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Network Working Group                                      H. Tschofenig
Request for Comments: 4081                                D. Kroeselberg
Category: Informational                                          Siemens
                                                               June 2005

          Security Threats for Next Steps in Signaling (NSIS)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This threats document provides a detailed analysis of the security
   threats relevant to the Next Steps in Signaling (NSIS) protocol
   suite.  It calls attention to, and helps with the understanding of,
   various security considerations in the NSIS Requirements, Framework,
   and Protocol proposals.  This document does not describe
   vulnerabilities of specific parts of the NSIS protocol suite.

Table of Contents

   1. Introduction ....................................................2
   2. Communications Models ...........................................3
   3. Generic Threats .................................................7
      3.1. Man-in-the-Middle Attacks ..................................8
      3.2. Replay of Signaling Messages ..............................11
      3.3. Injecting or Modifying Messages ...........................11
      3.4. Insecure Parameter Exchange and Negotiation ...............12
   4. NSIS-Specific Threat Scenarios .................................12
      4.1. Threats during NSIS SA Usage ..............................13
      4.2. Flooding ..................................................13
      4.3. Eavesdropping and Traffic Analysis ........................15
      4.4. Identity Spoofing .........................................15
      4.5. Unprotected Authorization Information .....................17
      4.6. Missing Non-Repudiation ...................................18
      4.7. Malicious NSIS Entity .....................................19
      4.8. Denial of Service Attacks .................................20
      4.9. Disclosing the Network Topology ...........................21
      4.10. Unprotected Session or Reservation Ownership .............21
      4.11. Attacks against the NTLP .................................23

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   5. Security Considerations ........................................23
   6. Contributors ...................................................24
   7. Acknowledgements ...............................................24
   8. References .....................................................25
      8.1. Normative References ......................................25
      8.2. Informative References ....................................25

1.  Introduction

   Whenever a new protocol is developed or existing protocols are
   modified, threats to their security should be evaluated.  To address
   security in the NSIS working group, a number of steps have been

      NSIS Analysis Activities (see [RSVP-SEC] and [SIG-ANAL])

      Security Threats for NSIS

      NSIS Requirements (see [RFC3726])

      NSIS Framework (see [RFC4080])

      NSIS Protocol Suite (see GIMPS [GIMPS], NAT/Firewall NSLP

   This document identifies the basic security threats that need to be
   addressed during the design of the NSIS protocol suite.  Even if the
   base protocol is secure, certain extensions may cause problems when
   used in a particular environment.

   This document cannot provide detailed threats for all possible NSIS
   Signaling Layer Protocols (NSLPs).  QoS [QOS-NSLP], NAT/Firewall
   [NATFW-NSLP], and other NSLP documents need to provide a description
   of their trust models and a threat assessment for their specific
   application domain.  This document aims to provide some help for the
   subsequent design of the NSIS protocol suite.  Investigations of
   security threats in a specific architecture or context are outside
   the scope of this document.

   We use the NSIS terms defined in [RFC3726] and in [RFC4080].

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2.  Communications Models

   The NSIS suite of protocols is envisioned to support various
   signaling applications that need to install and/or manipulate state
   at nodes along the data flow path through the network.  As such, the
   NSIS protocol suite involves the communication between different

   This section offers terminology for common communication models that
   are relevant to securing the NSIS protocol suite.

   An abstract network topology with its administrative domains is shown
   in Figure 1, and in Figure 2 the relationship between NSIS entities
   along the path is shown.  For illustrative reasons, only end-to-end
   NSIS signaling is depicted, yet it might be used in other variations
   as well.  Signaling can start at any place and might terminate at any
   other place within the network.  Depending on the trust relationship
   between NSIS entities and the traversed network parts, different
   security problems arise.

   The notion of trust and trust relationship used in this document is
   informal and can best be captured by the definition provided in
   Section 1.1 of [RFC3756].  For completeness we include the definition
   of a trust relationship, which denotes a mutual a priori relationship
   between the involved organizations or parties wherein the parties
   believe that the other parties will behave correctly even in the

   An important observation for NSIS is that a certain degree of trust
   has to be placed into intermediate NSIS nodes along the path between
   an NSIS Initiator and an NSIS Responder, specifically so that they
   perform message processing and take the necessary actions.  A
   complete lack of trust between any of the participating entities will
   cause NSIS signaling to fail.

   Note that it is not possible to describe a trust model completely
   without considering the details and behavior of the NTLP, the NSLP
   (e.g., QoS NSLP), and the deployment environment.  For example,
   securing the communication between an end host (which acts as the
   NSIS Initiator) and the first NSIS node (which might be in the
   attached network or even a number of networks away) is impacted by
   the trust relationships between these entities.  In a corporate
   network environment, a stronger degree of trust typically exists than
   in an unmanaged network.

   Figure 1 introduces convenient abbreviations for network parts with
   similar properties: first-peer, last-peer, intra-domain, or

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

                 Figure 1: Communication patterns in NSIS

   First-Peer/Last-Peer Communication:

      The end-to-end communication scenario depicted in Figure 1
      includes the communication between the end hosts and their nearest
      NSIS hops.  "First-peer communications" 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.
      This "first-peer communications" commonly comes with specific
      security requirements that are especially important for addressing
      security issues between the end host (and a user) and the network
      it is attached to.

