Copier _subdirectory allows template root escape via parent-directory traversal
Summary
Copier's _subdirectory setting is documented as the subdirectory to use as the template root. However, the current implementation accepts parent-directory traversal such as .. and uses it directly when selecting the template root.
As a result, a template can escape its own directory and make Copier render files from the parent directory without --UNSAFE.
Details
The relevant code path is:
1. the template defines _subdirectory
2. Copier renders that string
3. template_copy_root returns self.template.local_abspath / subdir
4. Copier walks that directory as the template root
Relevant code:
- <https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/copier/_main.py#L1056-L1062>
- <https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/copier/_template.py#L504-L513>
The effective sink is:
```python
subdir = self._render_string(self.template.subdirectory) or ""
return self.template.local_abspath / subdir
```
There is no check that the resulting path stays inside the template directory.
The documentation for _subdirectory describes it as:
> Subdirectory to use as the template root when generating a project.
and explains it as a way to separate template metadata from template source code:
<https://github.com/copier-org/copier/blob/7aa7021bd73797c982492bac3535515d4484fdb7/docs/configuring.md#L1582-L1646>
That description fits values like template or poetry, but not ...
PoC
PoC 1: _subdirectory: .. escapes to the parent directory
```sh
mkdir -p root/template dst
echo 'loot' > root/loot.txt
printf '%s\n' '_subdirectory: ..' > root/template/copier.yml
copier copy --overwrite root/template dst
find dst -maxdepth 3 -type f | sort
cat dst/loot.txt
```
Expected output includes:
```text
dst/loot.txt
dst/template/copier.yml
loot
```
This shows Copier is rendering from root/ rather than from root/template/.
Impact
If a user runs Copier on an untrusted template, that template can change the effective template root and make Copier render files from outside the intended template directory.
Practical impact:
- template-root escape via
.. - rendering of parent-directory files that were not meant to be part of the template
- possible without
--UNSAFE
