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Zero Trust Networks by Doug Barth, Evan Gilman

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Chapter 4. Making Authorization Decisions

Authorization is arguably the most important process occurring within a zero trust network, and as such, making an authorization decision should not be taken lightly. Every flow and/or request will ultimately require a decision be made.

The databases and supporting systems we will discuss here are the key systems that come together to make and affect those decisions. Together, they are authoritative for access control and thus need to be rigorously isolated. Careful distinction should be made between these responsibilities, particularly when deciding whether to collapse them into a single system, which should generally be avoided if possible.

The zero trust model is still very new, and this area is undergoing rapid evolution. Some of the content included in this chapter is considered state of the art at the time of this writing. Known implementations still vary wildly in their approaches, and most are not publicly available. That being said, the major components and responsibilities are understood.

Taking reality into account, this chapter will focus on high-level architectural arrangement of the components required to make zero trust authorization decisions, as well as how they fit together and enforce said decisions.

Authorization Architecture

The zero trust authorization architecture comprises four main components, as shown in Figure 4-1:

  • Enforcement
  • Policy engine
  • Trust engine
  • Data stores

These four components are distinct in their responsibilities, and as a result, we treat them as separate systems. From a security standpoint, it is highly desirable that these components be isolated from each other. These systems represent the practical crown jewels of the zero trust security model, so special care should be taken in their maintenance and security posture. Carefully evaluate any proposals that suggest collapsing these responsibilities into a single system.

Figure 4-1. Zero trust authorization systems

The enforcement component will exist in large numbers throughout the system and should be as close to the workload as possible. It is the one that actually affects the outcome of the authorization decision. It is typically manifested as a load balancer, proxy, or even a firewall. This component interacts with the policy engine, which is the piece that we use to make the actual decision. The enforcement component ensures that clients are authenticated, and passes the context of each flow/request to the policy engine. The policy engine compares the request and its context to policy, and informs the enforcer whether the request will be permitted or not.

The trust engine is leveraged by the policy engine for risk analysis purposes. It leverages multiple data sources in order to compute a risk score, similar to a credit score. This score can be used to protect against unknown unknowns, and helps keep policy strong and robust without complicating it with edge cases and signatures. It is used by the policy engine as an additional component by which an authorization decision can be made. Google’s BeyondCorp is widely recognized as having pioneered this technology.

Finally, we have the various data stores that represent the source of truth for the data being used to inform authorization. This data is used to paint a full contextual picture of a particular flow/request, using small authenticated bits of data as the primary lookup keys (i.e., a username or a device’s serial number). These data stores, be they user data, device data, or otherwise, are heavily leveraged by both the policy engine and trust engine, and represent the backing against which all decisions are measured.

Enforcement

The enforcement component (depicted in Figure 4-2) is a natural place to start. It sits on the “front line” of the authorization flow and is responsible for carrying out decisions made by the rest of the authorization system.

Figure 4-2. An agent receives a pre-authorization signal to grant access to a system using traditional enforcement mechanisms. These systems together form the enforcement component.

Enforcement can be broken down into two primary responsibilities. First, an interaction with the policy engine must occur. This is generally the authorization request itself (e.g., a load balancer has received a request and needs to know whether it is authorized or not). The second is the actual installation and ongoing enforcement of the decision. While these two responsibilities represent a single component in the zero trust authorization architecture, you can choose whether they are fulfilled together or separately.

The way you choose to handle this will likely depend on your use case. For instance, an identity-aware proxy can call the policy engine to actively authorize a request it has received, and in the same step use the response to either service or reject the request. This is an example of treating the concerns as unified. Alternatively, perhaps a pre-authorization daemon receives a request for access to a particular service, which then calls the policy engine for authorization. Upon successful authorization, the daemon can manipulate local firewall rules to allow the specific request. With this approach, we rely on “standard” enforcement mechanisms that are informed/programmed by the zero trust control plane. It should be noted, however, that this approach requires a client-side hook in order to notify the control plane of the authorization request. This may or may not be acceptable, depending on the level of control over your devices and applications.

Placement of the enforcement component is very important. Since it represents our control point within the data plane, we must ensure that enforcement components are placed as close to the endpoints as possible. Otherwise, trust can pool “behind” the enforcement component, undermining zero trust security. Luckily, the enforcement component can be modeled as a client of sorts and applied liberally throughout the system. This is in contrast to the rest of the authorization components, which are modeled as services.

Policy Engine

The policy engine is the component that has the power to make a decision. It compares the request coming from the enforcement component against policy in order to determine whether the request is authorized or not. Once determined, the result is returned to the enforcement piece for actual realization.

