/var/lib/roundcube/plugins/ is in roundcube-plugins 1.3.6+dfsg.1-1.
This file is owned by root:root, with mode 0o755.
..
/var/lib/roundcube/plugins/acl
/var/lib/roundcube/plugins/additional_message_headers
/var/lib/roundcube/plugins/archive
/var/lib/roundcube/plugins/autologon
/var/lib/roundcube/plugins/database_attachments
/var/lib/roundcube/plugins/debug_logger
/var/lib/roundcube/plugins/emoticons
/var/lib/roundcube/plugins/enigma
/var/lib/roundcube/plugins/help
/var/lib/roundcube/plugins/hide_blockquote
/var/lib/roundcube/plugins/http_authentication
/var/lib/roundcube/plugins/managesieve
/var/lib/roundcube/plugins/markasjunk
/var/lib/roundcube/plugins/new_user_dialog
/var/lib/roundcube/plugins/new_user_identity
/var/lib/roundcube/plugins/newmail_notifier
/var/lib/roundcube/plugins/password
/var/lib/roundcube/plugins/show_additional_headers
/var/lib/roundcube/plugins/squirrelmail_usercopy
/var/lib/roundcube/plugins/subscriptions_option
/var/lib/roundcube/plugins/userinfo
/var/lib/roundcube/plugins/vcard_attachments
/var/lib/roundcube/plugins/virtuser_file
/var/lib/roundcube/plugins/virtuser_query
/var/lib/roundcube/plugins/zipdownload