      To illustrate this, in roaming environments, it is difficult to
      assume the existence of a pre-established security association
      directly available for NSIS peers involved in first-peer
      communications, because these peers cannot be assumed to have any
      pre-existing relationship with each other.  In contrast, in
      enterprise networks 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 infrastructure already

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      available, and per-session negotiation of security mechanisms is
      often not required (although, in contrast, it is required in a
      roaming environment).

      Last-Peer communication is a variation of First-Peer communication
      in which the roles are reversed.

   Intra-Domain Communication:

      After verification of the NSIS signaling message at the border of
      an administrative domain, an NSIS signaling message traverses the
      network within the same administrative domain to which the first
      peer belongs.  It might not be necessary to repeat the
      authorization procedure of the NSIS initiator again at every NSIS
      node within this domain.  Key management within the administrative
      domain might also be simpler.

      Security protection is still required to prevent threats by
      non-NSIS nodes in this network.

   Inter-Domain Communication:

      Inter-Domain communication deals with the interaction between
      administrative domains.  For some NSLPs (for example, QoS NSLP),
      this interaction is likely to take place between neighboring
      domains, whereas in other NSLPs (such as the NAT/Firewall NSLP),
      the core network is usually not involved.

      If signaling messages are conveyed transparently in the core
      network (i.e., if they are neither intercepted nor processed in
      the core network), then the signaling message communications
      effectively takes place between access networks.  This might place
      a burden on authorization handling and on the key management
      infrastructure required between these access networks, which might
      not know of each other in advance.

   To refine the above differentiation based on the network parts that
   NSIS signaling may traverse, we subsequently consider relationships
   between involved entities.  Because a number of NSIS nodes might
   actively participate in a specific protocol exchange, a larger number
   of possible relationships need to be analyzed than in other
   protocols.  Figure 2 illustrates possible relationships between the
   entities involved in the NSIS protocol suite.

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

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

                   Figure 2: Possible NSIS Relationships

   End-to-Middle Communications:

      The scenario in which one NSIS entity involved is an end-entity
      (Initiator or Responder) and the other entity is any intermediate
      hop other than the immediately adjacent peer is typically called
      the end-to-middle scenario (see Figure 2).  A motivation for
      including this scenario can, for example, be found in SIP

      An example of end-to-middle interaction might be an explicit
      authorization from the NSIS Initiator to some intermediate node.
      Threats specific to this scenario may be introduced by some
      intermediate NSIS hops that are not allowed to eavesdrop or modify
      certain objects.

   Middle-to-Middle Communications:

      Middle-to-middle communication refers to the exchange of
      information between two non-neighboring NSIS nodes along the path.
      Intermediate NSIS hops may have to deal with specific security
      threats that do not involve the NSIS Initiator or the NSIS
      Responder directly.

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   End-to-End Communications:

      NSIS aims to signal information from an Initiator to some NSIS
      nodes along the path to a data receiver.  In the case of
      end-to-end NSIS signaling, the last node is the NSIS Responder, as
      it is the data receiver.  The NSIS protocol suite is not an
      end-to-end protocol used to exchange information purely between
      end hosts.

      Typically, it is not required to protect NSIS messages
      cryptographically between the NSIS Initiator and the NSIS
      Responder.  Protecting the entire signaling message end-to-end
      might not be feasible since intermediate NSIS nodes need to add,
      inspect, modify, or delete objects from the signaling message.

3.  Generic Threats

   This section provides scenarios of threats that are applicable to
   signaling protocols in general.  Note that some of these scenarios
   use the term "user" instead of "NSIS Initiator".  This is mainly
   because security protocols allow differentiation between entities
   that are hosts and those that are users (based on the identifiers

   For the following subsections, we use the general distinction in two
   cases in which attacks may occur.  These are according to the
   separate steps, or phases, normally encountered when applying
   protocol security (with, e.g., IPsec, TLS, Kerberos, or SSH).
   Therefore, this section starts by briefly describing a motivation for
   this separation.

   Security protection of protocols is often separated into two steps.
   The first step primarily provides entity authentication and key
   establishment (which result in a persistent state often called a
   security association), whereas the second step provides message
   protection (some combination of data origin authentication, data
   integrity, confidentiality, and replay protection) using the
   previously established security association.  The first step tends to
   be more expensive than the second, which is the main reason for the
   separation.  If messages are transmitted infrequently, then these two
   steps may be collapsed into a single and usually rather costly one.
   One such example is e-mail protection via S/MIME.  The two steps may
   be tightly bound into a single protocol, as in TLS, or defined in
   separate protocols, as with IKE and IPsec.  We use this separation to
   cover the different threats in more detail.

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3.1.  Man-in-the-Middle Attacks

   This section describes both security threats that exist if two peers
   do not already share a security association or do not use security
   mechanisms at all, and threats that are applicable when a security
   association is already established.

