Copier safe template has filesystem write access outside destination path

Vulnerability Description

Copier library and CLI app for rendering project templates. From 7.1.0 to before 9.9.1, 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. This vulnerability is fixed in 9.9.1.

References

• https://github.com/copier-org/copier/security/advisories/GHSA-p7q8-grrj-3m8w
• https://github.com/copier-org/copier/commit/fdbc0167cc22780b497e4db176feaf6f024757d6