diff --git a/src/google/adk/artifacts/file_artifact_service.py b/src/google/adk/artifacts/file_artifact_service.py
index b0078e27ce..ff6fff0361 100644
--- a/src/google/adk/artifacts/file_artifact_service.py
+++ b/src/google/adk/artifacts/file_artifact_service.py
@@ -133,6 +133,31 @@ def _resolve_scoped_artifact_path(
   return candidate, relative
 
 
+def _validate_path_segment(parent: Path, segment: str, label: str) -> Path:
+  """Validates that a path segment does not escape its parent directory.
+
+  Args:
+    parent: The directory the segment must stay within.
+    segment: Caller-supplied value used as a single path component.
+    label: Human-readable name for error messages (e.g. ``"User ID"``).
+
+  Returns:
+    The resolved path ``parent / segment``.
+
+  Raises:
+    InputValidationError: If the resulting path escapes ``parent``.
+  """
+  parent_resolved = parent.resolve(strict=False)
+  candidate = (parent_resolved / segment).resolve(strict=False)
+  try:
+    candidate.relative_to(parent_resolved)
+  except ValueError as exc:
+    raise InputValidationError(
+        f"{label} {segment!r} resolves outside storage directory."
+    ) from exc
+  return candidate
+
+
 def _is_user_scoped(session_id: Optional[str], filename: str) -> bool:
   """Determines whether artifacts should be stored in the user namespace."""
   return session_id is None or _file_has_user_namespace(filename)
@@ -145,7 +170,11 @@ def _user_artifacts_dir(base_root: Path) -> Path:
 
 def _session_artifacts_dir(base_root: Path, session_id: str) -> Path:
   """Returns the path that stores session-scoped artifacts."""
-  return base_root / "sessions" / session_id / "artifacts"
+  sessions_dir = base_root / "sessions"
+  return (
+      _validate_path_segment(sessions_dir, session_id, "Session ID")
+      / "artifacts"
+  )
 
 
 def _versions_dir(artifact_dir: Path) -> Path:
@@ -220,7 +249,8 @@ def __init__(self, root_dir: Path | str):
 
   def _base_root(self, user_id: str, /) -> Path:
     """Returns the artifacts root directory for a user."""
-    return self.root_dir / "users" / user_id
+    users_dir = self.root_dir / "users"
+    return _validate_path_segment(users_dir, user_id, "User ID")
 
   def _scope_root(
       self,
diff --git a/tests/unittests/artifacts/test_artifact_service.py b/tests/unittests/artifacts/test_artifact_service.py
index 25294d4909..79dc6239a3 100644
--- a/tests/unittests/artifacts/test_artifact_service.py
+++ b/tests/unittests/artifacts/test_artifact_service.py
@@ -896,3 +896,62 @@ async def test_save_artifact_with_snake_case_dict(
   assert loaded is not None
   assert loaded.inline_data is not None
   assert loaded.inline_data.mime_type == "text/plain"
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "user_id",
+    [
+        "../../escape",
+        "../sibling",
+        "valid/../../../etc",
+    ],
+)
+async def test_file_save_rejects_traversal_user_id(tmp_path, user_id):
+  """FileArtifactService rejects user_id values that escape the storage root."""
+  artifact_service = FileArtifactService(root_dir=tmp_path / "artifacts")
+  part = types.Part(text="content")
+  with pytest.raises(InputValidationError):
+    await artifact_service.save_artifact(
+        app_name="myapp",
+        user_id=user_id,
+        session_id="sess1",
+        filename="file.txt",
+        artifact=part,
+    )
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "session_id",
+    [
+        "../../escape",
+        "../sibling",
+        "valid/../../../tmp",
+    ],
+)
+async def test_file_save_rejects_traversal_session_id(tmp_path, session_id):
+  """FileArtifactService rejects session_id values that escape the storage root."""
+  artifact_service = FileArtifactService(root_dir=tmp_path / "artifacts")
+  part = types.Part(text="content")
+  with pytest.raises(InputValidationError):
+    await artifact_service.save_artifact(
+        app_name="myapp",
+        user_id="user1",
+        session_id=session_id,
+        filename="file.txt",
+        artifact=part,
+    )
+
+
+@pytest.mark.asyncio
+async def test_file_delete_rejects_traversal_session_id(tmp_path):
+  """Traversal session_id is caught before shutil.rmtree can be reached."""
+  artifact_service = FileArtifactService(root_dir=tmp_path / "artifacts")
+  with pytest.raises(InputValidationError):
+    await artifact_service.delete_artifact(
+        app_name="myapp",
+        user_id="user1",
+        session_id="../../escape",
+        filename="file.txt",
+    )