   Attacks during NSIS SA Establishment:

      While establishing a security association, an adversary fools the
      signaling message Initiator with respect to the entity to which it
      has to authenticate.  The Initiator authenticates to the man-in-
      the-middle adversary, who is then able to modify signaling
      messages to mount DoS attacks or to steal services that get billed
      to the Initiator.  In addition, the adversary may be able to
      terminate the Initiator's NSIS messages and to inject messages to
      a peer itself, thereby acting as the peer to the Initiator and as
      the Initiator to the peer.  As a result, the Initiator wrongly
      believes that it is talking to the "real" network, whereas it is
      actually attached to an adversary.  For this attack to be
      successful, pre-conditions that are described in the following
      three cases have to hold:

      Missing Authentication:

         In the first case, this threat can be carried out because of
         missing authentication between neighboring peers: without
         authentication, an NI, NR, or NF is unable to detect an
         adversary.  However, in some practical cases, authentication
         might be difficult to accomplish, either because the next peer
         is unknown, because there are 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 communicating endpoints is unknown, then
         for some security mechanisms it is either impossible or
         impractical to apply appropriate security protection.
         Sometimes network administrators use intra-domain signaling
         messages without proper security.  This configuration allows 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 those caused by malicious NSIS nodes
         (described in Section 4.7).

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      Unilateral Authentication:

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

         Missing or unilateral authentication, as described above, is
         part of a general problem of network access with inadequate
         authentication, and it should not be considered something
         unique to the NSIS signaling protocol.  Obviously, there is a
         strong need to address this correctly in a future NSIS protocol
         suite.  The signaling protocols addressed by NSIS are different
         from other protocols in which only two entities are involved.
         Note that first-peer authentication is especially important
         because a security breach there could impact nodes beyond the
         entities directly involved (or even beyond a local network).

         Finally, note that the signaling protocol should be considered
         a peer-to-peer protocol, wherein the roles of Initiator and
         Responder can be reversed at any time.  Thus, unilateral
         authentication is not particularly useful for such a protocol.
         However, some form of asymmetry might be needed in the
         authentication process, whereby one entity uses an
         authentication mechanism different from that of the other one.
         As an example, the combination of symmetric and asymmetric
         cryptography should be mentioned.

      Weak Authentication:

         In the case of weak authentication, the threat can be carried
         out because information transmitted during the NSIS SA
         establishment process may leak passwords or allow offline
         dictionary attacks.  This threat is applicable to NSIS for the
         process of selecting certain security mechanisms.

   Finally, we conclude with a description of a man-in-the-middle (MITM)
   attack during the discovery phase.  This attack benefits from the
   fact that NSIS nodes are likely to be unaware of the network

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   topology.  Furthermore, an authorization problem might arise if an
   NSIS QoS NSLP node pretends to be an NSIS NAT/Firewall-specific node
   or vice versa.

   An adversary might inject a bogus reply message, forcing the
   discovery message initiator to start a messaging association
   establishment with either an adversary or with another NSIS node that
   is not along the path.  Figure 3 describes the attack in more detail
   for peer-to-peer addressed messages with a discovery mechanism.  For
   end-to-end addressed messages, the attack is also applicable,
   particularly if the adversary is located along the path and able to
   intercept the discovery message that traverses the adversary.  The
   man-in-the-middle adversary might redirect to another legitimate NSIS
   node.  A malicious NSIS node can be detected with the corresponding
   security mechanisms, but a legitimate NSIS node that is not the next
   NSIS node along the path cannot be detected without topology

                      +-----------+   Messaging Association
     Message          | Adversary |   Establishment
     Association +--->+           +<----------------+
     Establish-  |    +----+------+                 |(4)
      ment       |     IPx |                        |
              (3)|         |Discovery Reply         v
                 |         | (IPx)              +---+-------+
                 v         |  (2)               |  NSIS     |
          +------+-----+   |       /----------->+  Node B   +--------
          | NSIS       +<--+      / Discovery   +-----------+
          | Node A     +---------/  Request          IPr
          +------------+             (1)

            Figure 3: MITM Attack during the Discovery Exchange

   This attack assumes that the adversary is able to eavesdrop on the
   initial discovery message sent by the sender of the discovery
   message.  Furthermore, we assume that the discovery reply message by
   the adversary returns to the discovery message initiator faster than
   the real response.  This represents some race condition
   characteristics if the next NSIS node is very close (in IP-hop terms)
   to the initiator.  Note that the problem is self-healing since the
   discovery process is periodically repeated.  If an adversary is
   unable to mount this attack with every discovery message, then the
   correct next NSIS node along the path will be discovered again.  A
   ping-pong behavior might be the consequence.

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   As shown in message step (2) in Figure 3, the adversary returns a
   discovery reply message with its own IP address as the next NSIS-
   aware node along the path.  Without any additional information, the
   discovery message initiator has to trust this information.  Then a
   messaging association is established with an entity at a given IP
   address IPx (i.e., with the adversary) in step (3).  The adversary
   then establishes a messaging association with a further NSIS node and
   forwards the signaling message.  Note that the adversary might just
   modify the Discovery Reply message to force NSIS Node A to establish
   a messaging association with another NSIS node that is not along the
   path.  This can then be exploited by the adversary.  The interworking
   with NSIS-unaware NATs in particular might cause additional
   unexpected problems.

