Internet Engineering Task Force                                     NSIS
Internet Draft                             H. Tschofenig, D. Kroeselberg
                                                              Siemens AG
23 January 2003
Expires: August 2003

                       Security Threats for NSIS


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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This threats document provides a detailed analysis of the security
threats relevant for the NSIS working group. It motivates and helps to
understand various security considerations in the NSIS Requirements,
Framework and Protocol proposals. This document does not describe
vulnerabilities of specific NSIS protocols.

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1 Introduction

Section 1.1 introduces the overall process of addressing the security
work done in the NSIS working group. Section 1.2 gives a high-level
picture of the different network parts, which are traversed by NSIS
signaling. Each part is characterized by a different set of requirements
and different trust relationships. The threats described in Section 2
can be assigned to these individual parts.

1.1 NSIS Security Process

Whenever a new protocol has to be developed or existing protocols have
to be modified their security threats should be evaluated. The process
of securing protocols in separated into individual steps. To address
security in the NSIS working group a number of documents have been

          |            NSIS Analysis Activities          |
          |         (e.g. RSVP Security Properties)      |
          |            Security Threats for NSIS         |
          |                                              |
          |               NSIS Requirements              |
          |                                              |
          |               NSIS Framework                 |
          |                                              |
          |                                              |
          |             NSIS Protocol Proposals          |

Figure 1: NSIS Security related Documents

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All the documents depicted in Figure 1 contribute to the NSIS security
approach. The purpose of each of these documents is briefly described
below to give the reader insight into the development process.

     NSIS Analysis Activities:

          The primary goal of the NSIS analysis activity is the
          investigation of existing approaches in the area of quality of
          service signaling protocols. Several of the published
          approaches directly identify security threats and
          requirements, whereas other threats and requirements can be
          derived from the different scenarios in which these protocols
          are used. For instance, [1] points to the reduced complexity
          if RSVP is used without multicast support. This modification
          also results in simplified security requirements. In [2],
          security issues in some example configurations are given. In
          [3], the security properties of RSVP are described.
          Furthermore an analysis of the interaction between RSVP and
          Mobile IP is provided by Michael Thomas in [4] and an analysis
          of existing QoS protocols is described in [5].

     NSIS Requirements:

          To address the security threats relevant for NSIS described in
          this document, security requirements have been specified as
          part of the NSIS Requirements document [6]. In addition to
          these requirements [6] describes basic scenarios where the
          NSIS signaling protocol might be deployed.

     NSIS Framework:

          Signaling information to a number of devices located in
          different parts in the network with different trust
          assumptions and possible interactions with a large number of
          other protocols require some framework thoughts, which is
          especially true for security. In [7] a security framework is
          provided for NSIS.

     NSIS Protocol:

          Finally there are documents describing concrete protocol
          proposals. These proposals either rely on existing security
          mechanisms or develop their own if the existing mechanisms
          cannot counter all relevant security threats or if they are
          inappropriate for other reasons. In practice, a protocol
          proposal might use established security mechanisms or
          protocols for basic protection, but is likely to require some
          additional protection mechanisms, or a combination of both for

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          enhanced security.

          Note that the process of developing the above-mentioned
          documents is not linear. Instead it takes various iterations
          to reach a satisfactory NSIS security solution.

     Security Threats for NSIS:

          This document identifies the basic threats that need to be
          addressed by the NSIS signaling protocol design. In addition,
          although the base protocol might be secure, some extensions
          may cause problems when used in a particular environment.
          Furthermore it is necessary to investigate the context in
          which a signaling protocol is used and the architecture where
          it is integrated. As an example of such interaction accounting
          and charging are taken into account in this document, since
          without an appropriate integration of the two it is difficult
          to deploy any NSIS solution. This interaction is also subject
          of the NSIS framework and some aspects are discussed in [7].

1.2 Relevant communication models

Independent of the threat scenarios described in Section 2 signaling
messages traverse different network parts, which demand different
security means. The difference in security protection is mainly caused
by the fact that the NSIS signaling messages cross trust boundaries
where different trust relationships are prevalent. Often a
categorization into first-peer/last-peer, intra-domain and inter-domain
communication is applicable (see Figure 2). Depending on the concrete
security requirements end-to-end security protection across trust
boundaries might be required for certain scenarios but is usually not
easily addressable by standard means.  The main properties of the listed
network parts are briefly described in this section and the threat
scenarios of Section 2 are classified accordingly. Figure 2 depicts a
typical end-to-end communication scenario including an access part
between the NSIS end entities and the nearest NSIS hops, respectively.
This "first-peer communication" commonly comes with specific security
requirements, especially important for properly addressing security in
mobile scenarios. Differences in the trust relationship and the required
security for first-peer communication, compared to other parts of the
signaling path, might exist.

If signaling messages are not exchanged end-to-end and only parts of the
signaling path are affected, some threats may not be relevant.

