diff --git a/nanobot/channels/wecom/runtime.py b/nanobot/channels/wecom/runtime.py
index fb82f5de5..c31a63a61 100644
--- a/nanobot/channels/wecom/runtime.py
+++ b/nanobot/channels/wecom/runtime.py
@@ -32,10 +32,12 @@ _SAFE_NAME_RE = re.compile(r"[^\w.\-()\[\]（）【】\u4e00-\u9fff]+", re.UNICO
 
 def _sanitize_filename(name: str, fallback: str = "unnamed") -> str:
     """Sanitize filename to avoid traversal and problematic chars."""
-    name = (name or "").strip()
-    name = Path(name).name
-    name = _SAFE_NAME_RE.sub("_", name).strip("._ ")
-    return name or fallback
+    def _clean(value: str) -> str:
+        value = (value or "").strip()
+        value = Path(value).name
+        return _SAFE_NAME_RE.sub("_", value).strip("._ ")
+
+    return _clean(name) or _clean(fallback) or "unnamed"
 
 
 _IMAGE_EXTS = {".jpg", ".jpeg", ".png", ".gif", ".webp", ".bmp"}
diff --git a/nanobot/channels/wecom/tests/test_wecom_channel.py b/nanobot/channels/wecom/tests/test_wecom_channel.py
index 0d3c50b5d..e6b815c7e 100644
--- a/nanobot/channels/wecom/tests/test_wecom_channel.py
+++ b/nanobot/channels/wecom/tests/test_wecom_channel.py
@@ -99,8 +99,10 @@ def test_sanitize_filename_empty_input() -> None:
 def test_sanitize_filename_empty_or_dots_fallback() -> None:
     assert _sanitize_filename("...") == "unnamed"
     assert _sanitize_filename("..", fallback="fallback.txt") == "fallback.txt"
+    assert _sanitize_filename("...", fallback="../../outside.txt") == "outside.txt"
     assert _sanitize_filename("") == "unnamed"
 
+
 def test_guess_wecom_media_type_image() -> None:
     for ext in (".jpg", ".jpeg", ".png", ".gif", ".webp", ".bmp"):
         assert _guess_wecom_media_type(f"photo{ext}") == "image"
@@ -149,6 +151,27 @@ async def test_download_and_save_success() -> None:
     os.unlink(path)
 
 
+@pytest.mark.asyncio
+async def test_download_and_save_sanitizes_sdk_fallback(tmp_path: Path) -> None:
+    """An unsafe SDK filename cannot escape the channel media directory."""
+    channel = WecomChannel(WecomConfig(bot_id="b", secret="s", allow_from=["*"]), MessageBus())
+    client = _FakeWeComClient()
+    client.download_file.return_value = (b"payload", "../../outside.txt")
+    channel._client = client
+
+    with patch("nanobot.channels.wecom.runtime.get_media_dir", return_value=tmp_path):
+        path = await channel._download_and_save_media(
+            "https://example.com/file",
+            "aes_key",
+            "file",
+            "...",
+        )
+
+    assert path is not None
+    assert Path(path) == tmp_path / "outside.txt"
+    assert Path(path).read_bytes() == b"payload"
+
+
 @pytest.mark.asyncio
 async def test_download_and_save_oversized_rejected() -> None:
     """Data exceeding 200MB is rejected → returns None."""