   As a variant of this attack, an adversary not able to eavesdrop on
   transmitted discovery requests could flood a node with bogus
   discovery reply messages.  If the discovery message sender
   accidentally accepts one of those bogus messages, then a MITM attack
   as described in Figure 3 is possible.

3.2.  Replay of Signaling Messages

   This threat scenario covers the case in which an adversary
   eavesdrops, collects signaling messages, and replays them at a later
   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 if there is
   replay protection) requires that the adversary crash an NSIS-aware
   node, causing it to lose state information (sequence numbers,
   security associations, etc.), and then replay old signaling messages.
   This attack takes advantage of re-synchronization deficiencies.

3.3.  Injecting or Modifying Messages

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

   An adversary may inject a signaling message requesting a large amount
   of resources (possibly using a different user's identity).  Other
   resource requests may then be rejected.  In combination with identity

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   spoofing, it is possible to carry out fraud.  This attack is only
   feasible in the absence of authentication and signaling message

   Some threats directly related to these are described in Sections 4.4,
   4.7, and 4.8.

3.4.  Insecure Parameter Exchange and Negotiation

   First, protocols may be useful in a variety of scenarios with
   different security requirements.  Second, different users (e.g., a
   university, a hospital, a commercial enterprise, or a government
   ministry) have inherently different security requirements.  Third,
   different parts of a network (e.g., within a building, across a
   public carrier's network, or over a private microwave link) may need
   different levels of protection.  It is often difficult to meet these
   (sometimes conflicting) requirements with a single security mechanism
   or fixed set of security parameters, so often a selection of
   mechanisms and parameters is offered.  Therefore, a protocol is
   required to agree on certain security mechanisms and parameters.  An
   insecure parameter exchange or security negotiation protocol can help
   an adversary to mount a downgrading attack to force selection of
   mechanisms weaker than those mutually desired.  Thus, without binding
   the negotiation process to the legitimate parties and protecting it,
   an NSIS protocol suite might only be as secure as the weakest
   mechanism provided (e.g., weak authentication), and the benefits of
   defining configuration parameters and a negotiation protocol are

4.  NSIS-Specific Threat Scenarios

   This section describes eleven threat scenarios in terms of attacks on
   and security deficiencies in the NSIS signaling protocol.  A number
   of security deficiencies might enable an attack.  Fraud is an example
   of an attack that might be enabled by missing replay protection,
   missing protection of authorization tokens, identity spoofing,
   missing authentication, and other deficiencies that help an adversary
   steal resources.  Different threat scenarios based on deficiencies
   that could enable an attack are addressed in this section.

   The threat scenarios are not independent.  Some of them (e.g., denial
   of service) are well-established security terms and, as such, need to
   be addressed, but they are often enabled by one or more deficiencies
   described under other scenarios.

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4.1.  Threats during NSIS SA Usage

   Once a security association is established (and used) to protect
   signaling messages, many basic attacks are prevented.  However, a
   malicious NSIS node is still able to perform various attacks as
   described in Section 4.7.  Replay attacks may be possible when an
   NSIS node crashes, restarts, and performs state re-establishment.
   Proper re-synchronization of the security mechanism must therefore be
   provided to address this problem.

4.2.  Flooding

   This section describes attacks that allow an adversary to flood an
   NSIS node with bogus signaling messages to cause a denial of service

   We will discuss this threat at different layers in the NSIS protocol

   Processing of Router Alert Options:

      The processing of Router Alert Option (RAO) requires that a router
      do some additional processing by intercepting packets with IP
      options, which might lead to additional delay for legitimate
      requests, or even rejection of some of them.  A router being
      flooded with a large number of bogus messages requires resources
      before finding out that these messages have to be dropped.

      If the protocol is based on using interception for message
      delivery, this threat cannot be completely eliminated, but the
      protocol design should attempt to limit the processing that has to
      be done on the RAO-bearing packet so that it is as similar as
      possible to that for an arbitrary packet addressed directly to one
      of the router interfaces.

   Attacks against the Transport Layer Protocol:

      Certain attacks can be mounted against transport protocols by
      flooding a node with bogus requests, or even to finish the
      handshake phase to establish a transport layer association.  These
      types of threats are also addressed in Section 4.11.

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   Force NTLP to Do More Processing:

      Some protocol fields might allow an adversary to force an NTLP
      node to perform more processing.  Additionally it might be
      possible to interfere with the flow control or the congestion
      control procedure.  These types of threats are also addressed in
      Section 4.11.

      Furthermore, it might be possible to force the NTLP node to
      perform some computations or signaling message exchanges by
      injecting "trigger" events (which are unprotected).

   Force NSLP to Do More Processing:

      An adversary might benefit from flooding an NSLP node with
      messages that must be stored (e.g., due to fragmentation handling)
      before verifying the correctness of signaling messages.

      Furthermore, causing memory allocation and computational efforts
      might allow an adversary to harm NSIS entities.  If a signaling
      message contains, for example, a digital signature, then some
      additional processing is required for the cryptographic
      verification.  An adversary can easily create a random bit
      sequence instead of a digital signature to force an NSIS node into
      heavy computation.