To further refine the above differentiation based on network parts that
NSIS signaling may traverse, we consider trust relationships between

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  +------------------+   +---------------+   +------------------+
  |                  |   |               |   |                  |
  |  Administrative  |   | Intermediate  |   |  Administrative  |
  |     Domain A     |   |   Domains     |   |     Domain B     |
  |                  |   |               |   |                  |
  |                 (Inter-domain Communication)                |
  |        +---------+---+---------------+---+---------+        |
  |  (Intra-domain   |   |               |   | (Intra-domain    |
  |   Communication) |   |               |   |  Communication)  |
  |        |         |   |               |   |         |        |
  |        |         |   |               |   |         |        |
  +--------+---------+   +---------------+   +---------+--------+
           ^                                           v
           |                                           |
  First Peer Communication               Last Peer Communication
           |                                           |
     +-----+-----+                               +-----+-----+
     |   NSIS    |                               |   NSIS    |
     | Initiator |                               | Responder |
     +-----------+                               +-----------+

Figure 2: Involved Network Parts

NSIS hops.  Additional threats may apply to NSIS communication where one
entity involved is an end-entity (initiator or responder) and the other
entity is any intermediate hop not being the first peer. This is
typically called end-to-middle scenario. The motivation for including
this configuration stems for example from the SIP [8] protocol. Any
intermediate SIP proxy may request a SIP end entity (UA) to
authenticate, countering a number of specific security threats. Such
functionality in general seems to be useful for intermediaries at the
borders of trust domains that signaling messages need to traverse.
Intermediate NSIS hops as well may have to deal with specific security
threats that do not (directly) relate to end-entities. Between such
intermediate hops, other such NSIS hops will typically be in the
signaling path. This scenario is called middle-to-middle. A generic
example are two NSIS hops at the border of their respective trust
domains with some form of trust relation. NSIS messages between these
hops may have to traverse one or more intermediate untrusted hops.
Figure 3 illustrates these additional scenarios. The first-peer case
discussed further above is covered by the peer-to-peer trust
relationships between end entity and closest hop, respectively.

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              *                                      *
         +----+-----+       +----------+        +----+-----+
   +-----+  NSIS    +-------+  NSIS    +--------+  NSIS    +-----+
   |     |  Node 1  |       |  Node 2  |        |  Node 3  |     |
   |     +----------+       +----+-----+        +----------+     |
   |                             ~                               |
   |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~                               |
   |  ~                                                          |
+--+--+-----+                                          +---------+-+
|   NSIS    +//////////////////////////////////////////+   NSIS    |
| Initiator |                                          | Responder |
+-----------+                                          +-----------+

