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RFC 5209

Network Endpoint Assessment (NEA): Overview and Requirements

Pages: 53
Informational
Part 3 of 3 – Pages 34 to 53
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7. Requirements

This section describes the requirements that will be used by the NEA WG to assess and compare candidate protocols for PA, PB, and PT. These requirements frequently express features that a candidate protocol must be capable of offering so that a deployer can decide whether to make use of that feature. This section does not state requirements about what features of each protocol must be used during a deployment. For example, a requirement (MUST, SHOULD, or MAY) might exist for cryptographic security protections to be available from each protocol but this does not require that a deployer make use of all or even any of them should they deem their environment to offer other protections that are sufficient.

7.1. Common Protocol Requirements

The following are the common requirements that apply to the PA, PB, and PT protocols in the NEA reference model: C-1 NEA protocols MUST support multiple round trips between the NEA Client and NEA Server in a single assessment. C-2 NEA protocols SHOULD provide a way for both the NEA Client and the NEA Server to initiate a posture assessment or reassessment as needed. C-3 NEA protocols including security capabilities MUST be capable of protecting against active and passive attacks by intermediaries and endpoints including prevention from replay based attacks. C-4 The PA and PB protocols MUST be capable of operating over any PT protocol. For example, the PB protocol must provide a transport independent interface allowing the PA protocol to operate without change across a variety of network protocol environments (e.g., EAP/802.1X, TLS, and Internet Key Exchange Protocol version 2 (IKEv2)).
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   C-5  The selection process for NEA protocols MUST evaluate and prefer
        the reuse of existing open standards that meet the requirements
        before defining new ones.  The goal of NEA is not to create
        additional alternative protocols where acceptable solutions
        already exist.

   C-6  NEA protocols MUST be highly scalable; the protocols MUST
        support many Posture Collectors on a large number of NEA Clients
        to be assessed by numerous Posture Validators residing on
        multiple NEA Servers.

   C-7  The protocols MUST support efficient transport of a large number
        of attribute messages between the NEA Client and the NEA Server.

   C-8  NEA protocols MUST operate efficiently over low bandwidth or
        high latency links.

   C-9  For any strings intended for display to a user, the protocols
        MUST support adapting these strings to the user's language
        preferences.

   C-10 NEA protocols MUST support encoding of strings in UTF-8 format
        [UTF8].

   C-11 Due to the potentially different transport characteristics
        provided by the underlying candidate PT protocols, the NEA
        Client and NEA Server MUST be capable of becoming aware of and
        adapting to the limitations of the available PT protocol.  For
        example, some PT protocol characteristics that might impact the
        operation of PA and PB include restrictions on: which end can
        initiate a NEA connection, maximum data size in a message or
        full assessment, upper bound on number of roundtrips, and
        ordering (duplex) of messages exchanged.  The selection process
        for the PT protocols MUST consider the limitations the candidate
        PT protocol would impose upon the PA and PB protocols.

7.2. Posture Attribute (PA) Protocol Requirements

The Posture Attribute (PA) protocol defines the transport and data model to carry posture and validation information between a particular Posture Collector associated with the NEA Client and a Posture Validator associated with a NEA Server. The PA protocol carries collections of standard attributes and vendor-specific attributes. The PA protocol itself is carried inside the PB protocol.
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   The following requirements define the desired properties that form
   the basis for comparison and evaluation of candidate PA protocols.
   These requirements do not mandate the use of these properties, but
   merely that the candidate protocols are capable of offering the
   property if it should be needed.

   PA-1 The PA protocol MUST support communication of an extensible set
        of NEA standards defined attributes.  These attributes will be
        distinguishable from non-standard attributes.

   PA-2 The PA protocol MUST support communication of an extensible set
        of vendor-specific attributes.  These attributes will be
        segmented into uniquely identified vendor-specific namespaces.

   PA-3 The PA protocol MUST enable a Posture Validator to make one or
        more requests for attributes from a Posture Collector within a
        single assessment.  This enables the Posture Validator to
        reassess the posture of a particular endpoint feature or to
        request additional posture including from other parts of the
        endpoint.

