailable()) {
-    return safeStorage.encryptString(data).toString('base64');
-  }
-  return data;
+ipcMain.handle('safe-storage:encrypt', (event, data) => {
+  assertTrustedSender(event);
+  if (!safeStorage.isEncryptionAvailable()) throw new Error('Secure credential storage is unavailable.');
+  return safeStorage.encryptString(data).toString('base64');
 });
-
-ipcMain.handle('safe-storage:decrypt', (_event, encrypted) => {
-  if (safeStorage.isEncryptionAvailable()) {
-    return safeStorage.decryptString(Buffer.from(encrypted, 'base64'));
-  }
-  return encrypted;
-});
-
-ipcMain.handle('get-backend-url', () => {
-  return `http://localhost:${BACKEND_PORT}`;
+ipcMain.handle('safe-storage:decrypt', (event, encrypted) => {
+  assertTrustedSender(event);
+  if (!safeStorage.isEncryptionAvailable()) throw new Error('Secure credential storage is unavailable.');
+  return safeStorage.decryptString(Buffer.from(encrypted, 'base64'));
 });
-
-ipcMain.handle('app:getStartupStatus', () => ({
-  backendError: backendStartupError,
-}));
-
-ipcMain.handle('app:getInfo', () => ({
-  version: app.getVersion(),
-  platform: process.platform,
-  arch: process.arch,
-  packaged: app.isPackaged,
-  electron: process.versions.electron,
-}));
-
-ipcMain.handle('app:quit', () => {
-  app.quit();
-  return true;
+ipcMain.handle('get-backend-url', (event) => { assertTrustedSender(event); return BACKEND_ORIGIN; });
+ipcMain.handle('app:getStartupStatus', (event) => { assertTrustedSender(event); return { backendError: backendStartupError }; });
+ipcMain.handle('app:getInfo', (event) => {
+  assertTrustedSender(event);
+  return { version: app.getVersion(), platform: process.platform, arch: process.arch, packaged: app.isPackaged, electron: process.versions.electron };
 });
+ipcMain.handle('app:quit', (event) => { assertTrustedSender(event); app.quit(); return true; });
 
-ipcMain.handle('project:read', async (_event, filePath) => {
+ipcMain.handle('project:read', async (event, filePath) => {
+  assertTrustedSender(event);
   assertProjectPath(filePath);
-  if (fs.statSync(filePath).size > MAX_PROJECT_FILE_BYTES) {
-    throw new Error('Project file is larger than 50 MB.');
-  }
+  if (fs.statSync(filePath).size > MAX_PROJECT_FILE_BYTES) throw new Error('Project file is larger than 50 MB.');
   return fs.readFileSync(filePath, 'utf-8');
 });
-
-ipcMain.handle('project:write', async (_event, filePath, content) => {
+ipcMain.handle('project:write', async (event, filePath, content) => {
+  assertTrustedSender(event);
   assertProjectPath(filePath);
   assertTextContent(content);
-  fs.writeFileSync(filePath, content, 'utf-8');
+  fs.writeFileSync(filePath, content, { encoding: 'utf-8', mode: 0o600 });
   return true;
 });
-
-ipcMain.handle('clip-manifest:write', async (_event, filePath, content) => {
+ipcMain.handle('clip-manifest:write', async (event, filePath, content) => {
+  assertTrustedSender(event);
   assertClipManifestPath(filePath);
   assertTextContent(content);
-  fs.writeFileSync(filePath, content, 'utf-8');
+  fs.writeFileSync(filePath, content, { encoding: 'utf-8', mode: 0o600 });
   return true;
 });
-
-ipcMain.handle('shell:revealPath', async (_event, filePath) => {
+ipcMain.handle('shell:revealPath', async (event, filePath) => {
+  assertTrustedSender(event);
   shell.showItemInFolder(filePath);
   return true;
 });
-
-ipcMain.handle('shell:openPath', async (_event, filePath) => {
+ipcMain.handle('shell:openPath', async (event, filePath) => {
+  assertTrustedSender(event);
   const error = await shell.openPath(filePath);
   return error || true;
-});
+});
\ No newline at end of file
diff --git a/electron/python-bridge.js b/electron/python-bridge.js
index b76f659..9eba4a5 100644
--- a/electron/python-bridge.js
+++ b/electron/python-bridge.js
@@ -6,11 +6,11 @@ const { resolvePythonRuntime } = require('./python-runtime');
 const { bundledToolEnv } = require('./bundled-tools');
 