  -----: Peer-to-Peer Trust Relationship
  /////: End-to-End Trust Relationship
  *****: Middle-to-Middle Trust Relationship
  ~~~~~: End-to-Middle Trust Relationship

Figure 3: Trust Relationships

     First-Peer Communication:

          First peer communication refers to the peer-to-peer
          interaction between a signaling message originator, the NSIS
          Initiator (NI), and the first NSIS aware entity along the
          path. Assumptions about the threats, security requirements and
          the available trust relationships may be difficult here.  To
          illustrate this, in many mobility environments it is difficult
          to assume the existence of a pre-established security
          association directly available for NSIS peers involved in
          first-peer communication, as these peers cannot be assumed to
          have any relation between each other in advance. For
          enterprise networks, in contrast, the situation is different.
          Usually there is a fairly strong (pre-established) trust
          relationship between the peers. Enterprise network
          administrators usually have some degree of freedom to select
          the appropriate security protection and to enforce it. The
          choice of selecting a security mechanism is therefore often
          influenced by the already available infrastructure. Per-
          session negotiation of security mechanisms is therefore often
          not required (which, in contrast, is required for the mobility

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          For first-peer communication, especially threats related to
          initial security association setup, replay attacks, lack of
          confidentiality, denial of service, integrity violation,
          identity spoofing and fraud are applicable.

     End-to-Middle Communication:

          End-to-middle interaction in signaling may be required to e.g.
          grant end-entities access to, or specific services in trust
          domains different from the one the first peer belongs to.
          Threats, in addition to these already discussed for first-hop
          communication, may be untrusted intermediate NSIS hops that
          maliciously alter NSIS signaling. These threats are still
          relevant if security mechanisms are in place between the NSIS
          hops, but terminate at each hop (e.g. IPsec hop-by-hop

     Intra-Domain Communication:

          After having been verified at the first peer, an NSIS
          signaling message traverses the network within the same
          administrative domain the first peer belongs to. Since the
          request has already been authenticated and authorized threats
          are different to those described above in a). To differentiate
          first-peer communication with the intra-domain communication
          (i.e. communication internally within one administrative
          domain) we assume that no end hosts have direct access to the
          internal network nodes, except the first peer. We furthermore
          assume that NSIS peers within the same administrative domain
          have at least some sort of trust relationship.

     Inter-Domain Communication:

          The threat assumptions between the borders of different
          administrative domains largely depend on accounting procedures
          (and therefore business relationships) in case of QoS
          signaling, which is an important example application of NSIS
          signaling. If one domain transmits forged QoS reservations
          (for example stating a higher QoS reservation than a
          aggregated number of user did) to the next domain then the
          originating domain may also have to pay for the reservation.
          Hence in this case, there is no real benefit for the first
          network domain to forge a QoS reservation. If an end host is
          directly charged by domains different to the first peer's
          domain, then such an attack may be quite a reasonable threat.
          However, security protection of messages transmitted between

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          different administrative domains is still necessary to tackle
          attacks like spoofing, integrity violation, or denial of
          service between these domains, e.g. to allow for proper
          accounting. In case of securing signaling messages between
          administrative domains, the number of domains is usually
          rather limited (compared to first-peer communication) which
          causes fewer problems for the key management.

          Signaling information other than QoS service parameters such
          as policy rules in case of middlebox communication demands
          different assumptions for inter-domain communication. Trust
          assumptions and business relationships are of particular
          importance for their communication.

          If signaling messages are transparent in the core network
          (i.e. the are not intercepted and processed in the core
          network) then the signaling message communication effectively
          takes place between access networks. This might place a burden
          on the key management infrastructure because of the global PKI
          requirements. Hence this can be seen as a serious deployment
          threat since it might be unacceptable for an access network
          service provider to perform processing (QoS reservations,
          policy rule installation at firewalls) due to unprotected
          incoming signaling messages.

     End-to-End Communication:

          Providing end-to-end signaling message protection for NSIS
          would cause difficulties for authentication and key
          establishment procedures. It would furthermore limit the
          flexibility of a signaling protocol in general. Functionality
          such as terminating at an arbitrary location along the path,
          delegating a signaling message exchange to other nodes, etc.
          would be difficult to achieve in a secure fashion. Protecting
          signaling messages end-to-end (in addition to peer-to-peer
          security) is in our opinion rarely required. This is based on
          the observation that end-to-end issues like charging and
          payment selection (i.e. which user has to pay for which part
          of a QoS reservation) are already securely negotiated by
          preceding upper layer protocols (for example SIP). Information
          carried within an NSIS signaling protocol for the purpose of
          charging is therefore assumed opaque to the NSIS protocol
          itself. Note that this observation makes some assumptions
          about the charging model and about the existence of a protocol
          interaction with AAA, QoS and an application layer protocol.

          It is however possible to imagine a charging solution that
          requires end-to-end protection of information delivered within

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          the NSIS signaling protocol. The following example requires
          some sort of end-to-end protection: Alice wants Bob to pay for
          a QoS reservation (reverse charging). Bob wants to be assured
          that the QoS signaling message he receives was transmitted by
          Alice because he is only willing to pay for particular users
          and not for everyone. Hence Bob requires Alice to protect the
          reservation request.

          Regarding end-to-end security one additional issue needs to be
          addressed - delegation. Whenever a signaling is addressed end-
          to-end and an arbitrary node along the path acts as a proxy on
          behalf of the other endpoint a delegation mechanism is
          required to provide secure interaction. This obviously leads
          to additional complexity in the area of end-to-end security,
