A system should ensure that multiple conditions are met before granting permissions to an object. Checking access on only one condition may not be adequate for strong security. If an attacker is able to obtain one privilege but not a second, he or she may not be able to launch a successful attack. If a software system largely consists of one component, the idea of having multiple checks to access different components cannot be implemented. Compartmentalizing software into separate components that require multiple checks for access can inhibit an attack or potentially prevent an attacker from taking over an entire system.
Detailed Description Excerpts
According to Saltzer and Schroeder [Saltzer 75] in "Basic Principles of Information Protection" on page 9:
Separation of privilege: Where feasible, a protection mechanism that requires two keys to unlock it is more robust and flexible than one that allows access to the presenter of only a single key. The relevance of this observation to computer systems was pointed out by R. Needham in 1973. The reason is that, once the mechanism is locked, the two keys can be physically separated and distinct programs, organizations, or individuals made responsible for them. From then on, no single accident, deception, or breach of trust is sufficient to compromise the protected information. This principle is often used in bank safe-deposit boxes. It is also at work in the defense system that fires a nuclear weapon only if two different people both give the correct command. In a computer system, separated keys apply to any situation in which two or more conditions must be met before access should be permitted. For example, systems providing user-extendible protected data types usually depend on separation of privilege for their implementation.
According to Bishop [Bishop 03] in Chapter 13, "Design Principles," in the "Principle of Separation of Privilege" section on pages 347-348:
This principle is restrictive because it limits access to system entities.
Definition 13-6, The principle of separation of privilege states that a system should not grant permission based upon a single condition.
This principle is equivalent to the separation of duty principle discussed in Section 6.1 [of Computer Security]. Company checks for over $75,000 must be signed by two officers of the company. If either does not sign, the check is not valid. The two conditions are the signatures of both officers.
Similarly, systems and programs granting access to resources should do so when more than one condition is met. This provides a fine grained control over the resource, and additional assurance that the access is authorized.
Example 1. On Berkeley-based versions of the UNIX operating system, users are not allowed to change from their account to the root account unless two conditions are met. The first is that the user knows the root password. The second is that the user is in the wheel group (the group with GID 0). Meeting either condition is not sufficient to acquire root access. Meeting both conditions is required.
Separation of privilege is defined differently by Howard and LeBlanc [Howard 02]. We include their definition to show the importance of having multiple processes working together with different levels of privileges. This excerpt is from Chapter 3, "Security Principles to Live By," in the "Separation of Privilege" section on pages 61-62:
An issue related to using least privilege is support for separation of privilege. This means removing high privilege operations to another process and running that process with the higher privileges required to perform its tasks. Day-to-day interfaces are executed in a lower privileged process.
In June 2002, a severe exploit in OpenSSH v2.3.1 and v3.3, which ships with versions of Apple Mac OS X, FreeBSD and OpenBSD, was mitigated in v3.3 because it supports separation of privilege by default. The code that contained the vulnerability ran with lower capabilities because the UsePrlvilegeSeparation option was set in sshd_config. You can read about the issue at http://www.openssh.com/txt/preauth.adv.
Another example or privilege separation is Microsoft Internet Information Services (lIS) 6, which ships in Windows .NET Server. Unlike lIS 5, it does not execute user code in elevated privileges by default. All user mode HTTP requests are handled by external worker processes (named w3wp.exe) that run under the Network Service account, not under the more privileged Local System account. However, the administration and process management process, inetinfo.exe, which has no direct interface to HTTP requests, runs as Local System.
The Apache Web Server is another example. When it starts up, it starts the main Web server process, httpd, as root and then spawns new httpd processes that run as the low privilege nobody account to handle the Web requests.
Bishop, Matt. Computer Security: Art and Science. Boston, MA: Addison-Wesley, 2003.
Howard, Michael & LeBlanc, David. Writing Secure Code, Second Edition. Redmond, WA: Microsoft Press, 2002.
McGraw, Gary & Viega, John. "Divide and Conquer." Software Development. CMP Media LLC, April, 2003.
NIST. Engineering Principles for Information Technology Security. Special Publication 800-27. US Department of Commerce, National Institute of Standards and Technology, 2001.
Saltzer, Jerome H. & Schroeder, Michael D. "The Protection of Information in Computer Systems," 1278-1308. Proceedings of the IEEE 63, 9 (September 1975).
Viega, John & McGraw, Gary. Building Secure Software: How to Avoid Security Problems the Right Way. Boston, MA: Addison-Wesley, 2002.
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