Physical Security Passwords UNIX Lines of Defense

Chapter 8: UNIX System Security

8.1 UNIX Lines of Defense

System security is an extremely important issue, especially today, when computer systems are networked and directly exposed to an unknown number of intruders. UNIX designers could not anticipate such extensive development of computer technologies, but they have paid significant attention to system security and have provided a decent level of basic system protection. Standard UNIX offered two basic ways to prevent security problems: Passwords were designed to prevent unauthorized users from obtaining access to the system at all. 1. File permissions were designed to allow access to the various commands, files, programs, and system resources only to designated individuals or groups of authorized users. 2. On a stand−alone system, which is isolated from the external world, this approach was sufficient. On a system connected to the network, however, which communicates with other known and unknown computer systems, everything is more complex and there are additional risks. For example, under some circumstances network access can bypass the regular password authentication procedures, so the system may be only as secure as the other trusted systems on the network. Passwords and file permissions are certainly useful and necessary, but they should be only a part of an overall security strategy for the system itself, based upon its needs and potential threats. Various lines of defense may be set to protect the system; each of them should be seriously considered, and most of them are relatively easy to implement. We will discuss the most common types of system defense. Although all of them are not exclusive to UNIX, they can certainly be used in UNIX systems. Some of them are part of the generic UNIX security and others are optional, but they are all widely implemented across all UNIX platforms. The UNIX security features we will discuss here are not perfect. There are third−party add−on security packages available on the market for sites that require a higher level of security, but they are out of the scope of this text.

8.1.1 Physical Security

The first line of defense is the physical access to the UNIX system the computer itself. From todays point of view, users do not need physical access to the system at all. They can use the system extensively without being physically near it. Visual contact between a user and the system is not a condition for successful communication however, this is not the rule for successful system administration. Some of the most common issues related to the physical security of the system are: Preventing theft and vandalism by locking the door or locking the equipment to a table or desk • Restricting access to the system console and computer itself. To prevent the system from crashing and rebooting to the single−user mode which is an unsecured system mode, lock the key in the secure key position if applicable and keep the key safe • 189 Restricting or monitoring access to other parts of the system terminals, modems, network facilities, and printers to prevent vandalism on these exposed parts which is a frequent problem • Restricting access to backup tapes, in particular, to protect system data •

8.1.2 Passwords

If an unauthorized individual gains physical access to the system, user authentication is the next line of defense; a password keeps the system closed off, preventing unauthorized users to access the systems files programs and data. One weakness of passwords is that if someone breaks into an account by finding out its password, the intruder has all the rights and privileges of the legitimate user. There are a variety of methods for adding additional stumbling blocks if a password is broken, such as: Secondary authentication programs, which require additional input before granting access to the system • Dialup passwords, which act as a second password when logging in via a modem • Enhanced network authentication systems like Kerberos designed to protect networked systems and fileservers; some of these systems are very complex to install and maintain • Additional authentication−based security identification devices tokens synchronized with the system • The system administrator must be sure that all available measures for system protection are implemented before the decision is made to upgrade a systems security. In doing this, special attention should be paid to the password−related files. It is crucial that each entry in these files includes an encrypted password or asterisk. Entries with empty password fields are extremely dangerous for the system and they represent large security holes in the systems defenses.

8.1.3 File Permissions