          as an additional set of threats becomes relevant.


          We do not explicitly consider the middle-to-middle case here,
          as this is already covered by either intra- or inter-domain
          communication depending on the location of the involved

2 Threat Scenarios

This section provides threat scenarios that are applicable to signaling
protocols. Note that some threat scenarios use the term user instead of
NSIS Initiator. This is mainly because security protocols allow a
differentiation between entities being hosts and users (based on the
identities used).

2.1 MITM Attacks

Security protection of protocols is often separated into two steps. The
first step provides entity authentication and key establishment whereas
the second step provides message protection using the previously
established security association. The first step usually tends to be
more expensive than the second which is also the main reason for
separation. If messages are transmitted very infrequently then these two
steps are collapsed into a single and usually rather costly step. One
such example is e-mail protection via S/MIME. A good example for an
efficient two-step approach is provided by IPsec [9]. We use this
separation to cover the different threats in more detail.  The first
paragraph describes security threats where two peers do not already
share a security association, or do not use security mechanisms at all.
The next paragraph describes threats which are applicable when a
security association is already established. Finally a denial of service
attack is described which is applicable to a signaling message when no

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separation between SA establishment and signaling protection takes

Various security threat are caused by a protocol performing dynamic node
discovery. These threats include Denial of Service attacks, which are
among other threats described in Section 2.9. Note that the threats are
largely independently of the discovery procedure (path discovery, next
peer discovery or topology discovery).

     1.   Attacks during NSIS SA Establishment

          During the process of establishing a security association an
          adversary fools the signaling message initiator with respect
          to the entity to which it has to authenticate. The man-in-the-
          middle adversary is able to modify signaling messages to mount
          e.g. DoS attacks. In addition, it may be able to terminate
          NSIS messages of the Initiator and inject messages to a peer
          itself, therefore acting as the peer to the initiator and as
          the initiator to the peer. This results in the initiator
          wrongly believing that it talks to the "real" network whereas
          it is actually attached to an adversary.  For this attack to
          be successful, pre-conditions have to hold which are described
          with the following two cases:

          - Missing Authentication

            The first case addresses missing authentication between the
            neighboring peers: Without authentication a NI, NR or NF is
            unable to detect an adversary. However in some cases
            protection available might be difficult to accomplish in a
            practical environment either because the next peer is
            unknown, because of misbelieved trust relationships in parts
            of the network or because of the inability to establish
            proper security protection (inter-domain signaling messages,
            dynamic establishment of a security association, etc.). If
            one of the communication endpoints is unknown then for some
            security mechanisms it is either not possible or very
            difficult to apply appropriate security protection.
            Sometimes network administrators use intra-domain signaling
            messages without proper security. Such a configuration would
            then allow an adversary on a compromised non-NSIS aware node
            to interfere with nodes running an NSIS signaling protocol.
            Note that this type of threat goes beyond a threat caused by
            malicious NSIS nodes (described in Section 2.8).

          - Unilateral Authentication

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            In case of a unilateral authentication the NSIS entity that
            does not authenticate its peer is unable to discover the
            man-in-the-middle adversary. Although authentication of
            signaling messages should take place between each peer
            participating in the protocol operation special attention is
            given here to first-peer communication. Unilateral
            authentication between end hosts and the first peer is still
            common today, but certainly opens up many possibilities for
            MITM attackers impersonating either the end host or the
            (administrative domain represented by the) first peer.

            The two threats described above are a general problem of
            network access without appropriate authentication, not only
            for an NSIS signaling protocol. Obviously there is a strong
            need to correctly address them in a future NSIS protocol.
            The signaling protocols addressed by NSIS are different to
            other protocols where only two entities are involved. Note,
            that especially first-peer authentication is important, as
            the impacts of a security breach likely reach beyond the
            directly involved entities (or even beyond a local network).

            Finally it should be noted that the signaling protocol
            should be considered as a peer-to-peer protocol where the
            roles of initiator and responder can be reversed at any
            time. This leads to the conclusion that unilateral
            authentication is not very useful for such a protocol.
            However there might be a need to have some form of asymmetry
            in the authentication process whereby one entity uses a
            different authentication mechanism than the other one. As an
            example the combination of symmetric and asymmetric
            cryptography should be mentioned.

          - Weak Authentication

            This threat addresses weak authentication mechanisms whereby
            information transmitted during the NSIS SA establishment
            process may leak passwords and/or may allow offline
            dictionary attacks. This threat is not specific to NSIS
            signaling protocols but may also be applicable and
            countermeasures must be taken.

     2.   Attacks during NSIS SA Usage

          Once a security association is establish (and used to protect
          signaling messages) basic attacks are prevented. However, a
          malicious NSIS node is still able to perform various attacks
          as described in Section 2.8. Replay attacks, which can be a
          problem when a NSIS node crashes, restarts and performs state

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          re-establishment. Proper re-synchronization capability of the
          security mechanism must therefore address this problem.

     3.   Combining Signaling and SA Establishment

          This threat covers an attack which allows an adversary to
          flood an NSIS node with bogus signaling messages to cause a
          denial of service attack.

          When a signaling message arrives at a NSIS aware network
          element some processing is required. If this message contains
          security objects such as digital signatures and not security
          association is already available then some processing is
          required for the cryptographic verification. Since NSIS
          signaling should not require several roundtrips between two
          NSIS peers it is difficult to provide DoS protection