 class PythonBackend {
-  constructor(port, isDev) {
+  constructor(port, isDev, apiToken = null) {
     this.port = port;
     this.isDev = isDev;
     this.process = null;
-    this.apiToken = null;
+    this.apiToken = apiToken || crypto.randomBytes(32).toString('hex');
     this.lastBackendError = '';
     this.backendExitReason = '';
   }
@@ -18,11 +18,13 @@ class PythonBackend {
   async start() {
     this.lastBackendError = '';
     this.backendExitReason = '';
-    // In dev mode, check if a backend is already running (e.g. from `npm run dev:backend`)
-    // If so, reuse it instead of spawning a duplicate.
     if (this.isDev) {
       const alreadyRunning = await this._isPortOpen(2000);
       if (alreadyRunning) {
+        const authorized = await this._isAuthorizedBackend(2000);
+        if (!authorized) {
+          throw new Error(`Port ${this.port} is occupied by a backend that does not accept this ScriptCut session token.`);
+        }
         console.log(`[backend] Dev backend already running on port ${this.port} — reusing it.`);
         return;
       }
@@ -31,13 +33,8 @@ class PythonBackend {
     const backendDir = this.isDev
       ? path.join(__dirname, '..', 'backend')
       : path.join(process.resourcesPath, 'backend');
-
     const { command, argsPrefix } = resolvePythonRuntime();
 
-    // Packaged builds use a per-launch token so another local process cannot
-    // call the backend or stream arbitrary local files through it.
-    this.apiToken = this.isDev ? null : crypto.randomBytes(32).toString('hex');
-
     this.process = spawn(command, [
       ...argsPrefix,
       '-m', 'uvicorn', 'main:app',
@@ -49,15 +46,13 @@ class PythonBackend {
       env: {
         ...process.env,
         ...bundledToolEnv(this.isDev),
-        ...(this.apiToken ? { SCRIPTCUT_API_TOKEN: this.apiToken } : {}),
+        SCRIPTCUT_API_TOKEN: this.apiToken,
+        SCRIPTCUT_FILE_TOKEN_SECRET: this.apiToken,
         PYTHONUNBUFFERED: '1',
       },
     });
 
-    this.process.stdout.on('data', (data) => {
-      console.log(`[backend] ${data.toString().trim()}`);
-    });
-
+    this.process.stdout.on('data', (data) => console.log(`[backend] ${data.toString().trim()}`));
     this.process.stderr.on('data', (data) => {
       const output = data.toString().trim();
       if (output) {
@@ -65,13 +60,11 @@ class PythonBackend {
         console.error(`[backend] ${output}`);
       }
     });
-
     this.process.on('error', (err) => {
       this.backendExitReason = `Local backend could not start: ${err.message}`;
       this.lastBackendError = this.backendExitReason;
       console.error('[backend] Failed to start Python backend:', err.message);
     });
-
     this.process.on('exit', (code, signal) => {
       this.backendExitReason = signal
         ? `Local backend exited with signal ${signal}.`
@@ -84,17 +77,31 @@ class PythonBackend {
     console.log(`[backend] Ready on port ${this.port}`);
   }
 
-  _isPortOpen(timeoutMs) {
+  _request(pathname, timeoutMs, includeToken = false) {
     return new Promise((resolve) => {
-      const req = http.get(`http://127.0.0.1:${this.port}/health`, (res) => {
-        resolve(res.statusCode === 200);
+      const req = http.get({
+        hostname: '127.0.0.1',
+        port: this.port,
+        path: pathname,
+        headers: includeToken ? { 'X-ScriptCut-Token': this.apiToken } : {},
+      }, (res) => {
+        res.resume();
+        resolve(res.statusCode || 0);
       });
-      req.on('error', () => resolve(false));
-      req.setTimeout(timeoutMs, () => { req.destroy(); resolve(false); });
+      req.on('error', () => resolve(0));
+      req.setTimeout(timeoutMs, () => { req.destroy(); resolve(0); });
       req.end();
     });
   }
 
+  async _isPortOpen(timeoutMs) {
+    return (await this._request('/health', timeoutMs)) === 200;
+  }
+
+  async _isAuthorizedBackend(timeoutMs) {
+    return (await this._request('/system/diagnostics', timeoutMs, true)) === 200;
+  }
+
   stop() {
     if (this.process) {
       if (process.platform === 'win32') {
@@ -110,7 +117,7 @@ class PythonBackend {
   _waitForReady(timeoutMs) {
     const startTime = Date.now();
     return new Promise((resolve, reject) => {
-      const check = () => {
+      const check = async () => {
         if (this.backendExitReason) {
           const detail = this.lastBackendError ? ` ${this.lastBackendError}` : '';
           reject(new Error(`${this.backendExitReason}${detail}`));
@@ -121,32 +128,16 @@ class PythonBackend {
           reject(new Error(`Backend startup timed out.${detail}`));
           return;
         }
-        const remainingMs = timeoutMs - (Date.now() - startTime);
-        let completed = false;
-        const retry = () => {
-          if (completed) return;
-          completed = true;
-          setTimeout(check, 500);
-        };
-        const req = http.get(`http://127.0.0.1:${this.port}/health`, (res) => {
-          res.resume();
-          if (res.statusCode === 200) {
-            completed = true;
-            resolve();
-          } else {
-            retry();
-          }
-        });
-        req.on('error', retry);
-        req.setTimeout(Math.max(1, Math.min(2000, remainingMs)), () => {
-          req.destroy();
-          retry();
-        });
-        req.end();
+        const status = await this._request('/health', 2000);
+        if (status === 200) {
+          resolve();
+          return;
+        }
+        setTimeout(check, 500);
       };
       setTimeout(check, 1000);
     });
   }
 }
 
