The local file manager joined attacker-controlled path segments into the
uploads storage path without ensuring the result stayed under the uploads
root. Any authenticated user could read arbitrary files readable by the
Planka process (e.g. /etc/passwd, /app/.env) via `../` sequences on the
/user-avatars/*, /background-images/* and /favicons/* routes (CWE-22).
Resolve the path and enforce it stays within uploadsBasePath, centrally in
buildPath so every local file operation is covered. Additionally resolve
symlinks in read() and re-check containment, so a symlink inside the root
cannot be used to escape it.
Reported by Alpesh (github.com/Alpastx).
Daniel Hiller