   PA-4 The PA protocol MUST be capable of returning attributes from a
        Posture Validator to a Posture Collector.  For example, this
        might enable the Posture Collector to learn the specific reason
        for a failed assessment and to aid in remediation and
        notification of the system owner.

   PA-5 The PA protocol SHOULD provide authentication, integrity, and
        confidentiality protection for attributes communicated between a
        Posture Collector and Posture Validator.  This enables
        end-to-end security across a NEA deployment that might involve
        traversal of several systems or trust boundaries.

   PA-6 The PA protocol MUST be capable of carrying attributes that
        contain non-binary and binary data including encrypted content.

7.3. Posture Broker (PB) Protocol Requirements

The PB protocol supports multiplexing of Posture Attribute messages (based on PA protocol) between the Posture Collectors on the NEA Client to and from the Posture Validators on the NEA Server (in either direction). The PB protocol carries the global assessment decision made by the Posture Broker Server, taking into account the results of the Posture Validators involved in the assessment, to the Posture Broker Client.
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   The PB protocol also aggregates and transports advisories and
   notifications such as remediation instructions (e.g., patch
   references) from one or more Posture Validators.

   The requirements for the PB protocol are:

   PB-1 The PB protocol MUST be capable of carrying attributes from the
        Posture Broker Server to the Posture Broker Client.  This
        enables the Posture Broker Client to learn the posture
        assessment decision and if appropriate to aid in remediation and
        notification of the endpoint owner.

   PB-2 The PB protocol MUST NOT interpret the contents of PA messages
        being carried, i.e., the data it is carrying must be opaque to
        it.

   PB-3 The PB protocol MUST carry unique identifiers that are used by
        the Posture Brokers to route (deliver) PA messages between
        Posture Collectors and Posture Validators.  Such message routing
        should facilitate dynamic registration or deregistration of
        Posture Collectors and Validators.  For example, a dynamically
        registered anti-virus Posture Validator should be able to
        subscribe to receive messages from its respective anti-virus
        Posture Collector on NEA Clients.

   PB-4 The PB protocol MUST be capable of supporting a half-duplex PT
        protocol.  However this does not preclude PB from operating
        full-duplex when running over a full-duplex PT.

   PB-5 The PB protocol MAY support authentication, integrity and
        confidentiality protection for the attribute messages it carries
        between a Posture Broker Client and Posture Broker Server.  This
        provides security protection for a message dialog of the
        groupings of attribute messages exchanged between the Posture
        Broker Client and Posture Broker Server.  Such protection is
        orthogonal to PA protections (which are end to end) and allows
        for simpler Posture Collector and Validators to be implemented,
        and for consolidation of cryptographic operations possibly
        improving scalability and manageability.

   PB-6 The PB protocol MUST support grouping of attribute messages
        optimize transport of messages and minimize round trips.
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7.4. Posture Transport (PT) Protocol Requirements

The Posture Transport (PT) protocol carries PB protocol messages between the Posture Transport Client and the Posture Transport Server. PT is responsible for providing a protected transport for the PB protocol. The PT protocol may itself be transported by one or more concatenated sessions using lower layer protocols, such as 802.1X, RADIUS [RADIUS], TLS, or IKE. This section defines the requirements that candidate PT protocols must be capable of supporting. PT-1 The PT protocol MUST NOT interpret the contents of PB messages being transported, i.e., the data it is carrying must be opaque to it. PT-2 The PT protocol MUST be capable of supporting mutual authentication, integrity, confidentiality, and replay protection of the PB messages between the Posture Transport Client and the Posture Transport Server. PT-3 The PT protocol MUST provide reliable delivery for the PB protocol. This includes the ability to perform fragmentation and reassembly, detect duplicates, and reorder to provide in-sequence delivery, as required. PT-4 The PT protocol SHOULD be able to run over existing network access protocols such as 802.1X and IKEv2. PT-5 The PT protocol SHOULD be able to run between a NEA Client and NEA Server over TCP or UDP (similar to Lightweight Directory Access Protocol (LDAP)).

