diff --git a/git/cmd.py b/git/cmd.py
index 3665eb029..d6f8f946a 100644
--- a/git/cmd.py
+++ b/git/cmd.py
@@ -14,7 +14,6 @@
 import subprocess
 import threading
 from textwrap import dedent
-import unittest.mock
 
 from git.compat import (
     defenc,
@@ -24,7 +23,7 @@
     is_win,
 )
 from git.exc import CommandError
-from git.util import is_cygwin_git, cygpath, expand_path, remove_password_if_present
+from git.util import is_cygwin_git, cygpath, expand_path, remove_password_if_present, patch_env
 
 from .exc import GitCommandError, GitCommandNotFound, UnsafeOptionError, UnsafeProtocolError
 from .util import (
@@ -965,10 +964,10 @@ def execute(
                     '"kill_after_timeout" feature is not supported on Windows.',
                 )
             # Only search PATH, not CWD. This must be in the *caller* environment. The "1" can be any value.
-            patch_caller_env = unittest.mock.patch.dict(os.environ, {"NoDefaultCurrentDirectoryInExePath": "1"})
+            maybe_patch_caller_env = patch_env("NoDefaultCurrentDirectoryInExePath", "1")
         else:
             cmd_not_found_exception = FileNotFoundError  # NOQA # exists, flake8 unknown @UndefinedVariable
-            patch_caller_env = contextlib.nullcontext()
+            maybe_patch_caller_env = contextlib.nullcontext()
         # end handle
 
         stdout_sink = PIPE if with_stdout else getattr(subprocess, "DEVNULL", None) or open(os.devnull, "wb")
@@ -984,7 +983,7 @@ def execute(
             istream_ok,
         )
         try:
-            with patch_caller_env:
+            with maybe_patch_caller_env:
                 proc = Popen(
                     command,
                     env=env,
diff --git a/git/util.py b/git/util.py
index f6dedf0f2..dee467dd3 100644
--- a/git/util.py
+++ b/git/util.py
@@ -150,6 +150,7 @@ def wrapper(self: "Remote", *args: Any, **kwargs: Any) -> T:
 
 @contextlib.contextmanager
 def cwd(new_dir: PathLike) -> Generator[PathLike, None, None]:
+    """Context manager to temporarily change directory. Not reentrant."""
     old_dir = os.getcwd()
     os.chdir(new_dir)
     try:
@@ -158,6 +159,20 @@ def cwd(new_dir: PathLike) -> Generator[PathLike, None, None]:
         os.chdir(old_dir)
 
 
+@contextlib.contextmanager
+def patch_env(name: str, value: str) -> Generator[None, None, None]:
+    """Context manager to temporarily patch an environment variable."""
+    old_value = os.getenv(name)
+    os.environ[name] = value
+    try:
+        yield
+    finally:
+        if old_value is None:
+            del os.environ[name]
+        else:
+            os.environ[name] = old_value
+
+
 def rmtree(path: PathLike) -> None:
     """Remove the given recursively.
 
@@ -935,7 +950,7 @@ def _obtain_lock_or_raise(self) -> None:
             )
 
         try:
-            with open(lock_file, mode='w'):
+            with open(lock_file, mode="w"):
                 pass
         except OSError as e:
             raise IOError(str(e)) from e
diff --git a/test/fixtures/env_case.py b/test/fixtures/env_case.py
new file mode 100644
index 000000000..120e59289
--- /dev/null
+++ b/test/fixtures/env_case.py
@@ -0,0 +1,13 @@
+import subprocess
+import sys
+
+import git
+
+
+_, working_dir, env_var_name = sys.argv
+
+# Importing git should be enough, but this really makes sure Git.execute is called.
+repo = git.Repo(working_dir)  # Hold the reference.
+git.Git(repo.working_dir).execute(["git", "version"])
+
+print(subprocess.check_output(["set", env_var_name], shell=True, text=True))
diff --git a/test/test_git.py b/test/test_git.py
index 540ea9f41..f1d35a355 100644
--- a/test/test_git.py
+++ b/test/test_git.py
@@ -4,35 +4,23 @@
 #
 # This module is part of GitPython and is released under
 # the BSD License: http://www.opensource.org/licenses/bsd-license.php
-import contextlib
 import os
 import shutil
 import subprocess
 import sys
 from tempfile import TemporaryDirectory, TemporaryFile
-from unittest import mock
+from unittest import mock, skipUnless
 
 from git import Git, refresh, GitCommandError, GitCommandNotFound, Repo, cmd
 from test.lib import TestBase, fixture_path
 from test.lib import with_rw_directory
-from git.util import finalize_process
+from git.util import cwd, finalize_process
 
 import os.path as osp
 
 from git.compat import is_win
 
 
-@contextlib.contextmanager
-def _chdir(new_dir):
-    """Context manager to temporarily change directory. Not reentrant."""
-    old_dir = os.getcwd()
-    os.chdir(new_dir)
-    try:
-        yield
-    finally:
-        os.chdir(old_dir)
-
-
 class TestGit(TestBase):
     @classmethod
     def setUpClass(cls):
@@ -102,9 +90,26 @@ def test_it_executes_git_not_from_cwd(self):
                     print("#!/bin/sh", file=file)
                 os.chmod(impostor_path, 0o755)
 
-            with _chdir(tmpdir):
+            with cwd(tmpdir):
                 self.assertRegex(self.git.execute(["git", "version"]), r"^git version\b")
 
+    @skipUnless(is_win, "The regression only affected Windows, and this test logic is OS-specific.")
+    def test_it_avoids_upcasing_unrelated_environment_variable_names(self):
+        old_name = "28f425ca_d5d8_4257_b013_8d63166c8158"
+        if old_name == old_name.upper():
+            raise RuntimeError("test bug or strange locale: old_name invariant under upcasing")
+        os.putenv(old_name, "1")  # It has to be done this lower-level way to set it lower-case.
+
+        cmdline = [
+            sys.executable,
+            fixture_path("env_case.py"),
+            self.rorepo.working_dir,
+            old_name,
+        ]
+        pair_text = subprocess.check_output(cmdline, shell=False, text=True)
+        new_name = pair_text.split("=")[0]
+        self.assertEqual(new_name, old_name)
+
     def test_it_accepts_stdin(self):
         filename = fixture_path("cat_file_blob")
         with open(filename, "r") as fh: