Network World, 1/27/97

Some of the services you may have on your Web or Internet server should be secured, as their existence and corresponding vulnerabilities are known to many hackers. These include FTP, Telnet, Simple Mail Transfer Protocol (SMTP), Network File System (NFS), Trivial FTP (TFTP) and Network Information System (NIS). Here are some tips for dealing with each.

FTP should be configured to prevent remote access to all files on the server. Proper configuration should only allow access to a predetermined file structure. For example, improperly configured FTP can allow for access to the system password file, which a hacker can then use to grant any number of access privileges he desires.

Telnet on secure sites should be configured to filter environment variables, and to prevent hackers from determining the operating system, hardware platform, and access control mechanisms being used.

SMTP with its sendmail program capability is a serious vulnerability if not properly secured. These vulnerabilities can result in the SMTP server interpreting the body of a mail message as an executable shell script. A hacker can then program the server to send him the password file. So remove the prog mailer, disable the EXPN, VRFY, DEBUG, and WIZ commands (if applicable), and update the server with current security patches and versions of sendmail.

NFS, if not properly configured, can result in unauthorized viewing or modification of files on the exported volume of a host server. The system administrator, security manager, and IS auditor should coordinate policy with regards to this service.

TFTP service should be enhanced with a reliable authentication mechanism. Otherwise, unauthorized access to files, including system files, is possible. TFTP should be run in a secure mode.

NIS should be avoided as much as possible. Improperly configured, this service can let a hacker steal password files to a complete NIS domain, thus enabling him to gain access to all hosts in that domain.