-module.exports = { PythonBackend };
+module.exports = { PythonBackend };
\ No newline at end of file
diff --git a/package.json b/package.json
index 320ee01..8f8ccc4 100644
--- a/package.json
+++ b/package.json
@@ -4,26 +4,10 @@
   "private": true,
   "author": "Fernando Abishai",
   "description": "ScriptCut — Open-source AI-powered text-based video editor",
-  "keywords": [
-    "video-editor",
-    "text-based-video-editing",
-    "descript-alternative",
-    "transcript-editor",
-    "youtube-shorts",
-    "tiktok",
-    "reels",
-    "shorts-editor",
-    "social-video",
-    "video-transcription",
-    "creator-tools",
-    "captions",
-    "ffmpeg",
-    "local-first",
-    "electron"
-  ],
+  "keywords": ["video-editor", "text-based-video-editing", "descript-alternative", "transcript-editor", "youtube-shorts", "tiktok", "reels", "shorts-editor", "social-video", "video-transcription", "creator-tools", "captions", "ffmpeg", "local-first", "electron"],
   "main": "electron/main.js",
   "scripts": {
-    "dev": "concurrently \"npm run dev:backend\" \"npm run dev:frontend\" \"wait-on http://localhost:5173 && npm run dev:electron\"",
+    "dev": "concurrently \"npm run dev:frontend\" \"wait-on http://localhost:5173 && npm run dev:electron\"",
     "dev:frontend": "cd frontend && npm run dev",
     "dev:electron": "electron .",
     "dev:backend": "node electron/run-backend.js --reload --port 8642",
@@ -48,48 +32,20 @@
     "release:alpha": "node scripts/release-alpha.js",
     "release:trust": "node scripts/check-release-trust.js",
     "lint": "cd frontend && npm run lint",
-    "smoke:backend": "cd backend && (../.venv311/bin/python scripts/smoke_backend.py || ../.venv/bin/python scripts/smoke_backend.py || python scripts/smoke_backend.py)"
-  },
-  "devDependencies": {
-    "concurrently": "^9.1.0",
-    "electron": "^43.1.0",
-    "electron-builder": "^26.15.3",
-    "wait-on": "^8.0.0"
-  },
-  "dependencies": {
-    "python-shell": "^5.0.0"
+    "smoke:backend": "cd backend && (../.venv311/bin/python -m unittest discover -s scripts -p 'smoke_*.py' -v || ../.venv/bin/python -m unittest discover -s scripts -p 'smoke_*.py' -v || python -m unittest discover -s scripts -p 'smoke_*.py' -v)"
   },
+  "devDependencies": {"concurrently": "^9.1.0", "electron": "^43.1.0", "electron-builder": "^26.15.3", "wait-on": "^8.0.0"},
+  "dependencies": {"python-shell": "^5.0.0"},
   "build": {
     "appId": "com.fernandoabishai.scriptcut",
     "productName": "ScriptCut",
-    "files": [
-      "electron/**/*",
-      "frontend/dist/**/*",
-      "backend/**/*",
-      "shared/**/*"
-    ],
+    "files": ["electron/**/*", "frontend/dist/**/*", "shared/**/*"],
     "extraResources": [
-      {
-        "from": "backend",
-        "to": "backend"
-      },
-      {
-        "from": "build/bin",
-        "to": "bin",
-        "filter": [
-          "**/*"
-        ]
-      }
+      {"from": "backend", "to": "backend"},
+      {"from": "build/bin", "to": "bin", "filter": ["**/*"]}
     ],
-    "win": {
-      "target": "nsis"
-    },
-    "mac": {
-      "target": "dmg",
-      "icon": "build/icon.icns"
-    },
-    "linux": {
-      "target": "AppImage"
-    }
+    "win": {"target": "nsis"},
+    "mac": {"target": "dmg", "icon": "build/icon.icns"},
+    "linux": {"target": "AppImage"}
   }
-}
+}
\ No newline at end of file