8. Security Considerations

This document defines the functional requirements for the PA, PB, and PT protocols used for Network Endpoint Assessment. As such, it does not define a specific protocol stack or set of technologies, so this section will highlight security issues that may apply to NEA in general or to particular aspects of the NEA reference model. Note that while a number of topics are outside the scope of the NEA WG and thus this specification (see section 3.1), it is important that those mechanisms are protected from attack. For example, the methods of triggering an assessment or reassessment are out of scope but should be appropriately protected from attack (e.g., an attacker hiding the event indicating a NEA Server policy change has occurred).
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   NEA intends to facilitate detection and corrective actions for
   cooperating endpoints to become compliant with network compliance
   policies.  For example, it is envisioned that these policies will
   allow deployers to detect out-of-date, inactive, or absent security
   mechanisms on the endpoint that might leave it more vulnerable to
   known attacks.  If an endpoint is more vulnerable to compromise, then
   it is riskier to have this endpoint present on the network with other
   valuable assets.  By proactively assessing cooperating endpoints
   before their entrance to the network, deployers can improve their
   resilience to attack prior to network access.  Similarly,
   reassessments of cooperating endpoints on the network may be helpful
   in assuring that security mechanisms remain in use and are up to date
   with the latest policies.

   NEA fully recognizes that not all endpoints will be cooperating by
   providing their valid posture (or any posture at all).  This might
   occur if malware is influencing the NEA Client or policies, and thus
   a trustworthy assessment isn't possible.  Such a situation could
   result in the admission of an endpoint that introduces threats to the
   network and other endpoints despite passing the NEA compliance
   assessment.

8.1. Trust

Network Endpoint Assessment involves assessing the posture of endpoints entering or already on the network against compliance policies to assure they are adequately protected. Therefore, there must be an implied distrusting of endpoints until there is reason to believe (based on posture information) that they are protected from threats addressed by compliance policy and can be trusted to not propagate those threats to other endpoints. On the network provider side, the NEA Client normally is expected to trust the network infrastructure systems to not misuse any disclosed posture information (see section 9) and any remediation instructions provided to the endpoint. The NEA Client normally also needs to trust that the NEA Server will only request information required to determine whether the endpoint is safe to access the network assets. Between the NEA Client and Server there exists a network that is not assumed to be trustworthy. Therefore, little about the network is implicitly trusted beyond its willingness and ability to transport the exchanged messages in a timely manner. The amount of trust given to each component of the NEA reference model is deployment specific. The NEA WG intends to provide security mechanisms to reduce the amount of trust that must be assumed by a deployer. The following sections will discuss each area in more detail.
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8.1.1. Endpoint

For NEA to properly operate, the endpoint needs to be trusted to accurately represent the requested security posture of the endpoint to the NEA Server. By NEA WG charter, the NEA reference model does not explicitly specify how to detect or prevent lying endpoints that intentionally misrepresent their posture. Similarly, the detection of malware (e.g., root kits) that are able to trick the Posture Collectors into returning incorrect information is the subject for research and standardization outside the IETF (e.g., Trusted Computing Group [TCG]) and is not specifically addressed by the model. However, if such mechanisms are used in a deployment, the NEA reference model should be able to accommodate these technologies by allowing them to communicate over PA to Posture Validators or work orthogonally to protect the NEA Client from attack and assure the ability of Posture Collectors to view the actual posture. Besides having to trust the integrity of the NEA Client and its ability to accurately collect and report Posture Attributes about the endpoint, we try to limit other assumed trust. Most of the usage models for NEA expect the posture information to be sent to the NEA Server for evaluation and decision making. When PA and/or PT level security protections are used, the endpoint needs to trust the integrity and potentially confidentiality of the trust anchor information (e.g., public key certificates) used by the Posture Collector and/or Posture Transport Client. However, NEA implementations may choose to send or pre-provision some policies to the endpoint for evaluation that would assume more trust in the endpoint. In this case, the NEA Server must trust the endpoint's policy storage, evaluation, and reporting mechanisms to not falsify the results of the posture evaluation. Generally the endpoint should not trust network communications (e.g., inbound connection requests) unless this trust has been specifically authorized by the user or owner defined policy or action. The NEA reference model assumes the entire NEA Client is local to the endpoint. Unsolicited communications originating from the network should be inspected by normal host-based security protective mechanisms (e.g., firewalls, security protocols, Intrusion Detection/Prevention System (IDS/IPS), etc.). Communications associated with a NEA assessment or reassessment requires some level of trust particularly when initiated by the NEA Server (reassessment). The degree of trust can be limited by use of strong security protections on the messages as dictated by the network deployer and the endpoint user/owner policy.
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8.1.2. Network Communications