The arrangement of the enforcement layer and policy engine allows for dynamic, point-in-time decisions to be made, allowing revocation to occur rapidly. As such, it is important that these components be considered separately and independently. That is not to say, however, that they cannot be co-located.

Depending on a number of factors, a policy engine may be found hosted side by side with the enforcement mechanism. An example of this might be a load balancer that authorizes requests through inter-process communication (IPC) instead of a remote call. The most attractive benefit of this architecture is the lower latency to authorize the request. A low-latency authorization system enables fine-grained and comprehensive authorization of network activity; for example, individual HTTP requests could be authorized instead of the session-level authorization that commonly is deployed.

It should be noted that it is best to maintain process-level isolation between the policy engine and enforcement layer. The enforcement layer, being in the user’s data path, is more exposed; therefore, integrating the policy engine in the same process could expose it to unwanted risk. Deploying the policy engine as its own process goes a long way to ensure that bugs in the enforcement layer don’t result in a policy engine compromise.

What Ever Happened to RADIUS?

The relationship between the policy engine and the enforcement layer is a familiar one for most network engineers. In 1997, the IETF ratified a standard describing the RADIUS protocol, which provides authentication, authorization, and accounting for network services. RADIUS stands for Remote Authentication Dial-In User Service—the name alone shows its age. While the protocol itself is hopelessly insecure (it uses MD5 for authenticity assertions), it is specifically written for the task at hand. What would it look like to use RADIUS between the enforcement layer and the policy engine? RADIUS could be protected with other protocols discussed in this book, but that feels like a kludge. Perhaps there is an opportunity to create a RADIUS-like project, which takes into account the threat reality of today’s systems.

Policy Storage

The rules referenced by the policy engine need to be stored. These policy rules are ultimately loaded into the policy engine, but it is strongly recommended that the rules are captured outside of the policy engine itself. Storing the policy rules in a version control system is ideal and provides several benefits:

  • Changes to policy can be tracked over time.
  • Rationale for changing policy is tracked in the version control system.
  • The expected current policy state can be validated against the actual enforcement mechanisms.

Many of these benefits have historically been implemented using rigorous change management procedures. In that system, changes to the system’s configuration are requested and approved before ultimately being applied. The resulting change management log can be used to determine why the system is in the current state.

Moving policy definitions into version control is the logical conclusion of change management procedures when the system can be configured programmatically. Instead of relying on human system administrators to load desired policy into the system, we can instead capture the policy as data that a program can read and apply. In many ways, loading policy is then similar to deployable software. As a result, system administrators can use standard software development procedures (namely code review and promotion pipelines) to manage the changes in policy.

What Makes Good Policy?

Policy in a zero trust network is in some ways similar to traditional network security, and in other ways substantially different.

Zero Trust Policy Is Still Not Standardized

The reality today is that zero trust policy is still not standardized in the same way as a network-oriented policy. As a result, defining the standard policy language used in a zero trust network is a great opportunity.

Let’s look at what’s similar first. Good policy in a zero trust network is fine-grained. The level of granularity will vary based on the maturity of the network, but the desired goal is policy that is scoped to the individual resource being secured. This is not very different than a traditional network security model that aims to segment the network to decrease attack surface area.

The zero trust model starts to diverge from traditional network security in the control mechanisms that are used to define policy. Instead of defining policy in terms of network implementation details (IP addresses and ranges), policy is best defined in terms of logical components in the network. These components will generally consist of:

  • Network services
  • Device endpoint classes
  • User roles

Defining policy from logical components that exist in the network allows the policy engine to calculate the enforcement decisions based on its knowledge of the current state of the network. To put this in concrete terms, a web service running on one server today might be on a different server tomorrow, or might even move between servers automatically as directed by a workload scheduler. The policy that we define needs to be divorced from these implementation details to adapt to this reality. An example of this style of policy from the Kubernetes project is shown in Figure 4-3.

Figure 4-3. A snippet from a Kubernetes network policy. These policies use workload labels, computing the underlying IP-based enforcement rules when and where necessary.

Policy in a zero trust network also leans on trust scores to anticipate unknown attack vectors. By defining policy with a trust score component, administrators are able to mitigate risk that otherwise can’t be captured with a specific policy. Therefore, most policy should include a trust score component. We’ll talk more about the score component in the next section.

No Standard Exists

Currently,  mature zero trust networks implement their own policy language/format on a case-by-case basis, typically being developed fully in-house. Simpler zero trust networks may embed policy in an existing structure, such as in Figure 4-3. While the latter is generally acceptable, it is typically outgrown as the network evolves and adds features. The advantages of a standardized/interoperable policy language can be clearly seen. However, such work remains an open research question.

Policy should not rely on trust score alone. Specific characteristics of the request being authorized can also be part of the policy definition. An example of this might be certain user roles should only have access to a particular service.

