Monthly Archives: April 2007


For a over decade there has been a slow move towards building networks where only authenticated users can get on (wireless networks are a great example of this), this later moved towards building networks where onlyauthenticated users on managed machines get on, the most recent change to this trend has been to add checks that also require these machines meet a base-line configuration that gives the network administrators some idea of what kind of risks they are exposed to by letting these machines on their networks.

This all makes sense, after all machines that do not have up-to-date antivirus or the latest patches certainly represent more risk to the network than those that do.  A great example of this is actually SQL Slammer, even today five years after the patch for this vulnerability it is still negatively effecting customer environments.

Analysts have given solutions in this space the label of Network Admissions Control (NAC), these systems rely on two basic models one where the behavior of the host is externally monitored and its access is limited if it behaviors out of norm with the networks policy and another where the host makes claims about its configuration that is then evaluated to decide of the host is conformant with the networks policy to see if access should be limited.

Both of these models are examples of what I call “asking the drunk if he is drunk”,  I say this because in both cases the client can control if it displays a behavior (I used to work with a security consultant who would always say “give me your rules and I will comply with them”, this is a example of just that – warning pdf link) or produces a claim that will result in access being given or restricted. The core difference between the two approaches is how rich is the information that is available to make policy decisions on.

The problem with this is if we are to deploy solutions like this we must also accept that it is possible for malware (like root kits or other malicious host software) to change the behavior of the host so that it isn’t “caught” by these checks.

If thats really the case do these systems still have value? I would argue they do, you see they give systems administrators get something they have never really had before, insight into their networks risk profile (how many people on my network do not have this patch, are not running antivirus or have out of date signatures, etc.).

There are other values too, in general users actually “want to do the right thing™” but they don’t necessarily know what that means and these systems help them do just that, in addition these systems often implement concepts of “automatic remediation” so that you can remove the obligation of the user to do anything and instead you just fix it for the user without their interaction.

These reasons underscore why even in computing it’s important to “pull the drunk over and ask him to walk the line”, but does this mean we should accept the status quo? I would say no, although it will never be possible for a running system to evaluate its own state in without potentially being fooled there are lots of things we can do to make the host itself more trustworthy.

As a example many of the technologies in VISTA do just that (and I thinkSymantec would agree), some of the technologies in VISTA that help here today include:

  • Bitlocker – Protects the host from offline attacks of trusted software components and cryptographic keys.
  • Secure Startup – Increases the trustworthiness of the OS by checking to see that core components are authentic as part of the startup sequence.
  • Code Integrity – Increases the trustworthiness of the running state of the OS by checking to see if they are authentic before being loaded.
  • UAC – Reduces the negative impact that can be caused by careless users and/or malware by giving these processes a restricted token.
  • ASLR – Makes it more difficult for malware to attack running code by randomizing where code is running in memory.
  • DEP – Makes it more difficult for malicious code to get and keep running.

This represents just the beginning through, future versions of these conditional network access solutions will also leverage hardware that runs before the operating system (TPM) to verify that the components that implement the above are not tampered with before the operating system even is invoked.

Later systems will even be able to use virtual machines to add even more protection from these sorts of attacks, again that’s not to say that when we get here we will have made these systems fool proof; it’s just not possible(my friend Cem always has great analogies, one of my favorites is “Security is often a exercise of re-arranging deck chairs on a sinking ship” this problem is a example of just that) but that doesn’t mean there is not value in raising the bar and I think the industry is well on its way to do just that.

As a side note, we have deployed the Microsoft solution in this space (Network Access Protection) to over 75,000 hosts over the last few months pretty cool if you ask me.