Between the NEA Client and Server, there may exist a variety of types of devices to facilitate the communication path. Some of the devices may serve as intermediaries (e.g., simple L2 switches) so they may have the opportunity to observe and change the message dialogs. The intermediary devices may fall into a few major categories that impact our degree of trust in their operation. First, some intermediary devices may act as message forwarders or carriers for PT (e.g., L2 switches, L3 routers). For these devices we trust them not to drop the messages or actively attempt to disrupt (e.g., denial of service (DoS)) the NEA deployment. Second, some intermediary devices may be part of the access control layer of the network and as such, we trust them to enforce policies including remediation, isolation, and access controls given to them as a result on a NEA assessment. These devices may also fill other types of roles described in this section. Third, some devices may act as a termination point or proxy for the PT carrier protocol. Frequently, it is expected that the carrier protocol for PT will terminate on the NEA Client and Server so will be co-resident with the PT endpoints. If this expectation is not present in a deployment, we must trust the termination device to accurately proxy the PT messages without alteration into the next carrier protocol (e.g., if inner EAP method messages are transitioned from an EAP [EAP] tunnel to a RADIUS session). Fourth, many networks include infrastructure such as IDS/IPS devices that monitor and take corrective action when suspicious behavior is observed on the network. These devices may have a relationship with the NEA Server that is not within scope for this specification. Devices trusted by the NEA Server to provide security information that might affect the NEA Server's decisions are trusted to operate properly and not cause the NEA Server to make incorrect decisions. Finally, other types of intermediary devices may exist on the network between the NEA Client and Server that are present to service other network functions beside NEA. These devices might be capable of passively eavesdropping on the network, archiving information for future purposes (e.g., replay or privacy invasion), or more actively attacking the NEA protocols. Because these devices do not play a role in facilitating NEA, it is essential that NEA deployers not be forced to trust them for NEA to reliably operate. Therefore, it is required that NEA protocols offer security protections to assure these devices can't steal, alter, spoof or otherwise damage the reliability of the message dialogs.
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8.1.3. NEA Server

The NEA Server (including potentially remote systems providing posture validation services) is generally trusted to apply the specified assessment policies and must be protected from compromise. It is essential that NEA Server deployments properly safeguard these systems from a variety of attacks from the network and endpoints to assure their proper operation. While there is a need to trust the NEA Server operation to some degree, rigorous security architecture, analysis, monitoring, and review should assure its network footprint and internal workings are protected from attack. The network footprint would include communications over the network that might be subject to attack such as policy provisioning from the policy authoring systems and general security and system management protocols. Some examples of internal workings include protections from malware attacking the intra-NEA Server communications, NEA Server internal logic, or policy stores (particularly those that would change the resulting decisions or enforcements). The NEA Server needs to trust the underlying NEA and lower layer network protocols to properly behave and safeguard the exchanged messages with the endpoint. The NEA reference model does not attempt to address integrity protection of the operating system or other software supporting the NEA Server. One interesting example is where some components of the NEA Server physically reside in different systems. This might occur when a Posture Validator (or a remote backend server used by a local Posture Validator) exists on another system from the Posture Broker Server. Similarly, the Posture Broker Server might exist on a separate system from the Posture Transport Server. When there is a physical separation, the communications between the remote components of the NEA Server must ensure that the PB session and PA message dialogs are resistant to active and passive attacks, in particular, guarded against eavesdropping, forgery and replay. Similarly, the Posture Validators may also wish to minimize their trust in the Posture Broker Server beyond its ability to properly send and deliver PA messages. The Posture Validators could employ end-to-end PA security to verify the authenticity and protect the integrity and/or confidentiality of the PA messages exchanged. When PA security is used, each Posture Validator must be able to trust the integrity and potentially confidentiality of its trust anchor policies.
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8.2. Protection Mechanisms at Multiple Layers