      Idempotent signaling messages are particularly vulnerable to this
      type of attack.  The term "idempotent" refers to messages that
      contain the same amount of information as the original message.
      An example would be a refresh message that is equivalent to a
      create message.  This property allows a refresh message to create
      state along a new path, where no previous state is available.  For
      this to work, specific classes of cryptographic mechanisms
      supporting this behavior are needed.  An example is a scheme based
      on digital signatures, which, however, should be used with care
      due to possible denial of service attacks.

      Problems with the usage of public-key-based cryptosystems in
      protocols are described in [AN97] and in [ALN00].

      In addition to the threat scenario described above, an incoming
      signaling message might trigger communication with third-party
      nodes such as policy servers, LDAP servers, or AAA servers.  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
      other reservation messages from legitimate users.

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4.3.  Eavesdropping and Traffic Analysis

   This section covers threats whereby an adversary is able to eavesdrop
   on signaling messages.  The signaling packets collected may allow
   traffic analysis or be used later to mount replay attacks, as
   described in Section 3.2.  The eavesdropper might learn QoS
   parameters, communication patterns, policy rules for firewall
   traversal, policy information, application identifiers, user
   identities, NAT bindings, authorization objects, network
   configuration and performance information, and more.

   An adversary's capability to eavesdrop on signaling messages might
   violate a user's preference for privacy, particularly if unprotected
   authentication or authorization information (including policies and
   profile information) is exchanged.

   Because the NSIS protocol signals messages through a number of nodes,
   it is possible to differentiate between nodes actively participating
   in the NSIS protocol and those that do not.  For certain objects or
   messages, it might be desirable to permit actively participating
   intermediate NSIS nodes to eavesdrop.  On the other hand, it might be
   desirable that only the intended end points (NSIS Initiator and NSIS
   Responder) be able to read certain other objects.

4.4.  Identity Spoofing

   Identity spoofing relevant for NSIS occurs in three forms: First,
   identity spoofing can happen during the establishment of a security
   association based on a weak authentication mechanism.  Second, an
   adversary can modify the flow identifier carried within a signaling
   message.  Third, it can spoof data traffic.

   In the first case, Eve, acting as an adversary, may claim to be the
   registered user Alice by spoofing Alice's identity.  Eve thereby
   causes the network to charge Alice for the network resources
   consumed.  This type of attack is possible if authentication is 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.

   In the second case, an adversary may be able to exploit the
   established flow identifiers (required for QoS and NAT/FW NSLP).
   These identifiers are, among others, IP addresses, transport protocol
   type (UDP, TCP), port numbers, and flow labels (see [RFC1809] and
   [RFC3697]).  Modification of these flow identifiers allows
   adversaries to exploit or to render ineffective quality of service

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   reservations or policy rules at middleboxes.  An adversary could
   mount an attack by modifying the flow identifier of a signaling

   In the third case, an adversary may spoof data traffic.  NSIS
   signaling messages contain some sort of flow identifier that is
   associated with a specified behavior (e.g., a particular flow
   experiences QoS treatment or allows packets to traverse a firewall).
   An adversary might, therefore, use IP spoofing and inject data
   packets to benefit from previously installed flow identifiers.

   We will provide an example of the latter threat.  After NSIS nodes
   along the path between the NSIS initiator and the NSIS receiver
   processes a properly protected reservation request, transmitted by
   the legitimate user Alice, a QoS reservation is installed at the
   corresponding NSIS nodes (for example, the edge router).  The flow
   identifier is used for flow identification and allows data traffic
   originated from a given source to be assigned to this QoS
   reservation.  The adversary Eve now spoofs Alice's IP address.  In
   addition, Alice's host may be crashed by the adversary with a denial
   of service attack or may lose connectivity (for example, because of
   mobility).  If Eve is able to perform address spoofing, then she is
   able to receive and transmit data (for example, RTP data traffic)
   that receives preferential QoS treatment based on the previous
   reservation.  Depending on the installed flow identifier granularity,
   Eve might have more possibilities to exploit the QoS reservation or a
   pin-holed firewall.  Assuming the soft state paradigm, whereby
   periodic refresh messages are required, Alice's absence will not be
   detected until a refresh message is required, forcing Eve to respond
   with a protected signaling message.  Again, this attack is applicable
   not only to QoS traffic, but also to a Firewall control protocol,
   with a different consequence.

   The ability for an adversary to inject data traffic that matches a
   certain flow identifier established by a legitimate user and to get
   some benefit from injecting that traffic often also requires the
   ability to receive the data traffic or to have one's correspondent
   receive it.  For example, an adversary in an unmanaged network
   observes a NAT/Firewall signaling message towards a corporate
   network.  After the signaling message exchange was successful, the
   user Alice is allowed to traverse the company firewall based on the
   establish packet filter in order to contact her internal mail server.
   Now, the adversary Eve, who was monitoring the signaling exchange, is
   able to build a data packet towards this mail server that will pass
   the company firewall.  The packet will hit the mail server and cause
   some actions, and the mail server will reply with some response
   messages.  Depending on the exact location of the adversary and the

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   degree of routing asymmetry, the adversary might even see the
   response messages.  Note that for this attack to work, Alice does not
   need to participate in the exchange of signaling messages.

