fix(upload): normalize Windows separators before basename extraction

.jules/sentinel.md

@@ -1,3 +1,7 @@
+## 2026-07-28 - [Sentinel: Windows Path Traversal in Uploads]
+**Vulnerability:** Path Traversal (CWE-22) via Windows path separators in filenames.
+**Learning:** Python`s `Path(file.filename).name` on POSIX systems does not recognize `\` as a directory separator, allowing attackers to upload files containing backslashes (e.g., `..\..\etc\passwd`) that escape intended directories.
+**Prevention:** Explicitly sanitize filenames by replacing all backslashes with forward slashes (`filename.replace("\\", "/")`) before applying `Path().name`.
 ## 2026-05-28 - [Sentinel Fixes: Temp Files & Injection]
 **Vulnerability:** Predictable Temp Files (CWE-377) and Insecure Default Permissions (CWE-276), plus Command Injection via FFmpeg Filtergraph (CWE-20).
 **Learning:** Python's `Path.with_name` plus a suffix string to make a temp file opens a race condition because it's predictable and the permissions default to system `umask` which might expose secret `0600` data. Additionally, interpolating variables directly into FFmpeg filtergraph strings allows arbitrary filter injection.

CHANGELOG.md

@@ -11,3 +11,6 @@
 
 ### Fixed
 - 단일·일괄 대상 크기 입력을 비웠을 때 이전 custom validity와 `aria-invalid` 상태를 즉시 초기화해 현재 필수 입력 상태를 정확히 전달합니다.
+
+### 보안
+- 업로드된 파일 이름의 윈도우 디렉터리 구분자(`\`)를 악용한 경로 탐색 취약점을 수정했습니다.

saas_web.py

@@ -486,7 +486,7 @@ def _persist_upload(file: UploadFile) -> tuple[Path, Path, Path, Path]:
         input_dir.mkdir()
         output_dir.mkdir()
 
-        safe_filename = Path(file.filename).name
+        safe_filename = Path(str(file.filename).replace("\\", "/")).name
         if not safe_filename or safe_filename in (".", ".."):
             safe_filename = "upload.tmp"
 
@@ -619,7 +619,7 @@ def shrink_media_batch(
     try:
         with zipfile.ZipFile(zip_path, "w", compression=zipfile.ZIP_STORED) as archive:
             for index, upload in enumerate(files):
-                safe_filename = Path(upload.filename or "").name
+                safe_filename = Path(str(upload.filename or "").replace("\\", "/")).name
                 if not safe_filename or safe_filename in (".", ".."):
                     safe_filename = "upload.tmp"
                 entry = {

tests/test_saas_web.py

@@ -201,6 +201,26 @@ def test_shrink_media_handles_temp_dir_failure(self, _mock_mkdtemp):
 
         self.assertEqual(response, {"error": "Upload processing failed"})
 
+    @patch("saas_web.media_shrinker.convert_file")
+    def test_shrink_media_sanitizes_windows_paths(self, mock_convert_file):
+        import tempfile
+        with tempfile.TemporaryDirectory() as temp_dir:
+            output = Path(temp_dir) / "output.flac"
+            output.write_bytes(b"audio")
+            mock_result = MagicMock(spec=ConversionResult)
+            mock_result.output_path = output
+            mock_convert_file.return_value = [mock_result]
+
+            response = client.post(
+                "/shrink",
+                files={"file": ("..\\..\\..\\etc\\passwd", b"evil-content", "audio/wav")},
+                data={"target_bytes": 10000},
+            )
+
+            self.assertEqual(response.status_code, 200)
+            mock_convert_file.assert_called_once()
+            self.assertEqual(mock_convert_file.call_args.kwargs["source"].name, "passwd")
+
     @patch("saas_web.media_shrinker.convert_file")
     def test_shrink_media_uses_safe_fallback_filename(self, mock_convert_file):
         import tempfile
@@ -560,6 +580,20 @@ def test_shrink_batch_handles_archive_failure(self, _mock_zipfile):
         self.assertEqual(response.status_code, 500)
         self.assertEqual(response.json(), {"error": "Upload processing failed"})
 
+    @patch("saas_web.media_shrinker.convert_file")
+    def test_shrink_batch_sanitizes_windows_paths(self, mock_convert_file):
+        mock_convert_file.side_effect = self._fake_convert
+
+        response = client.post(
+            "/shrink-batch",
+            files=[("files", ("..\\..\\..\\etc\\passwd", b"evil", "audio/wav"))],
+            data={"target_bytes": 10000},
+        )
+        self.assertEqual(response.status_code, 200)
+        names, manifest, _ = self._read_zip(response)
+        self.assertIn("01_passwd.flac", names)
+        self.assertEqual(manifest["results"][0]["filename"], "passwd")
+
     @patch("saas_web.media_shrinker.convert_file")
     def test_shrink_batch_uses_safe_fallback_filename(self, mock_convert_file):
         mock_convert_file.return_value = []