Inherent in the requirements is a desire for NEA candidate protocols throughout the reference model to be capable of providing strong security mechanisms as dictated by the particular deployment. In some cases, these mechanisms may appear to provide overlapping or redundant protections. These apparent overlaps may be used in combination to offer a defense in depth approach to security. However, because of the layering of the protocols, each set of protections offers slightly different benefits and levels of granularity. For example, a deployer may wish to encrypt traffic at the PT layer to protect against some forms of traffic analysis or interception by an eavesdropper. Additionally, the deployer may also selectively encrypt messages containing the posture of an endpoint to achieve end-to-end confidentiality to its corresponding Posture Validator. In particular, this might be desired when the Posture Validator is not co-located with the NEA Server so the information will traverse additional network segments after the PT protections have been enforced or so that the Posture Validator can authenticate the corresponding Posture Collector (or vice versa). Different use cases and environments for the NEA technologies will likely influence the selection of the strength and security mechanisms employed during an assessment. The goal of the NEA requirements is to encourage the selection of technologies and protocols that are capable of providing the necessary protections for a wide variety of types of assessment.

8.3. Relevant Classes of Attack

A variety of attacks are possible against the NEA protocols and assessment technologies. This section does not include a full security analysis, but wishes to highlight a few attacks that influenced the requirement definition and should be considered by deployers selecting use of protective mechanisms within the NEA reference model. As discussed, there are a variety of protective mechanisms included in the requirements for candidate NEA protocols. Different use cases and environments may cause deployers to decide not to use some of these mechanisms; however, this should be done with an understanding that the deployment may become vulnerable to some classes of attack. As always, a balance of risk vs. performance, usability, manageability, and other factors should be taken into account.
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   The following types of attacks are applicable to network protocols
   defined in the reference model and thus should be considered by
   deployers.

8.3.1. Man-in-the-Middle (MITM)

MITM attacks against a network protocol exist when a third party can insert itself between two communicating entities without detection and gain benefit from involvement in their message dialog. For example, a malware infested system might wish to join the network replaying posture observed from a clean endpoint entering the network. This might occur by the system inserting itself into and actively proxying an assessment message dialog. The impact of the damage caused by the MITM can be limited or prevented by selection of appropriate protocol protective mechanisms. For example, the requirement for PT to be capable of supporting mutual authentication prior to any endpoint assessment message dialogs prevents the attacker from inserting itself as an active participant (proxy) within the communications without detection (assuming the attacker lacks credentials convincing either party it is legitimate). Reusable credentials should not be exposed on the network to assure the MITM doesn't have a way to impersonate either party. The PT requirement for confidentiality-protected (encrypted) communications linked to the above authentication prevents a passive MITM from eavesdropping by observing the message dialog and keeping a record of the conformant posture values for future use. The PT requirement for replay prevention stops a passive MITM from later establishing a new session (or hijacking an existing session) and replaying previously observed message dialogs. If a non-compliant, active MITM is able to trick a clean endpoint to give up its posture information, and the MITM has legitimate credentials, it might be able to appear to a NEA Server as having compliant posture when it does not. For example, a non-compliant MITM could connect and authenticate to a NEA Server and as the NEA Server requests posture information, the MITM could request the same posture from the clean endpoint. If the clean endpoint trusts the MITM to perform a reassessment and is willing to share the requested posture, the MITM could obtain the needed posture from the clean endpoint and send it to the NEA Server. In order to address this form of MITM attack, the NEA protocols would need to offer a strong (cryptographic) binding between the posture information and the authenticated session to the NEA Server so the NEA Server knows the posture originated from the endpoint that authenticated. Such a strong binding between the posture's origin and the authenticating endpoint may be feasible so should be preferred by the NEA WG.
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8.3.2. Message Modification