   We could imagine using attributes of a flow identifier that is not
   related to source and destination addresses.  For example, we could
   think of a flow identifier for which only the 21-bit Flow ID is used
   (without source and destination IP address).  Identity spoofing and
   injecting traffic is much easier since a packet only needs to be
   marked and an adversary can use a nearly arbitrary endpoint
   identifier to achieve the desired result.  Obviously, though, the
   endpoint identifiers are not irrelevant, because the messages have to
   hit some nodes in the network where NSIS signaling messages installed
   state (in the above example, they would have to hit the same

   Data traffic marking based on DiffServ is such an example.  Whenever
   an ingress router uses only marked incoming data traffic for
   admission control procedures, various attacks are possible.  These
   problems have been known in the DiffServ community for a long time
   and have been 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.

4.5.  Unprotected Authorization Information

   Authorization is an important criterion for providing resources such
   as QoS reservations, NAT bindings, and pinholes through 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 (as described in [RFC3182], for example).
   Depending on the chosen authentication protocol, certain threats may
   exist.  Section 3 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 [RFC3521].

   Achieving secure interaction between different protocols based on
   authorization tokens, however, requires some care.  By 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)

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   to steal resources.  An adversary might also use the token to monitor
   communication patterns.  Finally, an untrustworthy end host might
   also modify the token content.

   The Session/Reservation Ownership problem can also be regarded as an
   authorization problem.  Details are described in Section 4.10.  In
   enterprise networks, authorization is often coupled with membership
   in a particular class of users or groups.  This type of information
   either can 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 to determining authorization or the outcome of the
   authorization process itself, then theft of service might be

4.6.  Missing Non-Repudiation

   Signaling for QoS often involves three parties: the user, a network
   that offers QoS reservations (referred to as "service provider") and
   a third party that guarantees that the party making the reservation
   actually receives a financial compensation (referred to as "trusted
   third party").

   In this context,"repudiation" refers to a problem where either the
   user or the service provider later deny the existence or some
   parameters (e.g., volume or price) of a QoS reservation towards the
   trusted third party.  Problems stemming from a lack of non-
   repudiation appear in two forms:

   Service provider's point-of-view:
      A user may deny having issued a reservation request for which it
      was charged.  The service provider may then want to be able to
      prove that a particular user issued the reservation request in

   User's point-of-view:
      A service provider may claim to have received a number of
      reservation requests from a particular user.  The user in question
      may want to show that such reservation requests have never been
      issued and may want to see correct service usage records for a
      given set of QoS parameters.

   In today's networks, non-repudiation is not provided.  Therefore, it
   might be difficult to introduce with NSIS signaling.  The user has to
   trust the network operator to meter the traffic correctly, to collect
   and merge accounting data, and to ensure that no unforeseen problems

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   occur.  If a signaling protocol with the non-repudiation property is
   desired for establishing QoS reservations, then it certainly impacts
   the protocol design.

   Non-repudiation functionality places additional requirements on the
   security mechanisms.  Thus, a solution would normally increase the
   overhead of a security solution.  Threats related to missing non-
   repudiation are only considered relevant in certain specific
   scenarios and for specific NSLPs.

4.7.  Malicious NSIS Entity

   Network elements within a domain (intra-domain) experience a
   different trust relationship with regard to the security protection
   of signaling messages from that of edge NSIS entities.  It is assumed
   that edge NSIS entities are responsible for performing cryptographic
   processing (authentication, integrity and replay protection,
   authorization, and accounting) for signaling messages arriving from
   the outside.  This prevents unprotected signaling messages from
   appearing within the internal network.  If, however, an adversary
   manages to take over an edge router, then the security of the entire
   network is compromised.  An adversary is then able to launch a number
   of attacks, including denial of service; integrity violations; replay
   and reordering of objects and messages; bundling of messages;
   deletion of data packets; and various others.  A rogue firewall can
   harm other firewalls by modifying policy rules.  The chain-of-trust
   principle applied in peer-to-peer security protection cannot protect
   against a malicious NSIS node.  An adversary with access to an NSIS
   router is also able to get access to security associations and to
   transmit secured signaling messages.  Note that even non-peer-to-peer
   security protection might not be able to prevent this problem fully.
   Because an NSIS node might issue signaling messages on behalf of
   someone else (by acting as a proxy), additional problems need to be

   An NSIS-aware edge router is a critical component that requires
   strong security protection.  A strong security policy applied at the
   edge does not imply that other routers within an intra-domain network
   do not need to verify signaling messages cryptographically.  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 only as strong as the weakest link.  In the
   case under consideration, the edge router is the most critical
   component of this network, and it may also act as a security gateway
   or firewall for incoming and outgoing traffic.  For outgoing traffic,
   this device has to implement the security policy of the local domain
   and to apply the appropriate security protection.

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   For an adversary to mount this attack, either an existing NSIS-aware
   node along the path has to be attacked successfully, or an adversary
   must succeed in convincing another NSIS node to make it the next NSIS
   peer (man-in-the-middle attack).

