🐛 fix(discovery): skip relative PATH entries

docs/changelog/202.security.rst

@@ -0,0 +1,2 @@
+Skip relative ``PATH`` entries, including the working-directory entry in the Windows default search path, when
+discovering interpreters.

docs/how-to/standalone-usage.rst

@@ -61,6 +61,9 @@ shell. You can override these to control exactly where the library looks.
    env = {**os.environ, "PATH": "/usr/local/bin:/usr/bin"}
    result = get_interpreter("python3.12", env=env)
 
+Only absolute directories in ``PATH`` are searched. To select an interpreter in the working directory, use
+``./python`` as the spec on POSIX or ``./python.exe`` on Windows.
+
 Customize interpreter query timeout
 -------------------------------------
 

src/python_discovery/_discovery.py

@@ -432,14 +432,10 @@ 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:
-        # 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.
+        # Relative entries can execute an interpreter supplied by the working directory.
         for entry in map(Path, filter(None, path.split(os.pathsep))):
             with suppress(OSError):
-                if entry.is_dir() and next(entry.iterdir(), None):
+                if entry.is_absolute() and entry.is_dir() and next(entry.iterdir(), None):
                     yield entry
 
 

tests/test_discovery.py

@@ -291,8 +291,34 @@ def test_discovery_via_path_with_file(tmp_path: Path, monkeypatch: pytest.Monkey
 
 def test_get_paths_no_path_env(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.delenv("PATH", raising=False)
-    paths = list(get_paths({}))
-    assert paths
+    assert all(path.is_absolute() for path in get_paths({}))
+
+
+@pytest.mark.parametrize("relative", [".", "bin", "~/.local/bin"])
+def test_get_paths_skips_relative_entries(tmp_path: Path, monkeypatch: pytest.MonkeyPatch, relative: str) -> None:
+    monkeypatch.chdir(tmp_path)
+    relative_dir = tmp_path / relative
+    relative_dir.mkdir(parents=True, exist_ok=True)
+    (relative_dir / "python3").touch()
+    absolute_dir = tmp_path / "absolute"
+    absolute_dir.mkdir()
+    (absolute_dir / "python3").touch()
+
+    assert list(get_paths({"PATH": f"{relative}{os.pathsep}{absolute_dir}"})) == [absolute_dir]
+
+
+def test_get_paths_skips_relative_default(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.chdir(tmp_path)
+    (tmp_path / "python3").touch()
+    absolute_dir = tmp_path / "absolute"
+    absolute_dir.mkdir()
+    (absolute_dir / "python3").touch()
+
+    with (
+        patch("python_discovery._discovery.os.confstr", side_effect=AttributeError, create=True),
+        patch("python_discovery._discovery.os.defpath", f".{os.pathsep}{absolute_dir}"),
+    ):
+        assert list(get_paths({})) == [absolute_dir]
 
 
 @pytest.mark.parametrize(