Internet Engineering Task Force                                 NSIS
        Internet Draft                                         H. Tschofenig
                                                              D. Kroeselberg
        Expires: December 2003                                     June 2003
                              Security Threats for NSIS
     Status of this Memo
        This document is an Internet-Draft and is subject to all provisions
        of Section 10 of RFC2026.
        Internet-Drafts are working documents of the Internet Engineering
        Task Force (IETF), its areas, and its working groups. Note that other
        groups may also distribute working documents as Internet- Drafts.
        Internet-Drafts are draft documents valid for a maximum of six months
        and may be updated, replaced, or obsoleted by other documents at any
        time. It is inappropriate to use Internet-Drafts as reference
        material or to cite them other than as "work in progress."
        The list of current Internet-Drafts can be accessed at
        The list of Internet-Draft Shadow Directories can be accessed at
        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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     Table of Contents
        1. Introduction...................................................2
        2. Terminology....................................................3
        3. Relevant communication models..................................3
           3.1 First-Peer Communication...................................5
           3.2 End-to-Middle Communication................................6
           3.3 Intra-Domain Communication.................................6
           3.4 Inter-Domain Communication.................................6
           3.5 End-to-End Communication...................................7
           3.6 Middle-to-middle Communication.............................8
        4. Generic Threats................................................8
           4.1 Man-in-the-middle attacks..................................8
           4.2 Adversary being able to replay signaling messages.........10
           4.3 Adversary being able to inject/modify messages............10
           4.4 Security Parameter Exchange/Negotiation...................11
        5. Signaling specific Threats....................................11
           5.1 Attacks during NSIS SA Usage..............................11
           5.2 Combining Signaling and SA Establishment..................11
           5.3 Eavesdropping and Traffic Analysis........................12
           5.4 Identity Spoofing.........................................12
           5.5 Missing Protection of Authorization Information...........14
           5.6 Missing Non-Repudiation...................................15
           5.7 Malicious NSIS Entity.....................................15
           5.8 Denial of Service Attacks.................................16
           5.9 Disclosing the network topology...........................17
           5.10 Session/Reservation Ownership............................18
           5.11 Attacks against the signaling message transport mechanism19
        6. Security Considerations.......................................19
        7. Normative References..........................................19
        8. Informative References........................................20
        Author's Addresses...............................................21
        Full Copyright Statement.........................................21
     1. Introduction
        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 is separated into individual steps. To
        address security in the NSIS working group a number of steps have
        been taken:
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                  |            NSIS Analysis Activities          |
                  |         (e.g. RSVP Security Properties)      |
                  |            Security Threats for NSIS         |
                  |                                              |
                  |               NSIS Requirements              |
                  |                                              |
                  |               NSIS Framework                 |
                  |                                              |
                  |                                              |
                  |             NSIS Protocol Proposals          |
                            Figure 1: NSIS Security Steps
        This document identifies the basic security 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 to discussion within the NSIS framework.
     2. Terminology
        This document uses NSIS terms defined in [Bru03].
     3. Relevant communication models
        Signaling messages traverse different network parts, which demand
        different security protection and raise different security problems.
        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).
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        The main properties of the listed network parts are briefly described
        in this section and the threats of Section 4 and Section 5 classify
        them to generic threats and signaling specific threats. 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 (as described below), 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.
          +------------------+   +---------------+   +------------------+
          |                  |   |               |   |                  |
          |  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
        To further refine the above differentiation based on network parts
        that NSIS signaling may traverse, we consider trust relationships
        between 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 [RFC3261] protocol.
        Any intermediate SIP hop 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. This
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        scenario is called middle-to-middle. A typical example of middle-to-
        middle communication is between two NSIS hops at the border of their
        respective trust domains (i.e. inter-domain communication). NSIS
        messages may have to traverse one or more untrusted hops between
        these NSIS entities.
        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.
                      *                                      *
                 +----+-----+       +----------+        +----+-----+
           +-----+  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
     3.1 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
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        already available infrastructure. Per-session negotiation of security
        mechanisms is therefore often not required (which, in contrast, is
        required for the mobility case).
        For first-peer communication, especially threats related to initial
        security association setup, replay attacks, lack of confidentiality,
        denial of service, integrity violation, identity spoofing, theft of
        service and fraud are applicable.
     3.2 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 protection).
     3.3 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.
     3.4 Inter-Domain Communication
        The threat assumptions between the borders of different
        administrative domains largely depend on the authorization
        procedures. If one domain forges QoS reservations then this domain
        may also have to pay for the reservation. Hence in this case, there
        is no real benefit for this domain to forge a QoS reservation. If an
        end host is directly charged by intermediate domains (i.e. by a
        domain different from the malicious domain) such an attack may be
        quite a reasonable threat.
        However, security protection of messages transmitted between
        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 adjacent administrative domains,
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        the number of domains is usually rather limited (compared to first-
        peer communication) which causes fewer problems for the key
        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
        If signaling messages are conveyed transparently in the core network
        (i.e. they 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)
        triggered by unprotected incoming signaling messages.
