Take 3, talking authZ and TOCTOU with Gunnar

Here is part 3 of a conversation with Gunnar Peterson where we continue talking about externalized authorization, who in the organization is involved in an XACML system deployment – and it even includes a discussion of TOCTOU concerns as it relates to a XACML system. Thanks also to my colleagues, David Brossard and Pablo Giambiagi, for their input. You can also find part 1 and part 2 of the conversation on this blog.

GP: In our last conversation you mentioned “As with other access policies, administrators will work with application and security experts to construct XACML policies. Administrators learn this skill fairly quickly, but they do need some training and guidance at first.”

In your experience who authors these policies today? And where should this responsibility sit in the organization going forward? It seems there is a mix of application domain and security experience required, plus there may be some need to understand business processes and architecture. Is there a new organizational role, Security Policy manager emerging?

GG: I like the sound of Security Policy Manager, and it is a role that could appear over time. For now, we see security administrators working with a business analyst and/or application developer to construct the policies. This amounts to determining what resources/actions need to be protected, translating the business policy (doctors can only view records of patients they have a care relation with), determining what attributes are needed, sourcing the necessary attributes, etc.

GP: In looking at Infosec as a whole, it seems to me that we have two working mechanisms – Access Control and Crypto. Everything else is integration. What guide posts should people use to identify and locate the integration points for their authorization logic? Do you focus on the resource object, the user, or the Use Case context? Or is it a mix?

GG: Policies can focus on the resource, subject, action or environmental attributes – that is the beauty of an attribute based access control language like XACML. That also means there are many ways to model policies, here are some general guidelines:

- Make sure you start with the plain old english language business rules and policies

- Take a “divide and conquer” approach to building the policy structure. This could mean writing policies for different components of the application that can be viewed as logically separate. What you’re also doing here is taking into account the natural activity flows when people use an application or the types of requests that will come into the PDP. Upon analysis of the scenarios, you can place infrequently used policies in branches where they won’t impact performance of more frequently used policies.

- Evaluate frequently used policies first – seems intuitive but may not be apparent at first. Therefore, you need to continually evaluate how the system is being used to see if modifications to the policy structure are needed. As mentioned in the previous point, this will allow you to identify policies that are infrequently evaluated so you can ensure they are not in the path of frequently evaluated policies.

- Consider the sources of attributes that you will utilize in policies. Are the attributes readily available or spread across multiple repositories – great place to use a virtual directory if answer is yes.

It is only after analyzing the scenario that you can say whether policies will be targeted to users, resources or some mix of the two. This is an area where we do provide guidance to customers at the start of a deployment. However, we do find that customers are able to manage this aspect of the deployment pretty quickly. Another point to keep in mind is that access policies don’t change that frequently once an application is on-boarded into the system. What happens on a daily basis is the administration of attributes on the individual users that will be accessing the protected applications.

GP:The Time to Check Time of Use problem has been around for as long as distributed computing. The time between when tickets, tokens, assertions, claims, and/or cookies are created until the time(s) when they are used by the resource’s access control layer, gives an attacker a number of places to hide. Choosing the place to deal with this problem and crafting policies is an important engineering decision —  What new options and patterns does XACML bring to the table to help architects and developers deal with this problem?

You talked previously about the XACML Anywhere Architecture where a callback from a Cloud Provider queries PDP; this would enable the Cloud Provider to get the freshest attributes at runtime while still allowing the Cloud Consumer to benefit from the Cloud provider’s deployment. The callback idea has appeal to a wide variety of use cases, but do people need to update their attribute schemas with any additional data to mark the freshness of the attributes so the PEP/PDP can decide when or if to fire off these requests? Does the backed PDP have any responsibility to mark attributes? What are the emerging consensus for core patterns here if any?

GG: You are right to point out that having the ability to call an externalized authorization service provides a mechanism for the application to have the most current attributes and access decisions. However, there are a couple of places in the XACML architecture to consider as it relates to TOCTOU – freshness of attributes and caching of decisions.