          mechanisms commonly found in authentication and key agreement
          protocols. If signaling messages furthermore aim to be
          idempotent and no security association should be created then
          some cryptographic mechanisms should be used with precaution
          (for example public key cryptography).

          Additionally to the threat described above an incoming
          signaling message might require time consuming processing
          (computations, state maintenance, timer setting, etc) and
          communication with third-party nodes including policy servers,
          LDAP servers, etc. If an adversary is able to transmit a large
          number of signaling message (for example with QoS reservation
          requests) with invalid credentials then the verifying node may
          not be able to process further reservation messages by
          legitimate users.

2.2 Eavesdropping and Traffic Analysis

This threat cases covers adversaries which are able to eavesdrop
signaling messages but are unable to actively participate in signaling
message exchange (i.e. passive adversary). The collected signaling
packets may serve for the purpose of traffic analysis or to later mount
replay attacks as described in the Section 2.3. The eavesdropper might
learn QoS parameters, communication patterns, policy rules for firewall
traversal, policy information, application identifiers, user identities,
NAT bindings and more.

2.3 Adversary being able to replay signaling messages

This threat scenario covers the case where an adversary eavesdrops and
collects signaling messages and replays them at a latter point in time
(or at a different place, or uses parts of them at a different place or

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in a different way - e.g. cut and paste attacks). Without proper replay
protection an adversary might mount man-in-the-middle, denial of service
and theft of service attacks.

A more difficult attack that may cause problems even in case of replay
protection requires the adversary to crash an NSIS aware node to loose
state information (sequence numbers, security associations, etc.) and to
be able to replay old signaling messages. This attack addresses re-
synchronization deficiencies.

2.4 Missing Protection of Authorization Information

Authorization is an important step for providing resources such as QoS
reservations, NAT bindings and pin-holed firewalls. Authorization
information might be delivered to the NSIS participating entities in a
number of ways.

One such approach is to use a successful authorization step done by a
different protocol in a later NSIS signaling message by providing some
sort of token. The functionality and structure of such an authorization
token for RSVP is described in [10] and in [11].

The interaction between different protocols based on authorization
tokens, however, requires some care. Using such an authorization token
it is possible to link state information between different protocols.
Returning an unprotected authorization token to the end host might allow
an adversary (for example an eavesdropper) to steal resources. An
adversary might also use the token to learn communication patters. An
untrustworthy end host might also modify the token content.

Other authorization mechanisms might depend on availability of
sufficient funds and therefore real-time information. Deployment threats
of this kind are described in Section 2.14. The Session/Reservation
Ownership problem can also be considered as an authorization problem.
Details are described in Section 2.11. In enterprise networks
authorization is often coupled with membership to a particular class
user of users/groups. This type of information can either be delivered
as part of the authentication and key agreement procedure or has to be
retrieved via separate protocols from other entities. If an adversary
manages to modify information relevant for determining authorization or
the outcome of the authorization process itself then theft of service
might be the consequence.

2.5 Identity Spoofing

Identity spoofing relevant for NSIS appears in two flavors: First,
identity spoofing can appear during the establishment of a security
association if based on a weak authentication mechanism.

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Eve, acting as an adversary, claims to be the registered user Alice by
spoofing the identity of Alice. Thereby Eve causes the network to charge
Alice for the consumed network resources. This type of attack is
possible if authentication is done based on a simple username identifier
(i.e. in absence of cryptographic authentication) or if authentication
is provided for hosts and multiple users have access to a single host.
This attack could also be classified as theft of service.

Second, an adversary is able to perform identity spoofing on transmitted
data packets. This type of attack is often labeled as IP spoofing. Since
most NSIS signaling messages contain some sort of flow identifier for
which a certain behavior is performed (e.g. particular flow experiences
QoS treatment or is allowed to bypass a firewall, etc.) an adversary
could mount an attack by modifying the flow identifier of a signaling
message. The following example tries to show an adversary using identity
spoofing of the first category:

An adversary is able to exploit the established flow identifiers
(required for QoS and Midcom specific signaling protocols). Some
identifiers such as IP addresses, transport protocol identifiers, port
numbers, flow labels (see [12] and [13]) and others are communicated in
these protocols. Modification of these flow identifiers cause quality of
service reservations or policy rules at middleboxes to be either
ineffective or beneficial for adversaries.

The following paragraph describes a possible threat caused by identity
spoofing of transmitted data traffic. The spoofed identity is thereby
the source IP addresses. For this attack to be successful accounting
records are collected based on the source IP address and not on a SPI
due to IPSec protection. After the network receives a properly protected
reservation request, transmitted by the legitimate user Alice, Traffic
Selectors are installed at the corresponding devices (for example edge
router). These Traffic Selectors are used for flow identification and
allow to match data traffic originated from a given source address to be
assigned to a particular QoS reservation. The adversary Eve now spoofs
the IP address of the Alice. Additionally Alice's host may be crashed by
the adversary as a result of a denial of service attack or lost
connectivity for example because of mobility reasons. If both nodes are
located at the same link and use the same IP address then obviously a
duplicate IP address will be detected. Assuming that only Eve is present
at the link then she is able to receive and transmit data (for example
RTP data traffic), which receives preferential QoS treatment based on
the previous reservation. Depending on the installed Traffic Selector
granularity Eve might have more possibilities to exploit the QoS
reservation or a pin-holed firewall. Assuming the soft state paradigm,
where periodical refresh messages are required, the absence of Alice
will not be detected until the next signaling message appears and forces
Eve to respond with a protected signaling message. Again this issue is

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not only applicable to QoS traffic but the existence of QoS reservation
causes more difficulties since this type of traffic is more expensive.
The same procedure is also applicable to a Middlebox communication

The ability for an adversary to inject data traffic which matches a
certain Traffic Selector established by a legitimate user often requires
the ability to also receive the data traffic. This is, however, only
true if the Traffic Selector consists of values which contain addresses
used for routing. If we imagine to use attributes for a Traffic Selector