4.8.  Denial of Service Attacks

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

   Path Finding:

      Some signaling protocols establish state (e.g., routing state) and
      perform some actions (e.g., querying resources) at a number of
      NSIS nodes without requiring authorization (or even proper
      authentication) based on a single message (e.g., PATH message in

      An adversary can utilize this fact to transmit a large number of
      signaling messages to allocate state at nodes along the path and
      to cause resource consumption.

      An NSIS responder might not be able to determine the NSIS
      initiator and might even tend to respond to such a signaling
      message with a corresponding reservation message.

   Discovery Phase:

      Conveying 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 because it is
      difficult to secure.  An adversary can use the discovery
      mechanisms to convince one entity to signal information to another
      entity that is not along the data path, or to cause the discovery
      process to fail.  In the first case, the signaling protocol could
      appear to continue correctly, except that policy rules are
      installed at the 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.

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   Faked Error or Response Messages:

      An adversary may be able to inject false error or response
      messages as part of a DoS attack.  This could be at the signaling
      message protocol layer (NTLP), the layer of each client layer
      protocol (e.g., QoS NSLP or NAT/Firewall NSLP), or the transport
      protocol layer.  An adversary might cause unexpected protocol
      behavior or might succeed with a DoS attack.  The discovery
      protocol, especially, exhibits vulnerabilities with regard to this
      threat scenario (see the above discussion on discovery).  If no
      separate discovery protocol is used and signaling messages are
      addressed to end hosts only (with a Router Alert Option to
      intercept message as NSIS aware nodes), an error message might be
      used to indicate a path change.  Such a design combines a
      discovery protocol with a signaling message exchange protocol.

4.9.  Disclosing the Network Topology

   In some organizations or enterprises there is a desire not to reveal
   internal network structure (or other related information) outside of
   a closed community.  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, what capabilities
   nodes along a path have, etc.).  Discovery messages, traceroute,
   diagnostic messages (see [RFC2745] for a description of diagnostic
   message functionality for RSVP), and query messages, in addition to
   record route and route objects, provide potential assistance to an
   adversary.  Thus, the requirement of not disclosing a network
   topology might conflict with other requirements to provide means for
   discovering NSIS-aware nodes automatically or to provide diagnostic
   facilities (used for network monitoring and administration).

4.10.  Unprotected Session or Reservation Ownership

   Figure 4 shows an NSIS Initiator that has established state
   information at NSIS nodes along a path as part of the signaling
   procedure.  As a result, Access Router 1, Router 3, and Router 4 (and
   other nodes) have stored session-state information, including the
   Session Identifier SID-x.

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                                             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 or Reservation Ownership

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

   If an adversary were able to obtain the Session Identifier (for
   example, by eavesdropping on signaling messages), it would be able to
   add the same Session Identifier SID-x to a new signaling message.
   When the new signaling message hits Router 3 (as shown in Figure 4),
   existing state information can be modified.  The adversary can then
   modify or delete the established reservation and cause unexpected
   behavior for the legitimate user.

   The source of the problem is that Router 3 (a cross-over router) is
   unable to decide whether the new signaling message was initiated from
   the owner of the session or reservation.

   In addition, nodes other than the initial signaling message
   originator are 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 initiate a
   signaling message exchange.  It might, for example, be necessary to
   provide mobility support or to trigger a local repair procedure.  If
   only the initial signaling message originator were allowed to trigger
   signaling message exchanges, some protocol behavior would not be

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   If this threat scenario is not addressed, an adversary can launch
   DoS, theft of service, and various other attacks.

4.11.  Attacks against the NTLP

   In [2LEVEL], a two-level architecture is proposed, that would split
   an NSIS protocol into layers: a signaling message transport-specific
   layer and an application-specific layer.  This is further developed
   in the NSIS Framework [RFC4080].  Most of the threats described in
   this threat analysis are applicable to the NSLP application-specific
   part (e.g., QoS NSLP).  There are, however, some threats that are
   applicable to the NTLP.

   Network and transport layer protocols lacking protection mechanisms
   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.

   Threats that address parts of the NTLP that are not related to
   attacks against the use of transport layer protocols are covered in
   various sections throughout this document, such as Section 4.2.

   If existing transport layer protocols are used for exchanging NSIS
   signaling messages, security vulnerabilities known for these
   protocols need to be considered.  A detailed threat description of
   these protocols is outside the scope of this document.

5.  Security Considerations

   This entire memo discusses security issues relevant for NSIS protocol
   design.  It begins by identifying the components of a network running
   NSIS (Initiator, Responder, and different Administrative Domains
   between them).  It then considers five cases in which communications
   take place between these components, and it examines the trust
   relationships presumed to exist in each case: First-Peer
   Communications, End-to-Middle Communications, Intra-Domain
   Communications, Inter-Domain Communications, and End-to-End
   Communications.  This analysis helps determine the security needs and
   the relative seriousness of different threats in the different cases.

