diff --git a/src/mcore_bridge/utils/safetensors.py b/src/mcore_bridge/utils/safetensors.py
index 0ad3ea3..1f4b561 100644
--- a/src/mcore_bridge/utils/safetensors.py
+++ b/src/mcore_bridge/utils/safetensors.py
@@ -30,6 +30,26 @@ def load_slice(self, slices):
         return self.loader()[slices]
 
 
+def checkpoint_path(root: str, name: str) -> str:
+    """Join a checkpoint-relative name and reject paths that leave ``root``.
+
+    ``os.path.join`` drops the root when ``name`` is absolute, and a weight-map
+    entry such as ``../../other.safetensors`` would otherwise open a file
+    outside the model directory.
+    """
+    if os.path.isabs(name):
+        raise ValueError(f'Checkpoint file {name!r} is outside {root}')
+    base = os.path.realpath(root)
+    path = os.path.realpath(os.path.join(base, name))
+    try:
+        inside = os.path.commonpath([base, path]) == base
+    except ValueError:
+        inside = False
+    if not inside:
+        raise ValueError(f'Checkpoint file {name!r} is outside {root}')
+    return path
+
+
 class SafetensorLazyLoader:
 
     def __init__(self, hf_model_dir: str, peft_format: bool = False):
@@ -42,7 +62,7 @@ def __init__(self, hf_model_dir: str, peft_format: bool = False):
     def _open_file(self, filename: str):
         """Open a safetensors file if not already open."""
         if filename not in self._file_handles:
-            file_path = os.path.join(self.hf_model_dir, filename)
+            file_path = checkpoint_path(self.hf_model_dir, filename)
             self._file_handles[filename] = safe_open(file_path, framework='pt')
         return self._file_handles[filename]
 
diff --git a/tests/test_checkpoint_path.py b/tests/test_checkpoint_path.py
new file mode 100644
index 0000000..bd0770e
--- /dev/null
+++ b/tests/test_checkpoint_path.py
@@ -0,0 +1,35 @@
+# Copyright (c) ModelScope Contributors. All rights reserved.
+"""Weight-map filenames cannot leave the checkpoint directory."""
+import ast
+import os
+from pathlib import Path
+
+
+def _join():
+    path = Path(__file__).resolve().parents[1] / 'src/mcore_bridge/utils/safetensors.py'
+    tree = ast.parse(path.read_text())
+    fn = next(node for node in tree.body if isinstance(node, ast.FunctionDef) and node.name == 'checkpoint_path')
+    module = ast.Module(body=[fn], type_ignores=[])
+    ast.fix_missing_locations(module)
+    namespace = {'os': os}
+    exec(compile(module, str(path), 'exec'), namespace)
+    return namespace['checkpoint_path']
+
+
+def test_relative_shard_stays_inside(tmp_path):
+    join = _join()
+    shard = tmp_path / 'model-00001.safetensors'
+    shard.write_bytes(b'x')
+    assert join(str(tmp_path), 'model-00001.safetensors') == os.path.realpath(shard)
+
+
+def test_parent_and_absolute_names_are_rejected(tmp_path):
+    join = _join()
+    outside = tmp_path.parent / 'outside.safetensors'
+    outside.write_bytes(b'x')
+    for name in ('../outside.safetensors', str(outside)):
+        try:
+            join(str(tmp_path), name)
+        except ValueError:
+            continue
+        raise AssertionError(name)