In SolarWinds Serv-U before 15.2.2 Hotfix 1, there is a directory containing user profile files (that include users' password hashes) that is world readable and writable. An unprivileged Windows user (having access to the server's filesystem) can add an FTP user by copying a valid profile file to this directory. For example, if this profile sets up a user with a C:\ home directory, then the attacker obtains access to read or replace arbitrary files with LocalSystem privileges.
SolarWinds Serv-U MFT before 15.1.6 HFv1 assigns authenticated users a low-entropy session token that can be included in requests to the application as a URL parameter in lieu of a session cookie. This session token's value can be brute-forced by an attacker to obtain the corresponding session cookie and hijack the user's session.