Who Defines Policy?

Zero trust network policy should be fine-grained, which can place an extraordinary burden on system administrators to keep the policy up to date. To help spread the load of this configuration burden, most organizations decide to distribute policy definition across the teams so they can help maintain policy for the services they own.

Opening up policy definition to an entire organization can present certain risks, like well-meaning users who create overly broad policies, thereby increasing the attack surface area of the system they intended to constrain. Zero trust systems lean on two organizational workflows to counteract this exposure.

First, since policy is typically stored under version control, having another person review changes to the policy helps ensure that changes are well considered. Security teams can additionally review the changes and ask probing questions to ensure that the policy being defined is as tightly scoped as possible. Since the policy is defined using logical intent instead of physical components, the policy will change less rapidly than if it was defined in physical terms.

The second organizational measure used is to layer broad infrastructure policy on top of fine-grained policy. For example, an infrastructure group might rightly require that only a certain set of roles be allowed to accept traffic from the internet. The infrastructure team will therefore define policy that enforces that restriction, and no user-defined policy will be allowed to circumvent it. Enforcing this constraint could take several forms: an automated test of proposed policy, or perhaps a policy engine that will simply refuse overly broad policy assertions from untrusted sources. Such enforcement can also be useful for compliance and regulatory requirements.

Trust Engine

The trust engine is the system in a zero trust network that performs risk analysis against a particular request or action. This system’s responsibility is to produce a numeric assessment of the riskiness of allowing a particular request/action, which the policy engine uses to make an ultimate authorization decision.

The trust engine will frequently pull from data contained in authoritative inventory systems to check attributes of an entity when computing its score. A device inventory, for example, could provide the trust engine with information like the last time a device was audited, or whether it has a particular hardware security feature.

Creating a numeric assessment of risk is a difficult task. A simple approach would be to define a set of ad hoc rules that score an entity’s riskiness. For example, a device that is missing the latest software patches could have its score reduced. Similarly, a user who is continually failing to authenticate could have their trust score reduced.

While ad hoc trust scoring might be simple to get started with, a set of statically defined rules will be insufficient to meet the desired goal of defending against unexpected attacks. As a result, in addition to using static rules, mature trust engines use machine learning techniques to derive a scoring function.

Machine learning derives a scoring function by calculating observable facts from a subset of activity data known as training data. The training data is raw observations that have been associated with trusted or untrusted entities. From this data, features are extracted and used to derive a computer-generated scoring function. This scoring function, a model in machine learning terms, is then run against a set of data that is in the same format as the training data. The resulting scores are compared against human-defined risk assessments, and the model’s quality can then be refined based on its ability to correctly predict risk of the data being analyzed. A model that has sufficient accuracy can then be said to be predictive of the riskiness of yet unseen requests in the network.

While machine learning is increasingly used to solve hard computational problems, it does not obviate the need for more explicit rules in the trust engine. Whether due to limitation of the derived scoring models or for desired customization of the scoring function, trust engines will typically use a mixture of ad hoc and machine learning scoring methods.

What Entities Are Scored?

Deciding which components of a zero trust network should be scored is an interesting consideration. Should scores be calculated for each individual entity (user, device, and application), for the network agent as a whole, or for both? Let’s look at some scenarios.

Imagine a user’s credentials are being brute forced by a malicious third party. Some systems will mitigate this threat by locking the user’s account, which can present a denial-of-service attack against that particular user. If we were to score a user negatively based on that activity, a zero trust network would suffer the same problem. A better approach is to realize that we’re authenticating the network agent, and so the attacker’s network agent is counteracted, leaving the legitimate user’s network agent unharmed. This scenario makes a case that the network agent is the entity that should be scored.

But just scoring the network agent can be insufficient against other attack vectors. Consider a device that has been associated with malicious activity. A user’s network agent on that device may be showing no signs of malicious behavior, but the fact that the agent is being formed with a suspected device should clearly have an impact on the trust score for all requests originating from that device. This scenario strongly suggests that the device should be scored.

Finally, consider a malicious human user (the infamous internal threat) is using multiple kiosk devices to exfiltrate trade secrets. We’d like the trust engine to recognize this behavior as the user hops across devices and to reflect the decreasing level of trust in their trust score for all future authorization decisions. Here again, we see that scoring the network agent alone is insufficient for mitigating common threats.

Taken as a whole, it seems like the right solution is to score both the network agent itself and the underlying entities that make up the agent. These scores can be exposed to the policy engine, which can choose the correct component(s) to authorize based on the policy being written.

