By design, the built-in FTP server for iSeries AS/400 systems does not support a restricted document root, which allows attackers to read or write arbitrary files, including sensitive QSYS databases, via a full pathname in a GET or PUT request.
https://exchange.xforce.ibmcloud.com/vulnerabilities/20260
http://www.venera.com/downloads/Canonicalization_problems_in_iSeries_FTP_security.pdf