Monday, April 24, 2017

A Tale of Two Viewpoints: Mixed Trust vs Shared Infrastructure

In NERC CIP Standards Drafting efforts, industry chatter, and auditing, there has been quite a bit of talk about “mixed trust”; meaning an environment that has both BES Cyber Systems and Cyber Assets not subject to CIP standards. Non-CIP Cyber Assets may be systems that are under the Responsible Entity’s control, and may even be providing functions related to Grid Operation, but not functions which “if rendered unavailable, degraded, or misused for 15 minutes will affect the reliability of the Bulk Electric System. For example, corporate business systems are not BES Cyber Assets, even though they are under the control of the same Responsible Entity as the BES Cyber Systems.

Here’s the thing; “mixed trust” is not the right term. Mixed trust would imply that Cyber Assets of different trust levels can access each other in an uncontrolled fashion. Nobody is proposing a relaxation of security controls between CIP and non-CIP assets. What is being proposed is “shared infrastructure”. At some level we all have shared infrastructure- the same building, the same power, the same Internet connection. Shared Infrastructure doesn’t mean “mixed trust”. Shared Infrastructure can have logical controls and isolation involved.

A few days back, I wrote about Streetlight Effect in relation to Lew Folkerth's "Lighthouse" article in the March-April issue of Reliability First's newsletter. In it he talks about “Zones of Authority” in relation to the audit process for NERC compliance. At the end of the article he makes an assertion about Virtualization being a bad idea not because of actual security concerns, but because of the auditor’s inability to look at things outside of the designated Electronic Security Perimeter (compliance concerns) required for BES Cyber Assets. While I respect Lew's experience and appreciate the viewpoint, I disagreed with that approach pretty strenuously.

Tom Alrich has another perspective on the article. He makes a point about security being enhanced if the RE’s entire network were in scope for NERC CIP Compliance audits. I don’t think I can agree with that either (although he does acknowledge that the average compliance specialist would rather repeatedly hit themselves in the head with a hammer than take this approach because of the burden of paperwork involved, so it doesn’t appear to be a serious suggestion.) Such an approach would only be a net gain if compliance evidence production didn’t overwhelm the efforts to secure things in the first place. And if all of the compliance requirements were strictly security requirements and not just designed to make audits easier.

But realistically, bringing security mechanisms that exist outside the ESP (Electronic Security Perimeter) into scope of auditing doesn’t require making the entire corporate network subject to inspection by the Regions. Here's the problem with CIP-005 and ESP. Most NERC CIP standards require you to have a program or process to accomplish X. CIP-005, on the other hand, requires everything peripherally related to BES Cyber Systems to reside inside the ESP. What we have here is the "hard crunchy shell" concept from 20 years ago. It provides “bright line” criteria for audit and categorizing assets as either in-scope or out, because the auditor can require a diagram with the asset shown inside a neatly drawn “dotted red line” (inside joke for NERC CIP compliance specialists), but it doesn’t provide good security on its own.

Some of this difficulty is definitional. A PCA "protected cyber asset" is any cyber asset "associated" with a BES Cyber System and can exist "in or on" an ESP, but this is only made specific in the NERC Glossary.  A BES Cyber Asset must be contained in an ESP (CIP-005). An Intermediate System must reside outside the ESP. An EACMS can be in or out.

Documenting a mechanism that provides a security function doesn’t require that everything else on that network be in scope for auditing. Authentication, Authorization, and Accounting (AAA) may be partly provided by an RSA Token system. That AAA system could reside inside or outside of the security zone where clients of the system exist. The documented controls for CIP AAA could bring that RSA server into scope without touching other, unrelated non-CIP servers on its network segment.

The key to providing good security for BES Cyber Assets doesn’t rely solely upon whether a device is inside a particular perimeter. It depends upon the controls applied to accessing that device. A layered defense of the device includes boundary identification & control, but it doesn’t stop at the outer boundary.

Let's address a couple specific points:

"Lew says (or implies) that auditors are having differences of opinion with entities on the security of mixed-trust switches. It seems these entities have switches that implement both ESP and non-ESP VLANs. When the auditors tell them this isn’t secure, the entities point out that the non-ESP VLANs have just as good security as the ESPs do. So why aren’t they safe?"

Let's examine a scenario with two VLANs, one named "CIP" and one named "non-CIP". The main point really isn't that the security of "non-CIP" is as good as "CIP". The most important thing is that network isolation is maintained between them. The configuration of devices in one VLAN are irrelevant to devices in the other VLAN. If it's just a Layer 2 VLAN, there is no "mixed trust" because the traffic from "non-CIP" doesn't mix with that of "CIP". If it's a Layer 3 switch, and routing takes place between the 2 VLANs, then an EAP must be identified (probably the VLAN's logical or virtual interface) and inbound/outbound access controls applied at that point. If it is a Hypervisor situation, then the same principle applies. vSwitch "non-CIP" doesn't have a traffic path that includes vSwitch "CIP" and the Guests cannot communicate with each other. If you create an EAP between them, then you apply the inbound and outbound access controls at that point.

And lest we forget, L2 VLAN is not the only way to have a shared switch infrastructure. Software Defined Networks and Network Overlays exist on a shared hardware infrastructure, but provide network isolation between the security zones as defined in configuration. This isolation, like that of the VLAN example above, is provided as a baseline function of the device, implemented in the control plane by code base. Access to modify configuration and code base is confined to a management plane interface. Traffic transiting the device doesn't have access to this function, only administrators accessing the device in the management plane do.

"Anything else, such as a VLAN that isn’t an ESP, is completely out of his or her purview; the auditor just has to assume these are completely insecure, and thus shouldn’t be found on the same switch as ESP VLANs."

Auditors aren't forced to assume anything.  The configuration of the switch is in scope, because it provides isolation to the "CIP" VLAN. (It doesn't provide ESP under the current model, because it doesn't provide EAP with in-bound/out-bound ACLS. It simply provides isolation, which is one flaw in CIP-005's prescriptive approach. CIP-005 would benefit from being changed to a security objective of isolating more critical assets and traffic from less critical.)The configuration of devices in the "non-CIP" VLAN are out of scope. If something is out of scope, they are not required to render an opinion on it.

"the auditors can’t look at what isn’t in an ESP, which limits the kinds of evidence an entity can show them. "

Currently, auditors can examine EACMS that may exist outside the ESP. They can examine Intermediate Systems which are required to be outside the ESP. I don't think this is a valid assertion. It's an auditing approach preference with deliberate blinders on.

"most entities, if told they could force CIP auditors to consider security controls they have implemented for non-ESP networks, but only in return for having at least some of the cyber assets on those non-ESP networks fall into scope for CIP, would say “Thanks but no thanks. We’ll leave things as they are.”

Perhaps the entity might choose that. But the issue isn't "security controls implemented for non-ESP networks." It is "security controls that exist outside the ESP, that are implemented to protect CIP Assets". ESP is a fundamentally limited construct, and must be considered fatally flawed when used without other other layered defenses.

1 comment:

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