Attributes: Fresh or Stale

First we’ll look at the freshness of attributes that you describe, because it is the attribute values that will be processed in the policy evaluation engine (PDP). The XACML 3.0 specification is clear about what an attribute is. In terms of XML schema, it is defined as follows:

<xs:element name=”AttributeDesignator” type=”xacml:AttributeDesignatorType” substitutionGroup=”xacml:Expression”/>

<xs:complexType name=”AttributeDesignatorType”> <xs:complexContent> <xs:extension base=”xacml:ExpressionType”>

<xs:attribute name=”Category” type=”xs:anyURI” use=”required”/> <xs:attribute name=”AttributeId” type=”xs:anyURI” use=”required”/>

<xs:attribute name=”DataType” type=”xs:anyURI” use=”required”/> <xs:attribute name=”Issuer” type=”xs:string” use=”optional”/>

<xs:attribute name=”MustBePresent” type=”xs:boolean” use=”required”/> </xs:extension> </xs:complexContent></xs:complexType>

There is no room for a timestamp or a freshness attribute in an attribute designator used in a XACML policy (set) which would indicate under which temporal terms a policy is relevant based on the freshness of incoming attributes.

Accordingly the incoming XACML request is made up of attributes and attribute values defined in the schema as follows:

<xs:element name=”Attribute” type=”xacml:AttributeType”/><xs:complexType name=”AttributeType”> <xs:sequence>

<xs:element ref=”xacml:AttributeValue” maxOccurs=”unbounded”/> </xs:sequence>

<xs:attribute name=”AttributeId” type=”xs:anyURI” use=”required”/>

<xs:attribute name=”Issuer” type=”xs:string” use=”optional”/>

<xs:attribute name=”IncludeInResult” type=”xs:boolean” use=”required”/>

</xs:complexType>

Again, there is no use or mention of a freshness / time attribute. This is coherent with the fact that the PDP is a stateless component in the architecture. It means that unless it is told so, it cannot know the freshness, the time of creation, or the expiry date of an attribute value.  It knows when it receives a request from a PEP or when it calls out to PIPs and can assume a date of retrieval but even so the value could be skewed by potential caching mechanisms. Even though it does know the time at which it receives a request from the PEP or the time at which it invokes the PIP(s), it does not mean it will be using those timestamps for access control unless, of course, the policy author explicitly asks to do so.

Let’s imagine the case of a role attribute. Let’s imagine we want to write a policy which says: grant access to data if

• the user is a manager and

• if the role attribute is no more than 5 minutes old (5 minutes being the maximum freshness time)

This uses 2 attributes: role and freshness. The incoming request says “joe access data” which is not enough for the PDP to reach a conclusion. The PDP queries an LDAP via a PIP connector to retrieve the roles for Joe: “give me Joe’s roles”. Joe may have multiple roles; some of which may have been cached in the PIP attribute cache. In addition, each role has a freshness value (which could be the time at which the LDAP was queried for the value or the number of minutes since it was last queried for the given attribute). In that case, we may have different freshness values. Which one should be used? This shows there is a ternary relationship between the user, the role, and the freshness of the role. The simplest way for XACML to handle this today is to encode the freshness and the role value into a single attribute value e.g. manager::0h04mn37s. In that case the policy must match the attribute value using regular expressions (or XPath) to extract the role value on one hand and the freshness value on the other.

Is TOCTOU masquerading as a governance issue?

Another aspect of attribute freshness is the administrative or governance processes behind them. If attributes are not fresh, isn’t this a governance failure? For example, how frequently does your provisioning process make updates? Once a day, once a week, every hour, or is it event driven and updates continuously? Your answer will provide additional input on how to manage the consumption of attributes by authorization systems. So this problem goes even beyond the Time to Check to the Time of Administration.

Decision Cache
Next up, PEP decision caching is another area to examine to stay clear of TOCTOU issues. For performance reasons, PEPs can cache decisions for reuse so they don’t have to call the PDP for every access request. Here, your TTL setting is the window of opportunity when the PEP could use an invalid decision if a privilege-granting attribute has changed.

In short, XACML is an attribute-based language and helps you express restrictions / conditions some of which can be freshness of attributes. It is easier in that sense than previous AC models which did not allow for that. However, there has yet to be a standardized effort around the overall freshness challenge. And it is no longer a problem constrained to policy modelling. Attribute retrieval strategies, PEP and PIP implementations, caching, and performance all impact freshness (or the impression of freshness).

GP: Thanks very much to Gerry Gebel, David Brossard and Pablo Giambiagi for the conversation on these important topics

Explore posts in the same categories: Architecture, Authorization, Standards, XACML

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