🐛 fix(discovery): skip empty PATH entries

docs/changelog/129.bugfix.rst

@@ -0,0 +1 @@
+Skip empty ``PATH`` entries during interpreter discovery - by :user:`gaborbernat`.

src/python_discovery/_discovery.py

@@ -423,7 +423,12 @@ def get_paths(env: Mapping[str, str]) -> Generator[Path, None, None]:
         except (AttributeError, ValueError):  # pragma: no cover # Windows only (no confstr)
             path = os.defpath
     if path:
-        for entry in map(Path, path.split(os.pathsep)):
+        # An empty component (a leading/trailing/doubled separator) means "the current directory", the
+        # same footgun a shell has when PATH is misconfigured - Path("") resolves to it. Since every
+        # yielded path here is later searched for interpreters to execute and interrogate, silently
+        # including the caller's CWD would let whatever directory virtualenv happens to run from smuggle
+        # in a candidate binary.
+        for entry in map(Path, filter(None, path.split(os.pathsep))):
             with suppress(OSError):
                 if entry.is_dir() and next(entry.iterdir(), None):
                     yield entry

tests/test_discovery.py

@@ -161,6 +161,38 @@ def test_get_paths_no_path_env(monkeypatch: pytest.MonkeyPatch) -> None:
     assert paths
 
 
+@pytest.mark.parametrize(
+    ("raw_path", "expected"),
+    [
+        pytest.param("{first}{sep}", ["first"], id="trailing"),
+        pytest.param("{sep}{first}", ["first"], id="leading"),
+        pytest.param("{first}{sep}{sep}{second}", ["first", "second"], id="doubled"),
+    ],
+)
+def test_get_paths_skips_empty_entries(tmp_path: Path, raw_path: str, expected: list[str]) -> None:
+    """An empty PATH entry means "current directory" - it must never be searched for interpreters."""
+    dirs = {}
+    for name in ("first", "second"):
+        dirs[name] = tmp_path / name
+        dirs[name].mkdir()
+        (dirs[name] / "dummy").touch()
+
+    paths = list(get_paths({"PATH": raw_path.format(first=dirs["first"], second=dirs["second"], sep=os.pathsep)}))
+
+    assert paths == [dirs[name] for name in expected]
+
+
+def test_get_paths_empty_entry_does_not_yield_cwd(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """Reproduces the exploit: an empty PATH entry used to resolve to the caller's CWD."""
+    cwd_marker = tmp_path / "python3.11"
+    cwd_marker.touch(mode=0o755)
+    monkeypatch.chdir(tmp_path)
+
+    paths = list(get_paths({"PATH": f"{os.pathsep}"}))
+
+    assert not any(path.resolve() == tmp_path.resolve() for path in paths)
+
+
 def test_lazy_path_dump_debug(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> None:
     monkeypatch.setenv("_VIRTUALENV_DEBUG", "1")
     a_dir = tmp_path