link_to_local_path in ebooks/conversion/plugins/html_input.py in calibre before 6.19.0 can, by default, add resources outside of the document root.
https://lists.debian.org/debian-lts-announce/2024/09/msg00005.html
https://github.com/kovidgoyal/calibre/compare/v6.18.1...v6.19.0