The current implementation of xdg-user-dir is basically eval echo \${XDG_${1}_DIR:-$HOME}, thus is can be easily used to achieve command execution.
It can be used to break out from restricted environments by spawning an interactive system shell.
xdg-user-dir '}; /bin/sh #'If the binary is allowed to run as superuser by sudo, it does not drop the elevated privileges and may be used to access the file system, escalate or maintain privileged access.
sudo xdg-user-dir '}; /bin/sh #'