/usr/share/ is in squirrelmail-spam-buttons 2.3.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/squirrelmail/
/usr/share/squirrelmail/plugins/
/usr/share/squirrelmail/plugins/spam_buttons/
/usr/share/squirrelmail/plugins/spam_buttons/functions.php
/usr/share/squirrelmail/plugins/spam_buttons/index.php
/usr/share/squirrelmail/plugins/spam_buttons/templates/
/usr/share/squirrelmail/plugins/spam_buttons/templates/index.php
/usr/share/squirrelmail/plugins/spam_buttons/templates/default/
/usr/share/squirrelmail/plugins/spam_buttons/templates/default/index.php
/usr/share/squirrelmail/plugins/spam_buttons/templates/default/confirmation_note.tpl
/usr/share/squirrelmail/plugins/spam_buttons/templates/default/redirect_preview_pane.tpl
/usr/share/squirrelmail/plugins/spam_buttons/templates/default/read_menubar_buttons.tpl
/usr/share/squirrelmail/plugins/spam_buttons/templates/default/redirect_standard.tpl
/usr/share/squirrelmail/plugins/spam_buttons/options.php
/usr/share/squirrelmail/plugins/spam_buttons/compose_functions-1.4.10.php
/usr/share/squirrelmail/plugins/spam_buttons/compose_functions-1.5.2.php
/usr/share/squirrelmail/plugins/spam_buttons/bounce_send.php
/usr/share/squirrelmail/plugins/spam_buttons/report.php
/usr/share/squirrelmail/plugins/spam_buttons/compose_functions-1.4.14.php
/usr/share/squirrelmail/plugins/spam_buttons/configtest.php
/usr/share/squirrelmail/plugins/spam_buttons/setup.php
/usr/share/squirrelmail/plugins/spam_buttons/compose_functions-1.4.11.php
/usr/share/squirrelmail/plugins/spam_buttons/buttons.php
/usr/share/squirrelmail/plugins/spam_buttons/bounce_identity.php
/usr/share/doc/
/usr/share/doc/squirrelmail-spam-buttons/
/usr/share/doc/squirrelmail-spam-buttons/sa-postfix-en.html
/usr/share/doc/squirrelmail-spam-buttons/sa-wrapper.html
/usr/share/doc/squirrelmail-spam-buttons/copyright
/usr/share/doc/squirrelmail-spam-buttons/README.gz
/usr/share/doc/squirrelmail-spam-buttons/changelog.Debian.gz
/usr/share/squirrelmail/plugins/spam_buttons/config.php