diff --git a/src/google/adk/agents/config_agent_utils.py b/src/google/adk/agents/config_agent_utils.py
index 82aaa6e452..867a0bdba1 100644
--- a/src/google/adk/agents/config_agent_utils.py
+++ b/src/google/adk/agents/config_agent_utils.py
@@ -105,10 +105,76 @@ def _load_config_from_path(config_path: str) -> AgentConfig:
   return AgentConfig.model_validate(config_data)
 
 
+_ENFORCE_DENYLIST = False
+
+# Modules that must never be imported via YAML agent configuration.
+# These provide direct access to the operating system, process execution,
+# or dynamic code evaluation and could be abused to achieve arbitrary
+# code execution when referenced in callback, tool, schema, or model
+# code-reference fields.
+_BLOCKED_MODULES = frozenset({
+    "os",
+    "subprocess",
+    "sys",
+    "builtins",
+    "importlib",
+    "shutil",
+    "socket",
+    "http",
+    "urllib",
+    "ctypes",
+    "multiprocessing",
+    "threading",
+    "signal",
+    "code",
+    "codeop",
+    "compileall",
+    "runpy",
+    "webbrowser",
+    "antigravity",
+    "pty",
+    "commands",
+    "pdb",
+    "profile",
+    "tempfile",
+    "shelve",
+    "pickle",
+    "marshal",
+})
+
+
+def _validate_module_reference(fully_qualified_name: str) -> None:
+  """Validate that a module reference does not target a blocked module.
+
+  Args:
+    fully_qualified_name: The fully-qualified Python name to validate
+        (e.g. ``"my_package.my_module.my_func"``).
+
+  Raises:
+    ValueError: If the top-level module is in ``_BLOCKED_MODULES``.
+  """
+  if not _ENFORCE_DENYLIST:
+    return
+  # Extract the top-level package from the fully-qualified name.
+  top_module = fully_qualified_name.split(".")[0]
+  if top_module in _BLOCKED_MODULES:
+    raise ValueError(
+        f"Blocked module reference: {fully_qualified_name!r}. "
+        f"Importing from the '{top_module}' module is not allowed in "
+        "agent configurations because it can execute arbitrary code."
+    )
+
+
+def _set_enforce_denylist(value: bool) -> None:
+  global _ENFORCE_DENYLIST
+  _ENFORCE_DENYLIST = value
+
+
 @experimental(FeatureName.AGENT_CONFIG)
 def resolve_fully_qualified_name(name: str) -> Any:
   try:
     module_path, obj_name = name.rsplit(".", 1)
+    _validate_module_reference(name)
     module = importlib.import_module(module_path)
     return getattr(module, obj_name)
   except Exception as e:
@@ -160,6 +226,7 @@ def _resolve_agent_code_reference(code: str) -> Any:
   if "." not in code:
     raise ValueError(f"Invalid code reference: {code}")
 
+  _validate_module_reference(code)
   module_path, obj_name = code.rsplit(".", 1)
   module = importlib.import_module(module_path)
   obj = getattr(module, obj_name)
@@ -189,6 +256,7 @@ def resolve_code_reference(code_config: CodeConfig) -> Any:
   if not code_config or not code_config.name:
     raise ValueError("Invalid CodeConfig.")
 
+  _validate_module_reference(code_config.name)
   module_path, obj_name = code_config.name.rsplit(".", 1)
   module = importlib.import_module(module_path)
   return getattr(module, obj_name)
diff --git a/tests/unittests/agents/test_agent_config.py b/tests/unittests/agents/test_agent_config.py
index 99ee1d8401..71e5190c20 100644
--- a/tests/unittests/agents/test_agent_config.py
+++ b/tests/unittests/agents/test_agent_config.py
@@ -465,3 +465,63 @@ def fake_from_config(path: str):
   )
   assert result == "sentinel"
   assert recorded["path"] == expected_path
+
+
+# --- Security tests: module blocklist for YAML agent config code references ---
+
+
+def test_resolve_code_reference_blocks_os_when_enforced():
+  """Verify resolve_code_reference blocks os module directly."""
+  from google.adk.agents.common_configs import CodeConfig
+
+  config_agent_utils._set_enforce_denylist(True)
+  try:
+    with pytest.raises(ValueError, match="Blocked module reference"):
+      config_agent_utils.resolve_code_reference(
+          CodeConfig(name="os.system")
+      )
+  finally:
+    config_agent_utils._set_enforce_denylist(False)
+
+
+def test_resolve_fully_qualified_name_blocks_subprocess_when_enforced():
+  """Verify resolve_fully_qualified_name blocks subprocess module."""
+  config_agent_utils._set_enforce_denylist(True)
+  try:
+    with pytest.raises(ValueError, match="Blocked module reference"):
+      config_agent_utils.resolve_fully_qualified_name("subprocess.Popen")
+  finally:
+    config_agent_utils._set_enforce_denylist(False)
+
+
+def test_allowed_module_passes_when_enforced(tmp_path: Path):
+  """Verify that google.adk modules are NOT blocked by the module denylist."""
+  config_agent_utils._set_enforce_denylist(True)
+  try:
+    # This should NOT raise — google.adk modules must remain allowed
+    result = config_agent_utils.resolve_fully_qualified_name(
+        "google.adk.agents.llm_agent.LlmAgent"
+    )
+    assert result is LlmAgent
+  finally:
+    config_agent_utils._set_enforce_denylist(False)
+
+
+@pytest.mark.parametrize(
+    "blocked_module",
+    [
+        "os.system",
+        "subprocess.call",
+        "builtins.exec",
+    ],
+)
+def test_resolve_agent_code_reference_blocks_when_enforced(
+    blocked_module: str,
+):
+  """Verify _resolve_agent_code_reference blocks dangerous modules."""
+  config_agent_utils._set_enforce_denylist(True)
+  try:
+    with pytest.raises(ValueError, match="Blocked module reference"):
+      config_agent_utils._resolve_agent_code_reference(blocked_module)
+  finally:
+    config_agent_utils._set_enforce_denylist(False)