Security Boundary

Full Title or Meme

An imaginary definable perimeter encompassing all the critical functions in an INFOSEC product and separating them from all other functions within the product.

Context

The Ideal

In an ideal world there will be computer systems that have protected data and other computer systems that have unprotected data, perhaps like an onion there will be several gradations of protections, each with their own access controls. All of the difficulties relate to the ways that data and control moves from one protection level to another. This problem was understood and analyzed at an early date in the US DoD. Even then it became clear that the flow of data and the flow of control resulted in contradictory requirements as shown in the two models described below.

• BIBA
• Bell LaPadula

The Real

Since the very first Trusted Execution Environment in 1961 there has been the desire to place only a part of the Resources of an enterprise in a location that is secure from attack.

Problems

All security boundaries have gateways that allow access to the protected Resources. Determination of what accesses are permitted across a boundary is never more than an approximation of an impenetrable boundary.

My Commentary

Security boundaries seem to be mixing two security features which basically describes why cross-site scripting attacks are so common. The basic rule is this - Don't mix control messages with data messages. The two rules are:

1. Bell–LaPadula model focuses on data confidentiality (privacy - hashtag#MAC)
2. Biba Integrity Model is for the protection of data integrity (control - hashtag#MIC)

Applying both rules means that no security boundary should be crossed without clear permission to do so. And a security policy that allows data flow (MAC) should not allow control flow (MIC).

So, security boundaries need to distinguish between the two. As a side note Microsoft Vista hashtag#UAC (user access control) is not a boundary of any sort, it is a UX that applies policy to override MAC. (I wrote the spec.)

It is the rainbow books that described MAC & MIC rules.

References

One interesting note about the Glossary term referenced at NIST. It actually comes from the Committee on National Security Systems - Instructions which is on a site with a DoD level 3 certificate, which is not trusted by any of the major browser vendors.