Without message integrity protection, an attacker capable of intercepting a message might be capable of modifying its contents and causing an incorrect decision to be made. For example, the attacker might change the Posture Attributes to always reflect incorrect values and thus prevent a compliant system from joining the network. Unless the NEA Server could detect this change, the attacker could prevent admission to large numbers of clean systems. Conversely, the attacker could allow a malware infested machine to be admitted by changing the sent Posture Attributes to reflect compliant values, thus hiding the malware from the Posture Validator. The attacker could also infect compliant endpoints by sending malicious remediation instructions that, when performed, would introduce malware on the endpoint or deactivate security mechanisms. In order to protect against such attacks, the PT includes a requirement for strong integrity protection (e.g., including a protected hash like a Hashed Message Authentication Code (HMAC) [HMAC] of the message) so any change to a message would be detected. PA includes a similar requirement to enable end-to-end integrity protection of the attributes, extending the protection all the way to the Posture Validator even if it is located on another system behind the NEA Server. It is important that integrity protection schemes leverage fresh secret information (not known by the attacker) that is bound to the authenticated session such as an HMAC using a derived fresh secret associated with the session. Inclusion of freshness information allows the parties to protect against some forms of message replay attacks using secret information from prior sessions.

8.3.3. Message Replay or Attribute Theft

An attacker might listen to the network, recording message dialogs or attributes from a compliant endpoint for later reuse to the same NEA Server or just to build an inventory of software running on other systems watching for known vulnerabilities. The NEA Server needs to be capable of detecting the replay of posture and/or the model must assure that the eavesdropper cannot obtain the information in the first place. For this reason, the PT protocol is required to provide confidentiality and replay prevention. The cryptographic protection from disclosure of the PT, PB, or PA messages prevents the passive listener from observing the exchanged messages and thus prevents theft of the information for future use. However, an active attacker might be able to replay the encrypted message if there is no strong link to the originating party or
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   session.  By linking the encrypted message dialog to the
   authentication event and leveraging per-transaction freshness and
   keying exchanges, this prevents a replay of the encrypted
   transaction.

8.3.4. Other Types of Attack

This section doesn't claim to present an exhaustive list of attacks against the NEA reference model. Several types of attack will become easier to understand and analyze once the NEA WG has created specifications describing the specific selected technologies and protocols to be used within NEA. One such area is Denial of Service (DoS). At this point in time, it is not practical to try to define all of the potential exposures present within the NEA protocols, so such an analysis should be included in the Security Considerations sections of the selected NEA protocols. However, it is important that the NEA Server be resilient to DoS attacks as an outage might affect large numbers of endpoints wishing to join or remain on the network. The NEA reference model expects that the PT protocol would have some amount of DoS resilience and that the PA and PB protocols would need to build upon that base with their own protections. To help narrow the window of attack by unauthenticated parties, it is envisioned that NEA Servers would employ PT protocols that enable an early mutual authentication of the requesting endpoint as one technique for filtering out attacks. Attacks occurring after the authentication would at least come from sources possessing valid credentials and could potentially be held accountable. Similarly, NEA protocols should offer strong replay protection to prevent DoS-based attacks based on replayed sessions and messages. Posture assessment should be strongly linked with the Posture Transport authentications that occurred to assure the posture came from the authenticated party. Cryptographic mechanisms and other potentially resource intensive operations should be used sparingly until the validity of the request can be established. This and other resource/protocol based attacks can be evaluated once the NEA technologies and their cryptographic use have been selected.

9. Privacy Considerations

While there are a number of beneficial uses of the NEA technology for organizations that own and operate networks offering services to similarly owned endpoints, these same technologies might enhance the potential for abuse and invasion of personal privacy if misused. This section will discuss a few of the potential privacy concerns raised by the deployment of this technology and offer some guidance to implementers.
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   The NEA technology enables greater visibility into the configuration
   of an endpoint from the network.  Such transparency enables the
   network to take into consideration the strength of the endpoint's
   security mechanisms when making access control decisions to network
   resources.  However, this transparency could also be used to enforce
   restrictive policies to the detriment of the user by limiting their
   choice of software or prying into past or present uses of the
   endpoint.