where such a property is not required then identity spoofing and
injecting traffic is much easier. An adversary can use a nearly
arbitrary endpoint identifier to experience the desired result.
Obviously the endpoint identifiers are still not irrelevant since the
messages have to travel the same path through the network. DiffServ
marking of IP packets is such an example and others can be constructed
very easily.

Data traffic marking based on DiffServ is such an example. Whenever an
ingress router uses only marked incoming data traffic for admission
control procedures then various attacks are possible. These problems are
known in the DiffServ community for a long time and documented in
various DiffServ related documents. The IPSec protection of DiffServ
Code Points is described in Section 6.2 of [14]. Related security issues
(for example denial of service attacks) are described in Section 6.1 of
the same document.

2.6 Adversary being able to inject/modify messages

This type of threat addresses integrity violations whereby an adversary
modifies signaling messages (e.g. by acting as a man-in-the-middle
attacker) to cause an unexpected network behavior. Possible actions an
adversary might consider for its attack are reordering, delaying,
dropping, injecting and modifying.

An adversary may inject a signaling message requesting a large amount of
resources (possibly using a different user identity). Other resource
requests could then be rejected. In combination with identity spoofing
it is also possible accomplish fraud. This attack is only successful in
absence of signaling message protection.

Some directly related threats are described in Section 2.8, 2.5, 2.8 and

2.7 Missing Non-Repudiation

Repudiation in this context refers to a problem where one party later
denies to have requested a certain action (such as a QoS reservation).

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The problem of a missing non-repudiation property appears in two

>From a service provider point-of-view the following threat may be worth
an investigation. A user may deny to have issued reservation request for
which it was charged. A service provider may then like to prove that a
particular user issued reservation requests.

The same threat can be interpreted from the users point-of-view. A
service provider claims to have received a number of reservation
requests. The user in question thinks that he never issued those
requests and wants to have a proof for correct service usage for a given
set of QoS parameters.

In today's telecommunication networks non-repudiation is not provided.
The user has to trust the network operator to correctly meter the
traffic, collect and merge accounting data and that no unforeseen
problems occur. If a signaling protocol is used to establish QoS
reservations with a higher volume (for example service level agreements)
then it might impact protocol design.

Looking at threats based on missing non-repudiation it must be carefully
considered whether non-repudiation is needed. Non-repudiation poses
additional requirements on the security mechanisms as it can only be
provided through public-key cryptography. As this would often increase
the overall cost for security, threats related to missing non-
repudiation are only considered relevant for certain specific scenarios
but not for the general NSIS scenario.

2.8 Malicious NSIS Entity

Network elements within a domain (intra-domain) experience a different
trust relationship with regard to the security protection of signaling
messages compared to edge routers. We assume that edge routers have the
responsibility to perform cryptographic processing (authentication,
integrity and replay protection, authorization and accounting) for
signaling message arriving from outside. This prevents signaling
messages to appear unprotected within the internal network. If however
an adversary manages to take over an edge router then the security of
the entire network is affected. An adversary is then able to launch a
number of attacks including denial of service, integrity violation,
replay attacks etc. In case of policy rule installation a rogue firewall
can cause harm to other firewalls by modifying the policy rules
accordingly. The chain-of-trust principle applied in the peer-to-peer
security protection cannot provide protection against a malicious NSIS
node. An adversary with access an NSIS router is then also able to get
access to security associations to transmit secured signaling messages.
Note that even non peer-to-peer security protection might not be able to

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fully prevent this problem. Since an NSIS node might issue signaling
message on behalf of someone else (by acting as a proxy) additional
problems are the consequence.

An NSIS aware edge router is a critical component that requires strong
security protection. A strong security policy applied at edge does not
imply that all routers within an intra-domain network do not need to
cryptographically verify signaling messages. If the chain-of-trust
principle is deployed then the security protection of the entire path
(in this case within the network of a single administrative domain) is
as strong as the weakest link. In our case the edge router is the most
critical component of this network that may also act as a security
gateway/firewall for incoming/outgoing traffic. For outgoing traffic
this device has to act according to the security policy of the local
domain to apply the appropriate security protection.

For an adversary to mount this attack either an existing NSIS aware node
along the path has to be successfully attacked or an adversary succeeds
to convince another NSIS node to be the next NSIS peer (man-in-the-
middle attack).

2.9 Denial of Service Attacks

A number of denial of service attacks can cause NSIS nodes to
malfunction. Other attacks that could lead to DoS, such as man-in-the-
middle attacks, replay attacks, injection or modification of signaling
messages etc., are mentioned throughout this document.

     1.   Path Finding

          This threat tries to address potential denial of service
          attacks when the reservation setup is split into two phases
          i.e. path and reservation (as for example used in receiver
          based reservation setup). For this example we assume that the
          node transmitting the path message is not charged for the path
          message itself and is able to issue a high number of
          reservation request (possibly in a distributed fashion).
          Charging is activated only after successful verification of
          the reservation request. The reservations are however never
          intended to be successful because of various reasons: the
          destination node cannot be reached; it is not responding or
          simply rejects the reservation. An adversary can benefit from
          the fact that resources are already consumed along the path
          for various processing tasks including path pinning.

     2.   Discovery Phase

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          Signaling information to a large number of entities along a
          data path requires some sort of discovery. This discovery
          process is vulnerable to a number of attacks since it is
          difficult to secure. An adversary can use the discovery
          mechanisms to convince an entity to signal information to
          another entity which is not along the data path or to cause
          the discovery process to fail. In the first case the signaling
          protocol could be correctly continued with the problem that
          policy rules are installed at incorrect firewalls or QoS
          resource reservations take place at the wrong entities. For an
          end host this means that the protocol failed for unknown

     3.   Faked Error/Response messages

          An adversary may be able to use false error/response messages