     3.5 End-to-End Communication
        NSIS aims to signal information between the initiator and the
        responder. This section refers to the trust relationships required
        between the end points in cases where security protection is
        required. Note that this security protection is likely to be required
        only for certain objects such as pricing and charging related
        information. Protecting the entire signaling message is not possible
        since intermediate NSIS nodes need to (a) inspect various objects and
        (b) need to add, modify or delete objects from the signaling message.
        The following example tries to illustrate a possible application of
        end-to-end protection for objects carried within the NSIS signaling
        protocol. Alice, the data sender, wants Bob, the data receiver, to
        pay for a QoS reservation (reverse charging). Bob wants to be assured
        that the QoS signaling message he receives was indeed transmitted by
        Alice because he is only willing to pay for particular users and not
        for everyone. Hence Bob wants to verify that the request came from
        Alice (authentication) and that the included parameters are
        unmodified. Additionally it might be necessary to secure a
        negotiation step and to secure deliver authorization information to
        the involved parties. Information which is required to compute an
        authorization decision (such as prices or QoS objects) also needs
        proper security protection.
        Typical threats in such a scenario range from modification of QoS
        objects or price information (i.e. Bob has to pay more), fraud (i.e.
        to force Bob always to pay for the reservations) to identity spoofing
        (i.e. the adversary claims to be Alice).
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        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 might lead to additional complexity.
     3.6 Middle-to-middle Communication
        We do not explicitly consider the middle-to-middle case here,
        although it is important, since it is already covered by either
        intra- or inter-domain communication depending on the location of the
        involved entities.
     4. Generic Threats
        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).
     4.1 Man-in-the-middle 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. An
        example for a two-step approach is provided by IKE/IPsec. 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 separation between SA establishment
        and signaling protection takes place. Particularly the discovery
        procedure is vulnerable against a number of attacks.
        - 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
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        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 5.7).
        - Unilateral Authentication
        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 an end host and the first peer
        (just authenticating the end host) 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).
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        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
        - 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 applicable to NSIS for the process of selecting certain
        security mechanisms.
     4.2 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 later point in
        time (or at a different place, or uses parts of them at a different
        place or 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.
     4.3 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.
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        Some directly related threats are described in Section 5.7, 5.4 and
     4.4 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 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.
     5. Signaling specific Threats
     5.1 Attacks during NSIS SA Usage
        Once a security association is established (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 5.7. Replay attacks, which can be a problem when a NSIS node
        crashes, restarts and performs state re-establishment. Proper re-
        synchronization capability of the security mechanism must therefore
        address this problem.
     5.2 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
        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 no 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).
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        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 messages (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.
        Further threats could be introduced by allowing an adversary to gain
        additional information by injecting error messages or by forcing the
        creation of error messages.
     5.3 Eavesdropping and Traffic Analysis
        This section covers threats whereby an adversary is able to eavesdrop
        signaling messages. The collected signaling packets may serve for the
        purpose of traffic analysis or to later mount replay attacks as
        described in the Section 4.2. The eavesdropper might learn QoS
        parameters, communication patterns, policy rules for firewall
        traversal, policy information, application identifiers, user
        identities, NAT bindings, authorization objects and more.
        Note, that such a threat is also applicable if the messages are
        integrity protected which is often considered sufficient for
        signaling protocols.
        Since the NSIS protocol signals messages through a number of nodes it
        is possible to differentiate between nodes actively participating in
        the NSIS protocol and others who do not actively participate in the
        NSIS protocol. For certain objects or messages it might be desirable
        to permit actively participating intermediate NSIS nodes to
        eavesdrop. As a further extension it might be desired that only the
        intended end points (NSIS initiator and NSIS responder) are able to
        read certain objects.
     5.4 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.
        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
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        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 [RFC1809] and [RC+03]) and others are
        communicated in these protocols. Modification of these flow
        identifiers causes quality of service reservations or policy rules at
        middleboxes to be either ineffective or exploidable for adversaries.
        An adversary could mount an attack by modifying the flow identifier
        of a signaling message.
        NSIS signaling messages contain some sort of flow identifier, which
        is associated with a specified behavior (e.g. a particular flow
        experiences QoS treatment or allows packets to traverse a firewall,
        etc.). An adversary might therefore use IP spoofing and inject data
        packets to benefit from previously installed flow identifiers.
        The following 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 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 protocol.
        The ability for an adversary to inject data traffic which matches a
        certain flow identifier established by a legitimate user often
        requires the ability to also receive the data traffic. This is,
        however, only true if the flow identifier consists of values which
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        contain addresses used for routing. If we imagine to use attributes
        for a flow identifier 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.
        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
        [RFC2745]. Related security issues (for example denial of service
        attacks) are described in Section 6.1 of the same document.
     5.5 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.
        Typically the authenticated identity is used to assist during the
        authorization procedure ass for example described in [RFC3812].
        Depending on the chosen authentication protocol certain attacks are
        possible. Section 4 discusses a number of issues related to this
        approach when the authentication and key exchange protocol is used to
        establish session keys for signaling message protection.
