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:

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

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