In the IPv4 PASV path, the FTP Client accepts whatever address was sent in the servers 227 reply. Validation only covers the parse and the non-zero values, thus a malicious server can name any address and direct the Client there.
Weakness
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.