   The scope of the NEA WG was limited to specifying protocols targeting
   the use cases where the endpoints and network are owned by the same
   party or the endpoint owner has established a clear expectation of
   disclosure/compliance with the network owner.  This is a familiar
   model for governments, institutions, and a wide variety of
   enterprises that provide endpoints to their employees to perform
   their jobs.  In many of these situations, the endpoint is purchased
   and owned by the enterprise and they often reserve the right to audit
   and possibly dictate the allowable uses of the device.  The NEA
   technologies allow them to automate the inspection of the contents of
   an endpoint and this information may be linked to the access control
   mechanisms on the network to limit endpoint use should the endpoint
   not meet minimal compliance levels.

   In these environments, the level of personal privacy the employee
   enjoys may be significantly reduced subject to local laws and
   customs.  However, in situations where the endpoint is owned by the
   user or where local laws protect the rights of the user even when
   using endpoints owned by another party, it is critical that the NEA
   implementation enable the user to control what endpoint information
   is shared with the network.  Such controls imposed by the user might
   prevent or limit their ability to access certain networks or
   protected resources, but this must be a user choice.

9.1. Implementer Considerations

The NEA WG is not defining NEA Client policy content standards nor defining requirements on aspects of an implementation outside of the network protocols; however, the following guidance is provided to encourage privacy friendly implementations for broader use than just the enterprise-oriented setting described above. NEA Client implementations are encouraged to offer an opt-in policy to users prior to sharing their endpoint's posture information. The opt-in mechanism should be on a per-user, per-NEA Server basis so each user can control which networks can access any posture information on their system. For those networks that are allowed to assess the endpoint, the user should be able to specify granular restrictions on what particular types and specific attributes Posture
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   Collectors are allowed to disclose.  Posture Validator
   implementations are discouraged from having the default behavior of
   using wild carded requests for posture potentially leading to
   overexposure of information (see section 9.2).  Instead Posture
   Validators, by default, should only request the specific attributes
   that are required to perform their assessment.

   Requests for attributes that are not explicitly allowed (or
   specifically disallowed) to be shared should result in a user
   notification and/or log record so the user can assess whether the
   service is doing something undesirable or whether the user is willing
   to share this additional information in order to gain access.  Some
   products might consider policy-driven support for prompting the user
   for authorization with a specific description of the posture
   information being requested prior to sending it to the NEA Server.

   It is envisioned that the owner of the endpoint is able to specify
   disclosure policies that may override or influence the user's
   policies on the attributes visible to the network.  If the owner
   disclosure policy allows for broader posture availability than the
   user policy, the implementation should provide a feedback mechanism
   to the user so they understand the situation and can choose whether
   to use the endpoint in those circumstances.

   In such a system, it is important that the user's policy authoring
   interface is easy to understand and clearly articulates the current
   disclosure policy of the system including any influences from the
   owner policy.  Users should be able to understand what posture is
   available to the network and the general impact of this information
   being known.  In order to minimize the list of restrictions
   enumerated, use of a conservative default disclosure policy such as
   "that which is not explicitly authorized for disclosure is not
   allowed" might make sense to avoid unintentional leakage of
   information.

   NEA Server implementations should provide newly subscribing endpoints
   with a disclosure statement that clearly states:

      o What information is required

      o How this information will be used and protected

      o What local privacy policies are applicable

   This information will empower subscribing users to decide whether the
   disclosure of this information is acceptable considering local laws
   and customs.
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9.2. Minimizing Attribute Disclosure

