Roundcube Webmail before 1.6.20 and 1.7.x before 1.7.5 is affected by a CSRF validation bypass in the mark-as-junk action of the markasjunk plugin. The plugin read the _uid parameter from POST data but passed it to rcmail::get_uids() without constraining the input mode, so values re-read inside that helper could be taken from the query string and the action carried out without a valid CSRF token. Upstream fixed this in the commit “Fix CSRF validation bypass in the mark-as-junk action”, which passes rcube_utils::INPUT_POST explicitly.