Presenting so many scores for consideration when writing policy, however, can make the task of crafting policy more difficult and error prone. In an ideal world, a single score would be sufficient, but that approach presents extra availability requirements on the trust engine. A system that tries to create a single score would likely need to move to an online model, where the trust engine is interactively queried during the policy decision making. The engine would be given some context about the request being authorized so it could choose the best scoring function for that particular request. This design is clearly more complex to build and operate. Additionally, for policy where a system administrator specifically wishes to target a particular component (say, only allow deploys from devices with a score above X), it seems rather roundabout.

Exposing Scores Considered Risky

While the scores assigned to entities in a zero trust network are not considered confidential, exposing the scores to end users of the system should be avoided. Seeing one’s score could be a signal to would-be attackers that they are increasing or decreasing their trustworthiness in the system. This desire to withhold information should be balanced against the frustration of end users’ ability to understand how their actions are affecting their own trust in the system. A good compromise from the fraud industry is to show users their scores infrequently, and to highlight contributing factors to their score determination.

Data Stores

The data stores used to make authorization decisions are very simply the sources of truth for the current and past state of the system. Information from these data stores flows through the control plane systems, providing a large portion of the basis on which authorization decisions are made, as demonstrated in Figure 4-4.

We previously spoke about the trust engine leveraging these data stores in order to produce a trust score, which in turn is considered by the policy engine. In this way, information from control plane data stores has flowed through the authorization system, finally reaching the policy engine where the decision was made. These data stores are used by the policy engine, both directly and indirectly, but they can be useful to other systems that need authoritative data about the state of the network.

Figure 4-4. Authoritative data stores are used by the policy engine both directly and indirectly through the trust engine

Zero trust networks tend to have many data stores, organized by function. There are two primary types: inventory and historical. An inventory is a single consistent source of truth, recording the current state of the resource(s) it represents. An example is a user inventory that stores all user information, or a device inventory that records information about devices known to the company.

In an inventory, a primary key exists which uniquely represents the tracked entity. In the case of a user, the likely choice is the username; for a device, perhaps it’s a serial number. When a zero trust agent undergoes authentication, it is authenticating its identity against this primary key in the inventory. Think about it like this: a user authenticates against a given username. The policy engine gets to know the username, and that the user was successfully authenticated. The username is then used as the primary key for lookup against the user inventory. Keeping this flow and purpose in mind will help you choose the right primary keys, depending on your particular implementation and authentication choices.

A historical data store is a little bit different. Historical data stores are kept primarily for risk analysis purposes. They are useful for examining recent/past behavior and patterns in order to assess risk as it relates to a particular request or action. Trust engine components are most likely to be consuming this data, as trust/risk determinations are the engine’s primary responsibility.

One can imagine many types of historical data stores, and when it comes to risk analysis, the sky’s the limit. Some common examples include user accounting records and sFlow data. Regardless of the data being stored, it must be queryable using the primary key from one of the inventory systems.

We will talk about various inventory and historical data stores as we introduce related concepts throughout this book.

Summary

This chapter focused on the systems that are responsible for making the ultimate decision of whether a particular request should be authorized in a zero trust network. This decision is a critical component of such a network, and therefore should be carefully designed and isolated to ensure it is trustworthy.

We broke this responsibility down into four key systems: enforcement, policy engine, trust engine, and data stores. These components are logical areas of responsibility. While they could be collapsed into fewer physical systems, the authors prefer an isolated design.

The enforcement system is responsible for ensuring that the policy engine’s authorization decision takes effect. This system, being in the data path of user traffic, is best implemented in a manner where the policy decision is referenced and then enforced. Depending on the architecture chosen, the policy engine might be notified before a request occurs, or during the processing of that same request.

The policy engine is the key system that computes the authorization decision based on data available to it and the policy definitions that have been crafted by the system administrators. This system should be heavily isolated. The policy that is defined should ideally be stored separately from the engine and should use good software development practices to ensure that changes are understood, reviewed, and not lost as the policy moves from being proposed to being implemented. Furthermore, since zero trust networks expect to have much finer-grained policy, mature organizations choose to distribute the responsibility of defining that policy into the organization with security teams reviewing the proposed changes.

The trust engine is a new concept in security systems. This engine is responsible for calculating a trust score of components of the system using static and inferred algorithms derived from past behavior. The trust score is a numerical determination of the trustworthiness of a component and allows the policy writers to focus on the level of trust required to access some resource instead of the particular details of what actions might reduce that trust.

The final component of this part of the system is the authoritative data sources that capture current and historical data that can be used to make the authorization decision. These data stores should focus on being sources of truth. The policy engine, the trust engine, and perhaps third-party systems can leverage this data so the collection of this data will have a decent return on investment from capturing it.

The next chapter will dig into how devices gain and maintain trust.

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