GitPython Remote.pull() refspec bypasses the --upload-pack unsafe-option guard, enabling arbitrary command execution

## Summary `Remote.pull()` in GitPython forwards its positional `refspec` argument directly to the `git pull` child process while its unsafe-option guard inspects only keyword arguments. An application that passes attacker-influenced data as the `refspec` (for example a user-supplied branch or refspec parameter in a web or CI service) can supply an option-shaped value such as `--upload-pack=<command>`. The value bypasses the `unsafe_git_pull_options` check, is placed after the `--` separator on the `git pull` command line, and is executed as the upload-pack program for local-path or SSH transports. This yields arbitrary command execution with the privileges of the calling process using the documented default arguments (`allow_unsafe_options=False`). ## Root Cause `Remote.pull()` is documented to reject dangerous options unless `allow_unsafe_options=True`, and `--upload-pack` is explicitly listed as such an option. The guard, however, only screens keyword arguments and never the positional `refspec`. The check builds its candidate list with `Git._option_candidates([], kwargs)`, passing an empty list in place of the positionals, so the `refspec` value is never compared against `unsafe_git_pull_options = ["--upload-pack"]`. The normalized `refspec` is then forwarded verbatim to the `git pull` invocation. The leading `--` separator that neutralizes an option-shaped refspec for `git fetch` and `git push` does not protect `git pull`, because `git pull` re-forwards its trailing positionals to an internal `git fetch` without re-inserting a `--` separator, so the positional becomes an option to that internal fetch. ```text Affected function: git.remote.Remote.pull Source: git/remote.py (guard at the check_unsafe_options call using _option_candidates([], kwargs); sink at the self.repo.git.pull("--", self, refspec, ...) invocation) ``` ## Impact An attacker who controls the `refspec` value passed to `Remote.pull()` achieves arbitrary operating-system command execution as the process invoking GitPython, with no additional privileges and no user interaction, on the default code path. Exploitation requires that the pulled remote uses a transport that runs upload-pack on the local side, namely a local filesystem path or SSH; over HTTPS the `--upload-pack` value is inert because the server ignores a client-supplied upload-pack program. This transport requirement narrows the population of vulnerable deployments but does not add work for the attacker once a vulnerable target is reached. Working checkouts commonly use local-path or SSH origins, so the exposed population is substantial. ## Proof of Concept ```text Environment: GitPython from the affected source tree; system git 2.43.0. The pulled origin uses a local filesystem transport (protocol.file.allow=always for git >= 2.38). ``` Reproduction steps: 1. Create an origin repository reachable over a local-path transport. 2. In a consumer repository, register that origin and call `Remote.pull()` with an attacker-controlled refspec. 3. Observe that the injected command executes. ```python import git repo = git.Repo("consumer") # has an origin using a local-path or ssh transport origin = repo.remote("origin") # Attacker-controlled refspec value; default arguments (allow_unsafe_options=False): origin.pull(refspec="--upload-pack=touch /tmp/pwned; git-upload-pack") ``` ```text cmdline: git pull -v -- origin --upload-pack=touch /tmp/pwned; git-upload-pack [attack] marker created (RCE) : True [isolate] upload_pack passed as KWARG blocked : True (only the positional refspec escapes) [sibling] same value via git fetch : safe (pull-specific behavior) [control] benign refspec : safe ``` ## Attack Chain 1. **Exposure:** An application calls `Remote.pull(refspec=<value>)` on the default path, where `<value>` is derived from attacker-influenced input such as a branch or refspec request parameter. 2. **Control:** The attacker supplies the string `--upload-pack=<command>; git-upload-pack`. The value passes through `Git._unpack_args`, which only flattens and stringifies it, preserving the option shape. 3. **Path:** The protocol loop calls `Git.check_unsafe_protocols` on the value, which only matches remote-helper transport URLs (a `<transport>::` form) and does not flag an `--upload-pack=` token. 4. **Guard:** The unsafe-option guard calls `check_unsafe_options(options=Git._option_candidates([], kwargs), ...)`. Because positionals are passed as an empty list, the positional `refspec` is never inspected against `unsafe_git_pull_options`. 5. **Primitive:** The value is forwarded to `git pull -v -- <remote> --upload-pack=<command>; git-upload-pack`; `git pull` re-forwards the positional to an internal `git fetch` without a `--` separator, so it is honored as the upload-pack program. 6. **Result:** For a local-path or SSH transport, git executes `<command>` as the upload-pack program, giving arbitrary command execution as the calling process user. ## Bypass Evidence The relevant guard is `Remote.pull()`'s own `check_unsafe_options` call, present in the current released artifact. It is incomplete: it inspects `Git._option_candidates([], kwargs)`, which only considers keyword arguments, so the positional `refspec` is never checked. This was confirmed empirically: passing the identical value as the `upload_pack=` keyword argument raises `UnsafeOptionError`, while passing it as the positional `refspec` does not, proving that only the positional escapes the guard. The `--` separator on the `git pull` command line was tested as a possible mitigation and refuted: with system git 2.43.0, `git pull -- <remote> '--upload-pack=<cmd>'` executed the injected command, while the identical value supplied to `git fetch -- ...` and `git push -- ...` did not execute, because those subcommands honor the `--` terminator whereas `git pull` re-forwards positionals to an internal fetch without one. The `check_unsafe_protocols` transport…