fix(core): harden check for txt files in deprecated prompt loading functions (#36471)

libs/core/langchain_core/prompts/loading.py

@@ -96,9 +96,12 @@ def _load_template(
         template_path = Path(config.pop(f"{var_name}_path"))
         if not allow_dangerous_paths:
             _validate_path(template_path)
+        # Resolve symlinks before checking the suffix so that a symlink named
+        # "exploit.txt" pointing to a non-.txt file is caught.
+        resolved_path = template_path.resolve()
         # Load the template.
-        if template_path.suffix == ".txt":
-            template = template_path.read_text(encoding="utf-8")
+        if resolved_path.suffix == ".txt":
+            template = resolved_path.read_text(encoding="utf-8")
         else:
             raise ValueError
         # Set the template variable to the extracted variable.

libs/core/tests/unit_tests/prompts/test_loading.py

@@ -278,6 +278,53 @@ def test_load_prompt_from_config_few_shot_rejects_absolute_example_prompt_path(
         load_prompt_from_config(config)
 
 
+def test_symlink_txt_to_py_is_blocked(tmp_path: Path) -> None:
+    """Test symlink redirects cannot get around file extension check."""
+    sensitive = tmp_path / "sensitive_source.py"
+    sensitive.write_text("INTERNAL_SECRET='ABC-123-XYZ'")
+    symlink = tmp_path / "exploit_link.txt"
+    symlink.symlink_to(sensitive)
+
+    config = {
+        "_type": "prompt",
+        "template_path": "exploit_link.txt",
+        "input_variables": [],
+    }
+    original_dir = Path.cwd()
+    try:
+        os.chdir(tmp_path)
+        with (
+            suppress_langchain_deprecation_warning(),
+            pytest.raises(ValueError),  # noqa: PT011
+        ):
+            load_prompt_from_config(config)
+    finally:
+        os.chdir(original_dir)
+
+
+def test_symlink_jinja2_rce_is_blocked(tmp_path: Path) -> None:
+    """Check jinja2 templates cannot be used to perform RCE via symlinks."""
+    payload = tmp_path / "rce_payload.py"
+    payload.write_text(
+        "{{ self.__init__.__globals__.__builtins__"
+        ".__import__('os').popen('id').read() }}"
+    )
+    symlink = tmp_path / "rce_bypass.txt"
+    symlink.symlink_to(payload)
+
+    config = {
+        "_type": "prompt",
+        "template_path": str(symlink),
+        "template_format": "jinja2",
+        "input_variables": [],
+    }
+    with (
+        suppress_langchain_deprecation_warning(),
+        pytest.raises(ValueError),  # noqa: PT011
+    ):
+        load_prompt_from_config(config, allow_dangerous_paths=True)
+
+
 def test_loading_few_shot_prompt_from_yaml() -> None:
     """Test loading few shot prompt from yaml."""
     with change_directory(EXAMPLE_DIR), suppress_langchain_deprecation_warning():