          as part of a denial of service attack. This could be either at
          the message signaling protocol level, at the level of each
          client layer protocol (QoS, Midcom, etc.) or at the transport
          level protocol. An adversary might cause unexpected protocol
          behavior or produce denial of service attacks. Especially the
          discovery protocol shows vulnerabilities with regard to this
          threat. In case that no separate discovery protocol is used by
          addressing signaling messages to end hosts only (with a Router
          Alert Option to intercept message as NSIS aware nodes) then an
          error message might be used to indicate a path change. Such a
          design is a combination of a discovery protocol together with
          a signaling message exchange protocol.

2.10 Disclosing the network topology

In some architectures there is a desire not to reveal the internal
network structure (or other related information) to the outside world.
An adversary might be able to use NSIS messages for network mapping
(e.g. discovering which nodes exist, which use NSIS, what version, what
resources are allocated, capabilities of nodes along a paths etc.).
Discovery messages, traceroute, diagnostic messages (see [14] for a
description of diagnostic message functionality for RSVP), query
messages in addition to record route and route objects provide the
potential to assist an adversary. Hence the requirement of not
disclosing a network topology might conflict with another requirement to
provide means for automatically discovering NSIS aware nodes or to
provide diagnostic facilities (used for network monitoring and

2.11 Session/Reservation Ownership

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Figure 4 shows an NSIS Initiator which established state information at
NSIS nodes along the path as part of the signaling procedure. As a
result the Access Router1 Router 3 and Router 4 (and other nodes) store
session state information including the Session Identifier SID-x.

                                         Session ID(SID-x)
                  +-----------------+ Router +------------>
 Session ID(SID-x)|                 |   4    |
              +---+----+            +--------+
              | Router |
       +------+   3    +*******
       |      +---+----+      *
       |                      *
       | Session ID(SID-x)    * Session ID(SID-x)
   +---+----+             +---+----+
   | Access |             | Access |
   | Router |             | Router |
   |   1    |             |   2    |
   +---+----+             +---+----+
       |                      *
       | Session ID(SID-x)    * Session ID(SID-x)
  +----+------+          +----+------+
  |  NSIS     |          | Adversary |
  | Initiator |          |           |
  +-----------+          +-----------+

Figure 4: Session/Reservation Ownership

The Session Identifier is included in signaling messages to reference to
the established state.

If an adversary was able to obtain the Session Identifier for example by
eavesdropping signaling messages it is able to add the same Session
Identifier SID-x to a new a signaling message. When the signaling
message hits Router3 (as shown in Figure 3) then existing state
information can be modified. The adversary can then modify or delete the
established reservation causing unexpected behavior for the legitimate

The source of the problem is that Router3 (cross-over router) is unable
to decide whether the new signaling message was initiated from the owner

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of the session/reservation.

To make processing even more difficult it must be mentioned that not
only the initial signaling message originator is allowed to signal
information during the lifetime of an established session. As part of
the protocol any NSIS aware node along the path (and the path might
change over time) could be involved in the signaling message exchange
and it might be necessary to provide mobility support or to trigger a
local repair procedure. Hence if only the initial signaling message
originator is allowed to trigger signaling message exchange some
protocol behavior will not be possible.

In case that this threat is not addressed an adversary can launch denial
of service, theft of service, and various other attacks.

2.12 Security Parameter Exchange/Negotiation

Protocols, which should be useful for a variety of scenarios, tend to
have different security requirements. It is often difficult to meet
these (sometimes conflicting requirements) with a single security
mechanism or a fixed security parameter. Hence often a few selected
mechanisms/parameters are supported. Therefore some protocol exchange is
required to agree on some security mechanisms/parameters. This protocol
exchanged can be the misused by an adversary to mount a downgrading
attack by selecting weaker mechanisms than desired. Hence without
protecting the negotiation process the security of an NSIS protocol
might be as secure as the weakest mechanism if no configuration
parameters (for example a security policy disallowing the weakest
mechanism, etc.) are used otherwise.

2.13 Attacks against the signaling message transport mechanism

In [15] a two-level architecture is proposed which suggests to split an
NSIS protocol into layers: a signaling message transport specific layer
and an application specific layer. This architectural assumptions is
also considered within the NSIS framework [7]. Most of the threats
described in this document are applicable to the application specific
part for signaling QoS or middlebox specific information. There are,
however, some threats which are applicable to the transport of signaling

Network or transport layer protocols which experience no protected are
vulnerable to certain attacks such as header manipulation, DoS, spoofing
of identities, session hijacking, unexpected aborts etc.

In case that an existing protocol is used for exchanging NSIS signaling
messages then threats known from these protocols are relevant.

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2.14 Deployment Threats

This section addresses problems which could appear during the deployment
of an NSIS protocol in a real-world environment. Although these problems
are theoretically not an obstacle for practical reasons they can
represent threats worth a consideration.

     Missing Authorization:

          Authentication is a very important step for providing the
          foundation of authorization and accounting. Unlike some other
          protocols (for example HTTPS) where an authorization
          verification step is fairly easy (and efficient) QoS and
          middlebox communication requires more care. First, there is
          the question what authorization means in the context of NSIS
          signaling. For quality of service signaling the possible range
          is broad and could range from pure monetary policies to
          traditional role-based access control policies. Second, there
          is a question where this authorization data can be retrieved.
          Especially in a mobile environment this might be more
          complicated to securely exchange this information between
          different network domains. Finally there is an issue of
          authorization representation (i.e. a language describing
          authorization policies). If authorization information is
          exchanged between a large number of networks then this issue
          deserves further consideration.

          In the discovery phase an additional issue of authorization
          was raised. Whenever a node wants to discover the next NSIS
          aware node then authentication might not be sufficient. In
          many cases the IP address or FQDN of a particular router in an
          unknown network does not add too much trust. An end host for
          example might want some assurance that this node belongs to a
          network which has some sort of business relationship which is
