Software Engineering Institute

RhinoSoft Serv-U is a shareware Windows FTP server that supports S/KEY one-time password (OTP) authentication using MD4 or MD5 hash algorithms. Cat Soft LLC is also involved in the development of Serv-U and is an affiliate of RhinoSoft. The Serv-U distribution includes an administration client that can be used to manage Serv-U servers remotely. Serv-U user accounts can be configured to use plaintext or S/KEY OTP authentication, and accounts can be granted several levels of administrative privilege on the server. When a user with administrative privileges attempts to log on to a Serv-U server using the remote administration client, and that user's account is configured on the server to use S/KEY OTP authentication, the server correctly sends an S/KEY OTP challenge, but the administration client ignores the challenge and sends the password entered by the user in plaintext. The server refuses the plaintext password and authentication fails, and the plaintext password is exposed on the network.

See RFC 1760 and RFC 2289 for more information on S/KEY and one-time password (OTP) authentication.

A properly located intruder using a sniffer can obtain administrative users' passwords. In addition, an administrative user account configured to use S/KEY OTP cannot log into a Serv-U server using a vulnerable remote administration client.

The CERT/CC is currently unaware of a practical solution to this problem.

IIIb. Workarounds

It may be possible to use other forms of encryption, such as a VPN, SSH, or IPsec, to secure a remote administration connection to a Serv-U server.