@@ -1108,6 +1108,20 @@ compatibility bug or reduction in CLI calls.
 | Config-backed remote/submodule maintenance | Missing config enumeration/multivar/removal operations (GIX-12), plus filesystem and worktree lifecycle requirements. |
 | Hook execution | Every explicit hook request, including an absent hook, retains Git-managed lookup, execution and diagnostics until GIX-23 is addressed. Native operations retain GitPython's default hook/fsmonitor/maintenance restrictions. |
 
+### Submodule transport policy
+
+Submodule clone and fetch stay on the CLI and pass `GIT_PROTOCOL_FROM_USER=0`
+for URLs from `.gitmodules`, matching `git submodule update --init` and
+`git submodule update --remote`. A `protocol.file.allow=user` policy therefore
+rejects the file transport, while `always` permits it. The existing explicit
+`allow_unsafe_protocols=True` opt-in preserves the caller's environment policy;
+`user` is accepted only when `GIT_PROTOCOL_FROM_USER` is unset or permits it.
+The regression compares both backends with Git 2.54.0, including unchanged
+checkouts after denied transfers.
+GixPython 0.2.0 still needs the clone/environment and fetch-result contracts
+listed above before these calls can move to Gix. This is a shared GitPython
+policy bug, not a demonstrated Gix compatibility mismatch.
+
 ### Downstream mirror-fetch result gap
 
 Airflow Git provider 1.0.1's bare-cache refspecs
 
@@ -371,6 +371,11 @@ def _module_abspath(cls, parent_repo: "Repo", path: PathLike, name: str) -> Path
             )
         raise NotADirectoryError()
 
+    @classmethod
+    def _transport_env(cls, allow_unsafe_protocols: bool = False) -> Dict[str, str]:
+        """Apply Git's non-user transport policy to URLs read from ``.gitmodules``."""
+        return {} if allow_unsafe_protocols else {"GIT_PROTOCOL_FROM_USER": "0"}
+
     @classmethod
     def _clone_repo(
         cls,
@@ -940,7 +945,10 @@ def update(
             They only take effect with the `init` option.
 
         :param allow_unsafe_protocols:
-            Allow unsafe protocols to be used, like ``ext``.
+            Allow unsafe protocols to be used, like ``ext``. Clone and fetch also
+            preserve the caller's ``GIT_PROTOCOL_FROM_USER`` policy instead of
+            forcing it to ``0``. A ``user`` transport such as ``file`` is then
+            accepted only when that environment policy permits it.
 
         :param allow_unsafe_options:
             Allow unsafe options to be used, like ``--upload-pack``.
@@ -992,7 +1000,8 @@ def fetch_remotes(module_repo: "Repo") -> None:
                 )
                 # ===============================
                 if not dry_run:
-                    remote.fetch(progress=progress)
+                    with module_repo.git.custom_environment(**self._transport_env(allow_unsafe_protocols)):
+                        remote.fetch(progress=progress)
                 # END handle dry-run
                 # ===============================
                 if i == len_rmts - 1:
@@ -1080,7 +1089,7 @@ def fetch_remotes(module_repo: "Repo") -> None:
                             self.path,
                             self.name,
                             n=True,
-                            env=env,
+                            env={**self._transport_env(allow_unsafe_protocols), **(env or {})},
                             multi_options=clone_multi_options,
                             allow_unsafe_options=allow_unsafe_options,
                             allow_unsafe_protocols=allow_unsafe_protocols,
 
@@ -10,6 +10,7 @@
     "with_rw_directory",
     "with_rw_repo",
     "with_rw_and_rw_remote_repo",
+    "patch_git_config",
     "PathLikeMock",
     "TestBase",
     "TestCase",
@@ -36,6 +37,7 @@
 import textwrap
 import time
 import unittest
+from unittest import mock
 from typing import Union, Type, Tuple
 
 import pytest
@@ -82,6 +84,26 @@ def fixture(name):
 # { Adapters
 
 
+@contextlib.contextmanager
+def patch_git_config(name, value):
+    """Temporarily add a git config name-value pair, using environment variables."""
+    pair_index = int(os.getenv("GIT_CONFIG_COUNT", "0"))
+
+    # This is recomputed each time the context is entered, for compatibility with
+    # existing GIT_CONFIG_* environment variables, even if changed in this process.
+    patcher = mock.patch.dict(
+        os.environ,
+        {
+            "GIT_CONFIG_COUNT": str(pair_index + 1),
+            f"GIT_CONFIG_KEY_{pair_index}": name,
+            f"GIT_CONFIG_VALUE_{pair_index}": value,
+        },
+    )
+
+    with patcher:
+        yield
+
+
 class StringProcessAdapter:
     """Allows strings to be used as process objects returned by subprocess.Popen.
 
 
@@ -7,7 +7,7 @@
 import os
 import os.path
 
-from test.lib import TestBase
+from test.lib import TestBase, patch_git_config
 from test.lib.helper import with_rw_directory, with_rw_repo
 from test.lib.fixtures import writable_fixture
 
@@ -25,6 +25,7 @@ def tearDown(self):
     #         "'C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\test_work_tree_unsupportedryfa60di\\master_repo\\.git\\objects\\pack\\pack-bc9e0787aef9f69e1591ef38ea0a6f566ec66fe3.idx")  # noqa: E501
     @with_rw_repo("HEAD")
     @with_rw_directory
+    @patch_git_config("protocol.file.allow", "always")
     def test_init_repo_object(self, rw_dir, rw_repo):
         # [1-test_init_repo_object]
         from git import Repo
@@ -484,6 +485,7 @@ def test_references_and_objects(self, rw_dir):
         repo.git.clear_cache()
 
     @with_rw_repo("c15a6e1923a14bc760851913858a3942a4193cdb")
+    @patch_git_config("protocol.file.allow", "always")
     def test_submodules(self, repo):
         # [1-test_submodules]
         sms = repo.submodules