          known and acceptable (from an accounting, charging, security
          and privacy point of view).

     Missing Cost Control:

          There is a risk that a large number of service providers with
          complex roaming agreements create a non-transparent cost-
          structure. In a traditional subscription-based scenario users
          are registered with their home networks and use this trust
          relationship to dynamically establishment other security
          associations. This is the typical AAA deployment scenario. In
          these scenarios users do not learn the identity of the access
          network as part of a regular authentication and key exchange
          protocol message exchange. The identity of the access network

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          is possibly never revealed (in a secure fashion). The user is
          therefore only authenticated to the home network (and
          hopefully vice versa). When issuing a QoS reservation request
          to the next NSIS peer (for example in the access network) the
          end host is typically unaware of the cost of such a
          reservation. Due to mobility and route changes along the path
          the cost for an end-to-end QoS reservation might not be
          transparent for the end host or even become unacceptable.

          Today there is no standarized protocol available which allows
          users to communicate cost limits, to request cost information
          for network resources or to learn already accumulated costs
          for a particular reservation.

          Especially in mobility environments where an end host is
          likely to have access to a large number of networks within a
          short time period cost control is even more complicated.

          Some mobility/QoS protocol proposals try to merge existing
          mobility protocols with QoS signaling (i.e. to apply in-band
          signaling). Thereby the access network is queried (towards the
          cross-over router or the MAP) for the possibility making a QoS
          reservation (without actually making the reservation itself).
          Without a query mechanism a user cannot take reservation costs
          into account when choosing between different access networks
          (or different access routers). Hence the user might not be
          unable to refuse a more expensive service provider. To allow a
          user to choose between different providers might be required
          not only because of the availability of different access
          technologies (e.g. IEEE 802.1x, Bluetooth, UTRAN) and the
          different service quality offered but also for cost reasons.

          Although real-time notifications of quality of service
          reservation costs (cost control) to the user are outside the
          scope of NSIS some interaction might be required.

3 Security Considerations

This entire memo discusses security issues relevant for NSIS. To counter
these threats security requirements have been defined and the framework
relevant topics have been described. Some additional threats applicable
for first peer communication in mobile environments are described in

4 Acknowledgements

We would like to thank (in alphabetical order) Marcus Brunner, Jorge
Cuellar, Mehmet Ersue, Xiaoming Fu and Robert Hancock for their comments

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to this draft. Jorge and Robert gave us an extensive list of comments
and provided information on additional threats.

5 Authors' Addresses

Hannes Tschofenig
Siemens AG
Otto-Hahn-Ring 6
81739 Munich

Dirk Kroeselberg
Siemens AG
Otto-Hahn-Ring 6
81739 Munich

6 Bibliography

[1] X. Fu, C. Kappler, and H. Tschofenig, "Analysis on RSVP regarding
multicast," Internet Draft, Internet Engineering Task Force, June 2002.
Work in progress.

[2] H. D. Meer et al.  , "Analysis of existing qos solutions," Internet
Draft, Internet Engineering Task Force, July 2002.  Work in progress.

[3] H. Tschofenig, "Rsvp security properties," Internet Draft, Internet
Engineering Task Force, 2002.  Work in progress.

[4] M. Thomas, "Analysis of mobile ip and rsvp interactions," Internet
Draft, Internet Engineering Task Force, 2002.  Work in progress.

[5] J. Manner and X. Fu, "Analysis of existing quality of service
signaling protocols," Internet Draft, Internet Engineering Task Force,
2002.  Work in progress.

[6] M. Brunner, "Requirements for QoS signaling protocols," Internet
Draft, Internet Engineering Task Force, July 2002.  Work in progress.

[7] R. Hancock, I. Freytsis, G. Karagiannis, J. Loughney, and S. V. den
Bosch, "Next steps in signaling: Framework," Internet Draft, Internet
Engineering Task Force, 2002.  Work in progress.

[8] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session

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initiation protocol," RFC 3261, Internet Engineering Task Force, June

[9] S. Kent and R. Atkinson, "Security architecture for the internet
protocol," RFC 2401, Internet Engineering Task Force, Nov. 1998.

[10] L. Hamer, B. Gage, M. Broda, B. Kosinski, and H. Shieh, "Session
authorization for RSVP," Internet Draft, Internet Engineering Task
Force, July 2002.  Work in progress.

[11] L. Hamer, B. Gage, and H. Shieh, "Framework for session set-up with
media authorization," Internet Draft, Internet Engineering Task Force,
July 2002.  Work in progress.

[12] C. Partridge, "Using the flow label field in IPv6," RFC 1809,
Internet Engineering Task Force, June 1995.

[13] J. Rajahalme, A. Conta, B. Carpenter, and S. Deering, "IPv6 flow
label specification," Internet Draft, Internet Engineering Task Force,
June 2002.  Work in progress.

[14] A. Terzis, B. Braden, S. Vincent, and L. Zhang, "RSVP diagnostic
messages," RFC 2745, Internet Engineering Task Force, Jan. 2000.

[15] B. Braden and B. Lindell, "A two-level architecture for internet
signaling," Internet Draft, Internet Engineering Task Force, Nov. 2001.
Work in progress.

[16] J. Kempf and E. Nordmark, "Threat analysis for IPv6 public multi-
access links," Internet Draft, Internet Engineering Task Force, June
2002.  Work in progress.

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                           Table of Contents

1          Introduction  . . . . . . . . . . . . . . . . . . . . . .   2
1.1        NSIS Security Process . . . . . . . . . . . . . . . . . .   2
1.2        Relevant communication models . . . . . . . . . . . . . .   4
2          Threat Scenarios  . . . . . . . . . . . . . . . . . . . .   9
2.1        MITM Attacks  . . . . . . . . . . . . . . . . . . . . . .   9
2.2        Eavesdropping and Traffic Analysis  . . . . . . . . . . .  12
2.3        Adversary being able to replay signaling messages . . . .  12
2.4        Missing Protection of Authorization Information . . . . .  13
2.5        Identity Spoofing . . . . . . . . . . . . . . . . . . . .  13
2.6        Adversary being able to inject/modify messages  . . . . .  15
2.7        Missing Non-Repudiation . . . . . . . . . . . . . . . . .  15
2.8        Malicious NSIS Entity . . . . . . . . . . . . . . . . . .  16
2.9        Denial of Service Attacks . . . . . . . . . . . . . . . .  17
2.10       Disclosing the network topology . . . . . . . . . . . . .  18
2.11       Session/Reservation Ownership . . . . . . . . . . . . . .  18
2.12       Security Parameter Exchange/Negotiation . . . . . . . . .  20
2.13       Attacks against the signaling message transport
mechanism  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
2.14       Deployment Threats  . . . . . . . . . . . . . . . . . . .  21
3          Security Considerations . . . . . . . . . . . . . . . . .  22
4          Acknowledgements  . . . . . . . . . . . . . . . . . . . .  22
5          Authors' Addresses  . . . . . . . . . . . . . . . . . . .  23
6          Bibliography  . . . . . . . . . . . . . . . . . . . . . .  23

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