fix(frontend): validate in-tree backend paths

src/pyproject_api/_frontend.py

@@ -211,7 +211,8 @@ def create_args_from_folder(
 
         :param folder: the python project folder
         :return: the frontend creation args
-        :raises ValueError: if ``build-system`` is not a table, or one of its keys has a type PEP-518 rules out
+        :raises ValueError: if ``build-system`` is not a table, one of its keys has a type PEP-518 rules out, or a
+            ``backend-path`` entry does not resolve inside the project (PEP-517 in-tree backends)
 
         E.g., to create a frontend from a python project folder:
 
@@ -228,7 +229,9 @@ def create_args_from_folder(
                 msg = f"build-system must be a table, got {build_system!r}"
                 raise ValueError(msg)
             if "backend-path" in build_system:
-                backend_paths: tuple[Path, ...] = tuple(folder / p for p in _str_list(build_system, "backend-path"))
+                backend_paths: tuple[Path, ...] = tuple(
+                    _in_tree_path(folder, entry) for entry in _str_list(build_system, "backend-path")
+                )
             else:
                 backend_paths = ()
             if "requires" in build_system:
@@ -539,6 +542,19 @@ def _str_list(build_system: dict[str, Any], key: str) -> list[str]:
     return value
 
 
+def _in_tree_path(folder: Path, entry: str) -> Path:
+    path = folder / entry
+    # Python 3.15 on Windows resolves a path containing NUL instead of raising
+    try:
+        inside = "\0" not in entry and not Path(entry).is_absolute() and path.resolve().is_relative_to(folder.resolve())
+    except (OSError, RuntimeError):  # RuntimeError: symlink loop before Python 3.13
+        inside = False
+    if not inside:
+        msg = f"build-system.backend-path entry {entry!r} must be a relative path inside the project {folder}"
+        raise ValueError(msg)
+    return path
+
+
 __all__ = [
     "BackendFailed",
     "CmdStatus",

tests/test_frontend.py

@@ -16,6 +16,8 @@
 if TYPE_CHECKING:
     from collections.abc import Callable
 
+    import pytest_mock
+
 
 @pytest.fixture
 def local_builder(tmp_path: Path) -> Callable[[str], Path]:
@@ -183,6 +185,38 @@ def test_empty_pyproject(tmp_path: Path) -> None:
         assert str(left) == str(right)
 
 
+@pytest.mark.parametrize(
+    "backend_path",
+    [
+        pytest.param("../outside", id="parent"),
+        pytest.param("{tmp}/outside", id="absolute-outside"),
+        pytest.param("{tmp}/project", id="absolute-inside"),
+        pytest.param("link", id="symlink-outside"),
+        pytest.param("\\u0000", id="nul"),
+    ],
+)
+def test_create_args_from_folder_rejects_out_of_tree_backend_path(tmp_path: Path, backend_path: str) -> None:
+    project = tmp_path / "project"
+    project.mkdir()
+    (project / "link").symlink_to(tmp_path, target_is_directory=True)
+    (project / "pyproject.toml").write_text(
+        f'[build-system]\nbackend-path = ["{backend_path.format(tmp=tmp_path.as_posix())}"]\nbuild-backend = "b"\n'
+    )
+
+    with pytest.raises(ValueError, match=r"backend-path entry .* must be a relative path inside the project"):
+        SubprocessFrontend.create_args_from_folder(project)
+
+
+def test_create_args_from_folder_rejects_unresolvable_backend_path(
+    tmp_path: Path, mocker: pytest_mock.MockerFixture
+) -> None:
+    (tmp_path / "pyproject.toml").write_text('[build-system]\nbackend-path = ["."]\nbuild-backend = "b"\n')
+    mocker.patch.object(Path, "resolve", autospec=True, side_effect=RuntimeError("Symlink loop"))
+
+    with pytest.raises(ValueError, match=r"backend-path entry '\.' must be a relative path inside the project"):
+        SubprocessFrontend.create_args_from_folder(tmp_path)
+
+
 @pytest.fixture(scope="session")
 def demo_pkg_inline() -> Path:
     return Path(__file__).absolute().parent / "demo_pkg_inline"