Environment variable exposure in GitPython separate_git_dir handling

### Summary `Repo._clone()`'s handling of `separate_git_dir` never disables `Git.polish_url()`'s environment-variable expansion, unlike every other caller of that helper that handles attacker-influenced data. Since `separate_git_dir` can be derived from an attacker-controlled `.gitmodules` submodule name, a completely ordinary `clone` + `submodule_update(init=True, recursive=True)` on a malicious repository expands an environment variable of the attacker's choosing and exposes its value both as a persistent on-disk directory name and, whenever DEBUG-level logging is enabled (a common CI/verbose-logging configuration), in a `Popen(...)` log line that is not redacted the way credential-bearing arguments are. ### Root cause / Details `git/repo/base.py`'s `_clone()` has: ```python sep_dir = kwargs.get("separate_git_dir") if sep_dir: kwargs["separate_git_dir"] = Git.polish_url(sep_dir) ``` This is the only remaining `Git.polish_url()` call site handling attacker-influenced data that does not pass `expand_vars=False` (there is one other unguarded call site, `clone_path` at line 1537, but its input is the caller's own local destination path, not attacker-controlled). The sibling fixes for `GHSA-94p4-4cq8-9g67`/`GHSA-rwj8-pgh3-r573` added `expand_vars=False` to `clone_url` (5 lines below this one, in the same function), `git/repo/base.py:1599`, `remote.py:814`, and `submodule/base.py:686` — this one did not receive it. (`submodule/root.py` also currently sets `expand_vars=False` at a related call site, but that was added incidentally by a later, unrelated commit, not as part of these security fixes.) `Git.polish_url()`'s docstring itself says: "Disable this for values obtained from an untrusted source, such as remote URLs." Its non-Cygwin branch does plain `os.path.expandvars()` on the entire string, including embedded variables anywhere in an absolute path. `separate_git_dir` reaches `_clone()` from `Submodule._clone_repo()` (`git/objects/submodule/base.py`) as `module_abspath = osp.join(parent_repo.git_dir, "modules", name)`, where `name` is the raw `.gitmodules` submodule section name — fully attacker-controlled content of a cloned repository. `_validated_name()` (added by the fix for `GHSA-hmq2-w58f-27jc`) only rejects names that are absolute, contain a drive letter, or contain `..` path components; it does not reject `$`. `_need_gitfile_submodules()` is true for git ≥ 1.7.5, i.e. the default path for virtually every real-world installation. ### Reproduction (live, against real, unmodified GitPython code, no monkeypatching) Built a hostile repository with a plain, ordinary git command: `git submodule add --name '$GPY_SECRET_MARKER' ../evil_submodule sub`. Set an environment variable in the "victim" process (standing in for a real CI secret such as `AWS_SECRET_ACCESS_KEY`): `GPY_SECRET_MARKER=AKIA_TOTALLY_FAKE_BUT_SECRET_LOOKING_VALUE_12345`. Then ran the exact ordinary workflow the `GHSA-hmq2-w58f-27jc` advisory's own PoC also uses: ```python repo = Repo.clone_from(evil_parent_url, victim_clone_dir) repo.submodule_update(init=True, recursive=True) ``` Observed result (`ls .git/modules/`): ``` .git/modules/AKIA_TOTALLY_FAKE_BUT_SECRET_LOOKING_VALUE_12345/ (a full git repo: HEAD, config, objects/, refs/, hooks/) ``` The literal `$GPY_SECRET_MARKER`-named directory was never created — only the expanded one exists, proving expansion happened before the value reached the `git clone --separate-git-dir=...` subprocess argument. The persistent, expanded-name directory under `.git/modules/` is itself the primary disclosure channel — it survives on disk for as long as the clone does. A second, independent channel: with the `git.cmd`/`git.repo.base` loggers at DEBUG level (a common CI/verbose-logging setting, not requiring any special flag), GitPython's own `Popen(...)` debug log line prints the fully expanded `--separate-git-dir=<value>` subprocess argument verbatim — `remove_password_if_present()` only redacts URL passwords/Authorization headers, not path-style arguments like this one, so the expanded value is not scrubbed. (The call also fails downstream with an `OSError`/`FileNotFoundError` referencing the original, unexpanded name, since `_write_git_file_and_module_config()` looks for a directory using the pre-expansion name — but that error message itself does not contain the expanded/secret value.) **Control**: the same call with an ordinary (no `$`) submodule name succeeds normally and creates `.git/modules/<name>` as expected — isolating the gap to submodule names containing environment-variable syntax specifically. ### Already-known check Checked all 35 published `gitpython-developers/GitPython` security advisories in full text. `GHSA-hmq2-w58f-27jc` covers the same `name` → `.git/modules/<name>` path but for path-traversal (`../../..`), not variable expansion. `GHSA-59cr-6r3x-644w` covers the sibling `path` field reaching unguarded `os.makedirs()`. `GHSA-94p4-4cq8-9g67`/`GHSA-rwj8-pgh3-r573` cover `expand_vars`, but only on URL arguments. `git log -p -S "separate_git_dir" -- git/repo/base.py` shows neither fix commit touches this line; it is unchanged since its 2016 introduction (commit `57d05379`). The closest topical match is `GHSA-8mcc-hrx5-hvxc` ("`clone_from()`/`clone()` omit `--separate-git-dir` from `unsafe_git_clone_options`") — already fixed, and its own text discusses this same `_clone()` handling of `separate_git_dir`. That fix only added `--separate-git-dir` to the `unsafe_git_clone_options` denylist, which gates the direct-kwarg vector; `Submodule._clone_repo()` explicitly sets `allow_unsafe_options=True` for the gitfile-submodules path, unconditionally bypassing that exact denylist (confirmed: the reproduction above succeeds against current `main`, which already has that fix). The bug this report describes is independent of that denylist — the *value* itself is expanded via `os.path.expandvars()` regardless of the denylist outcome — so the two are distinct, but flagging the relationship explicitly since a reviewer would otherwise reasonably ask about it. No advisory describes environment-variable expansion via the internal `separate_git_dir` path. ### Suggested fix Add `expand_vars=False` to the `Git.polish_url(sep_dir)` call in `_clone()`, matching the four other call sites already fixed for the same class of issue. ### Verification level Reproduced in a real environment: plain Linux, unmodified GitPython from current `main`, real git ≥ 1.7.5 behavior, an ordinary `git submodule add` + `clone` + `submodule_update(init=True, recursive=True)` workflow with no non-default configuration or monkeypatching required. Causation isolated via a control group (identical workflow, ordinary submodule name, no expansion). ### AI Use Disclosure AI (Claude) was used as a tool under my direction for code review, code summarization, and script development. A human reviewed and verified the AI-generated results before submission. Credit: please use "baeseungwon1010" (github.com/baeseungwon1010).