Internet Engineering Task Force NSIS
Internet Draft H. Tschofenig
D. Kroeselberg
Siemens
Document:
draft-ietf-nsis-threats-02.txt
Expires: December 2003 June 2003
Security Threats for NSIS
<draft-ietf-nsis-threats-02.txt>
Status of this Memo
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Abstract
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
Acknowledgments..................................................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 |
+-----------+ +-----------+
Legend:
-----: 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
management.
Signaling information other than QoS service parameters such as
policy rules in case of middlebox communication demands different
assumptions for inter-domain communication. Trust assumptions and
business relationships are of particular importance for their
communication.
If signaling messages are 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
mentioned.
- Weak Authentication
This threat addresses weak authentication mechanisms whereby
information transmitted during the NSIS SA establishment process may
leak passwords and/or may allow offline dictionary attacks. This
threat is 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
5.8.
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
attack.
When a signaling message arrives at a NSIS aware network element some
processing is required. If this message contains security objects
such as digital signatures and 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
service.
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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
protection.
For an adversary to mount this attack either an existing NSIS aware
node along the path has to be successfully attacked or an adversary
succeeds to convince another NSIS node to be the next NSIS peer (man-
in-the-middle attack).
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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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-
x.
Session ID(SID-x)
+--------+
+-----------------+ Router +------------>
Session ID(SID-x)| | 4 |
+---+----+ +--------+
| Router |
+------+ 3 +*******
| +---+----+ *
| *
| Session ID(SID-x) * Session ID(SID-x)
+---+----+ +---+----+
| Access | | Access |
| Router | | Router |
| 1 | | 2 |
+---+----+ +---+----+
| *
| Session ID(SID-x) * Session ID(SID-x)
+----+------+ +----+------+
| NSIS | | Adversary |
| Initiator | | |
+-----------+ +-----------+
Figure 4: Session/Reservation Ownership
The Session Identifier is included in signaling messages to reference
to the established state.
If an adversary was able to obtain the Session Identifier for example
by eavesdropping signaling messages it is able to add the same
Session Identifier SID-x to a new a signaling message. When the
signaling message hits Router3 (as shown in Figure 3) then existing
state information can be modified. The adversary can then modify or
delete the established reservation causing unexpected behavior for
the legitimate 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
relevant.
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
[HF+03].
7. Normative References
[Brun03] M. Brunner, "Requirements for QoS signaling protocols,"
Internet Draft, Internet Engineering Task Force, June 2003. Work in
progress.
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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.
Acknowledgments
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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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
Germany
EMail: Hannes.Tschofenig@siemens.com
Dirk Kroeselberg
Siemens AG
Otto-Hahn-Ring 6
81739 Munich
Germany
EMail: Dirk.Kroeselberg@siemens.com
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Acknowledgement
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Tschofenig, Kroeselberg Expires - December 2003 [Page 22]
