Warn before instantiating _target_ from algo_object.json (#9085)

### Description

`algo_from_json` resolves the `_target_` value from an
`algo_object.json` to an importable callable and invokes it, and adds
file-influenced directories to `sys.path`. Emit a trust-boundary warning
before instantiation so users only load trusted files
(GHSA-2wx3-8x3w-r8qv).

### Types of changes
- [x] Non-breaking change
- [x] New tests added to cover the changes.

---------

Signed-off-by: R. Garcia-Dias <rafaelagd@gmail.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: garciadias

monai/auto3dseg/utils.py

@@ -493,6 +493,14 @@ def algo_from_json(filename: str, template_path: PathLike | None = None, **kwarg
             if state_template_path:
                 algo_config["template_path"] = state_template_path
 
+            warnings.warn(
+                f"Loading {filename}: the file's `_target_` value is resolved to an imported callable and "
+                "invoked, and template directories from the file may be added to `sys.path`; only load "
+                "algo_object.json files from a source you trust "
+                "(see https://github.com/Project-MONAI/MONAI/security/advisories/GHSA-2wx3-8x3w-r8qv).",
+                stacklevel=2,
+            )
+
             parser = ConfigParser(algo_config)
             algo = parser.get_parsed_content()
             used_template_path = path

tests/apps/test_auto3dseg.py

@@ -11,9 +11,11 @@
 
 from __future__ import annotations
 
+import json
 import os
 import tempfile
 import unittest
+import warnings
 from copy import deepcopy
 from numbers import Number
 
@@ -36,6 +38,7 @@
     SampleOperations,
     SegSummarizer,
     SummaryOperations,
+    algo_from_json,
     datafold_read,
     verify_report_format,
 )
@@ -177,6 +180,20 @@ def __call__(self, data):
         return d
 
 
+class _DummyAlgo:
+    """Minimal stand-in for an Auto3DSeg Algo object used in warning tests."""
+
+    def __init__(self) -> None:
+        self.template_path: str | None = None
+        self.output_path = os.getcwd()
+
+    def load_state_dict(self, state: dict) -> None:
+        pass
+
+    def get_output_path(self) -> str:
+        return self.output_path
+
+
 class TestDataAnalyzer(unittest.TestCase):
     def setUp(self):
         self.test_dir = tempfile.TemporaryDirectory()
@@ -619,5 +636,23 @@ def tearDown(self) -> None:
         self.test_dir.cleanup()
 
 
+class TestAlgoFromJsonSecurityWarning(unittest.TestCase):
+    def test_warns_about_untrusted_target(self) -> None:
+        with tempfile.TemporaryDirectory() as tmpdir:
+            algo_file = os.path.join(tmpdir, "algo_object.json")
+            with open(algo_file, "w", encoding="utf-8") as f:
+                json.dump({"_target_": f"{__name__}._DummyAlgo"}, f)
+
+            with warnings.catch_warnings(record=True) as caught:
+                warnings.simplefilter("always")
+                algo_from_json(algo_file)
+
+            messages = [str(w.message) for w in caught]
+            self.assertTrue(
+                any("algo_object.json" in msg and "trust" in msg for msg in messages),
+                f"Keywords 'algo_object.json' and 'trust' not found in warning messages: {messages}",
+            )
+
+
 if __name__ == "__main__":
     unittest.main()