   The document points out the need for different protocol security
   measures: authentication, key exchange, message integrity, replay
   protection, confidentiality, authorization, and some precautions
   against denial of service.  The threats are subdivided into generic
   ones (e.g., man-in-the-middle attacks, replay attacks, tampering and
   forgery, and attacks on security negotiation protocols) and eleven
   threat scenarios that are particularly applicable to the NSIS

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   protocol.  Denial of service, for example, is covered in the
   NSIS-specific section, not because it cannot be carried out against
   other protocols, but because the methods used to carry out denial of
   service attacks tend to be protocol specific.  Numerous illustrative
   examples provide insight into what can happen if these threats are
   not mitigated.

   This document repeatedly points out that not all of the threats are
   equally serious in every context.  It does attempt to identify the
   scenarios in which security failures may have the highest impact.
   However, it is difficult for the protocol designer to foresee all the
   ways in which NSIS protocols will be used or to anticipate the
   security concerns of a wide variety of likely users.  Therefore, the
   protocol designer needs to offer a full range of security
   capabilities and ways for users to negotiate and select what they
   need, on a case-by-case basis.  To counter these threats, security
   requirements have been listed in [RFC3726].

6.  Contributors

   We especially thank Richard Graveman, who provided text for the
   security considerations section, as well as a detailed review of the

7.  Acknowledgements

   We would like to thank (in alphabetical order) Marcus Brunner, Jorge
   Cuellar, Mehmet Ersue, Xiaoming Fu, and Robert Hancock for their
   comments on an initial version of this document.  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, Cedric Aoun, John Loughney, Rene Soltwisch, Cornelia Kappler,
   Ted Wiederhold, Vishal Sankhla, Mohan Parthasarathy, and Andrew
   McDonald provided comments on more recent versions of this document.
   Their input helped improve the content of this document.  Roland
   Bless, Michael Thomas, Joachim Kross, and Cornelia Kappler, in
   particular, provided good proposals for regrouping and restructuring
   the material.

   A final review was given by Michael Richardson.  We thank him for his
   detailed comments.

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8.  References

8.1.  Normative References

   [RFC4080]     Hancock, R., Karagiannis, G., Loughney, J., and S. van
                 den Bosch, "Next Steps in Signaling (NSIS): Framework",
                 RFC 4080, June 2005.

   [RFC3726]     Brunner, M., "Requirements for Signaling Protocols",
                 RFC 3726, April 2004.

8.2.  Informative References

   [ALN00]       Aura, T., Leiwo, J., and P. Nikander, "Towards Network
                 Denial of Service Resistant Protocols, In Proceedings
                 of the 15th International Information Security
                 Conference (IFIP/SEC 2000), Beijing, China",
                 August 2000.

   [AN97]        Aura, T. and P. Nikander, "Stateless Connections", In
                 Proceedings of the International Conference on
                 Information and Communications Security (ICICS'97),
                 Lecture Notes in Computer Science 1334, Springer",

   [2LEVEL]      Braden, R. and B. Lindell, "A Two-Level Architecture
                 for Internet Signaling", Work in Progress,
                 November 2002.

   [RFC3697]     Rajahalme, J., Conta, A., Carpenter, B., and S.
                 Deering, "IPv6 Flow Label Specification", RFC 3697,
                 March 2004.

   [NATFW-NSLP]  Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer
                 Protocol (NSLP)", Work in Progress, February 2005.

   [GIMPS]       Schulzrinne, H., "GIMPS: General Internet Messaging
                 Protocol for Signaling", Work in Progress,
                 February 2005.

   [QOS-NSLP]    Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for
                 Quality-of-Service signaling", Work in Progress,
                 February 2005.

   [RSVP-SEC]    Tschofenig, H., "RSVP Security Properties", Work in
                 Progress, February 2005.

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RFC 4081               Security Threats for NSIS               June 2005

   [SIG-ANAL]    Manner, J. and X. Fu, "Analysis of Existing Quality-
                 of-Service Signaling Protocols", RFC 4094, May 2005.

   [RFC1809]     Partridge, C., "Using the Flow Label Field in IPv6",
                 RFC 1809, June 1995.

   [RFC2745]     Terzis, A., Braden, B., Vincent, S., and L. Zhang,
                 "RSVP Diagnostic Messages", RFC 2745, January 2000.

   [RFC3182]     Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore,
                 T., Herzog, S., and R. Hess, "Identity Representation
                 for RSVP", RFC 3182, October 2001.

   [RFC3261]     Rosenberg, J., Schulzrinne, H., Camarillo, G.,
                 Johnston, A., Peterson, J., Sparks, R., Handley, M.,
                 and E.  Schooler, "SIP: Session Initiation Protocol",
                 RFC 3261, June 2002.

   [RFC3520]     Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh,
                 "Session Authorization Policy Element", RFC 3520,
                 April 2003.

   [RFC3521]     Hamer, L-N., Gage, B., and H. Shieh, "Framework for
                 Session Set-up with Media Authorization", RFC 3521,
                 April 2003.

   [RFC3756]     Nikander, P., Kempf, J., and E. Nordmark, "IPv6
                 Neighbor Discovery (ND) Trust Models and Threats",
                 RFC 3756, May 2004.

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Authors' Addresses

   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739

   EMail: Hannes.Tschofenig@siemens.com

   Dirk Kroeselberg
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739

   EMail: Dirk.Kroeselberg@siemens.com

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Full Copyright Statement

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