One important issue in the design of the NEA reference model and protocols is enabling endpoints to disclose minimal information required to establish compliance with network policies. There are several models that could be considered as to how the disclosed attribute set is established. Each model has privacy related benefits and issues that should be considered by product developers. This section summarizes three potential models for how attribute disclosure might be provided within NEA products and some privacy implications potentially associated with each model. The first model is easy to implement and deploy but has privacy and potentially latency and scalability implications. This approach effectively defaults the local policy to send all known NEA Posture Attributes when an assessment occurs. While this might simplify deployment, it exposes a lot of information that is potentially not relevant to the security assessment of the system and may introduce privacy issues. For example, is it really important that the enterprise know whether Firefox is being used on a system instead of other browsers during the security posture assessment? The second model involves an out-of-band provisioning of the disclosure policy to all endpoints. This model may involve the enterprise establishing policy that a particular list of attributes must be provided when a NEA exchange occurs. Endpoint privacy policy may filter this attribute list, but such changes could cause the endpoint not to be given network or resource access. This model simplifies the network exchange as the endpoint always sends the filtered list of attributes when challenged by a particular network. However, this approach requires an out-of-band management protocol to establish and manage the NEA disclosure policies of all systems. The third model avoids the need for pre-provisioning of a disclosure policy by allowing the NEA Server to specifically request what attributes are required. This is somewhat analogous to the policy being provisioned during the NEA exchanges so is much easier to manage. This model allows for the NEA Server to iteratively ask for attributes based on the values of prior attributes. Note, even in this model the NEA protocols are not expected to be a general purpose query language, but rather allow the NEA Server to request specific attributes as only the defined attributes are possible to request. For example, an enterprise might ask about the OS version in the initial message dialog and after learning the system is running Linux ask for a different set of attributes specific to Linux than it would if the endpoint was a Windows system. It is envisioned that this
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   approach might minimize the set of attributes sent over the network
   if the assessment is of a complex system (such as trying to
   understand what patches are missing from an OS).

   In each model, the user could create a set of per-network privacy
   filter policies enforced by the NEA Client to prevent the disclosure
   of attributes felt to be personal in nature or not relevant to a
   particular network.  Such filters would protect the privacy of the
   user but might result in the user not being allowed access to the
   desired asset (or network) or being provided limited access.

10. References

10.1. Normative References

[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.

10.2. Informative References

[802.1X] IEEE Standards for Local and Metropolitan Area Networks: Port based Network Access Control, IEEE Std 802.1X-2001, June 2001. [CNAC] Cisco, Cisco's Network Admission Control Main Web Site, http://www.cisco.com/go/nac [EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [IPSEC] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [NAP] Microsoft, Network Access Protection Main Web Site, http://www.microsoft.com/nap [RADIUS] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
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   [TLS]    Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [TCG]    Trusted Computing Group, Main TCG Web Site,
            http://www.trustedcomputinggroup.org/

   [TNC]    Trusted Computing Group, Trusted Network Connect Main Web
            Site, https://www.trustedcomputinggroup.org/groups/network/

11. Acknowledgments

The authors of this document would like to acknowledge the NEA Working Group members who have contributed to previous requirements and problem statement documents that influenced the direction of this specification: Kevin Amorin, Parvez Anandam, Diana Arroyo, Uri Blumenthal, Alan DeKok, Lauren Giroux, Steve Hanna, Thomas Hardjono, Tim Polk, Ravi Sahita, Joe Salowey, Chris Salter, Mauricio Sanchez, Yaron Sheffer, Jeff Six, Susan Thompson, Gary Tomlinson, John Vollbrecht, Nancy Winget, Han Yin, and Hao Zhou.
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Authors' Addresses

Paul Sangster Symantec Corporation 6825 Citrine Dr Carlsbad, CA 92009 USA Phone: +1 760 438-5656 EMail: Paul_Sangster@symantec.com Hormuzd Khosravi Intel 2111 NE 25th Avenue Hillsboro, OR 97124 USA Phone: +1 503 264 0334 EMail: hormuzd.m.khosravi@intel.com Mahalingam Mani Avaya Inc. 1033 McCarthy Blvd. Milpitas, CA 95035 USA Phone: +1 408 321-4840 EMail: mmani@avaya.com Kaushik Narayan Cisco Systems Inc. 10 West Tasman Drive San Jose, CA 95134 Phone: +1 408 526-8168 EMail: kaushik@cisco.com Joseph Tardo Nevis Networks 295 N. Bernardo Ave., Suite 100 Mountain View, CA 94043 USA EMail: joseph.tardo@nevisnetworks.com
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