[3.15] gh-160279: Keep the venv directory off the import path when running pip (GH-160282)

Lib/test/test_venv.py

@@ -1003,7 +1003,13 @@ def test_devnull(self):
 
     def do_test_with_pip(self, system_site_packages):
         rmtree(self.env_dir)
+        os.mkdir(self.env_dir)
+        # Both ensurepip and its pip subprocess run with the venv as cwd.
+        for name in ('ensurepip.py', 'subprocess.py'):
+            pathlib.Path(self.env_dir, name).write_text(
+                'raise AssertionError(__file__)\n', encoding='utf-8')
         with EnvironmentVarGuard() as envvars:
+            envvars["PYTHONSAFEPATH"] = ""
             # pip's cross-version compatibility may trigger deprecation
             # warnings in current versions of Python. Ensure related
             # environment settings don't cause venv to fail.

Lib/venv/__init__.py

@@ -437,14 +437,15 @@ def setup_python(self, context):
     def _call_new_python(self, context, *py_args, **kwargs):
         """Executes the newly created Python using safe-ish options"""
         # gh-98251: We do not want to just use '-I' because that masks
-        # legitimate user preferences (such as not writing bytecode). All we
-        # really need is to ensure that the path variables do not overrule
-        # normal venv handling.
+        # legitimate user preferences (such as not writing bytecode).
         args = [context.env_exec_cmd, *py_args]
         kwargs['env'] = env = os.environ.copy()
         env['VIRTUAL_ENV'] = context.env_dir
         env.pop('PYTHONHOME', None)
         env.pop('PYTHONPATH', None)
+        # Keep the working directory off sys.path, including in the
+        # subprocess that ensurepip starts to run pip.
+        env['PYTHONSAFEPATH'] = '1'
         kwargs['cwd'] = context.env_dir
         kwargs['executable'] = context.env_exec_cmd
         subprocess.check_output(args, **kwargs)

Misc/NEWS.d/next/Security/2026-10-22-11-09-30.gh-issue-160279.9BtxeF.rst

@@ -0,0 +1,2 @@
+Prevent :mod:`venv` from importing modules from the root of an existing
+virtual environment when installing or upgrading pip.