        Another approach is to use some sort of authorization token. The
        functionality and structure of such an authorization token for RSVP
        is described in [RFC3520] and in [RFC3521].
        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 patterns. 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.
        The Session/Reservation Ownership problem can also be considered as
        an authorization problem. Details are described in Section 5.10. In
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        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.
     5.6 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). The problem of a missing non-repudiation property
        appears in two flavors:
        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 user's 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 the non-repudiation property for the
        authorized resources then it has an impact on the protocol design.
        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 (e.g. specific
        authorization mechanisms) and not for general NSIS signaling.
     5.7 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 the edge NSIS entity. We assume
        that edge NSIS entity have the responsibility to perform
        cryptographic processing (authentication, integrity and replay
        protection, authorization and accounting) for signaling message
        arriving from the outside. This prevents signaling messages to appear
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        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 to 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 fully prevent this problem. Since an
        NSIS node might issue signaling messages 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
        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).
     5.8 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.
        - 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 requests (possibly in a
        distributed fashion). Charging is activated only after successful
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        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
        state has already been allocated along the path for various
        processing tasks including path pinning.
        - Discovery Phase
        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 reasons.
        - 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.
     5.9 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
        [RFC2745] 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
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                           Security Threats for NSIS             June 2003
        nodes or to provide diagnostic facilities (used for network
        monitoring and administration).
     5.10 Session/Reservation Ownership
        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-
                                                 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 user.
        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 of the session/reservation.
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        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.
     5.11 Attacks against the signaling message transport mechanism
        In [BL01] 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
        assumption is also considered within the NSIS framework [HF+03].
        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 messages.
        Network or transport layer protocols which experience no protection
        are vulnerable to certain attacks such as header manipulation, DoS,
        spoofing of identities, session hijacking, unexpected aborts etc.
        Malicious nodes can attack the congestion control mechanism to force
        NSIS nodes into a congestion avoidance state.
        In case that an existing protocol is used for exchanging NSIS
        signaling messages then threats known from these protocols are
     6. Security Considerations
        This entire memo discusses security issues relevant for NSIS. To
        counter these threats security requirements have been listed in
        [Brun03]. Framework relevant topics have been incorporated into
     7. Normative References
        [Brun03] M. Brunner, "Requirements for QoS signaling protocols,"
        Internet Draft, Internet Engineering Task Force, June 2003.  Work in
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                           Security Threats for NSIS             June 2003
        [HF+03] R. Hancock, I. Freytsis, G. Karagiannis, J. Loughney, and S.
        V. den Bosch, "Next steps in signaling: Framework," Internet Draft,
        Internet Engineering Task Force, March 2003.  Work in progress.
     8. Informative References
        [RFC1809] C. Partridge, "Using the flow label field in IPv6," RFC
        1809, Internet Engineering Task Force, June 1995.
        [RFC2745] A. Terzis, B. Braden, S. Vincent, and L. Zhang, "RSVP
        Diagnostic Messages," RFC 2745, Internet Engineering Task Force,
        Jan. 2000.
        [RFC3182]   Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore,
        T., Herzog, S., Hess, R.: "Identity Representation for RSVP", RFC
        3182, October, 2001.
        [RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston,
        J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
        initiation protocol," RFC 3261, Internet Engineering Task Force,
        June 2002.
        [RFC3521]   L. Hamer, B. Gage, and H. Shieh, "Framework for session
        set-up with media authorization," RFC 3521, Internet Engineering
        Task Force, April 2003.
        [RFC3520] L. Hamer, B. Gage, B. Kosinski, and H. Shieh, "Session
        Authorization Policy Element", RFC 3520, Internet Engineering Task
        Force, April 2003.
        [RC+03] J. Rajahalme, A. Conta, B. Carpenter, and S. Deering, "IPv6
        Flow Label Specification," Internet Draft, Internet Engineering Task
        Force, April 2003.  Work in progress.
        [BL01] B. Braden and B. Lindell, "A two-level architecture for
        internet signaling," Internet Draft, Internet Engineering Task
        Force, Nov. 2001. Work in progress.
        We would like to thank (in alphabetical order) Marcus Brunner, Jorge
        Cuellar, Mehmet Ersue, Xiaoming Fu and Robert Hancock for their
        comments to an initial version of this draft. Jorge and Robert gave
        us an extensive list of comments and provided information on
        additional threats.
        Jukka Manner, Martin Buechli, Roland Bless, Marcus Brunner, Michael
        Thomas and Mohan Parthasarathy provided comments to a recent version
        of this draft. Their input helped to improve the content of this
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                           Security Threats for NSIS             June 2003
        document. Particularly Roland Bless and Michael Thomas provided
        proposals for regrouping and restructuring.
     Author's Addresses
        Hannes Tschofenig
        Siemens AG
        Corporate Technology
        CT IC 3
        Otto-Hahn-Ring 6
        81739 Munich
        Dirk Kroeselberg
        Siemens AG
        Otto-Hahn-Ring 6
        81739 Munich
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                           Security Threats for NSIS             June 2003
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