Security Threats for Next Steps in Signaling (NSIS)
draft-ietf-nsis-threats-06
The information below is for an old version of the document that is already published as an RFC.
| Document | Type | RFC Internet-Draft (nsis WG) | |
|---|---|---|---|
| Authors | Dirk Kroeselberg , Hannes Tschofenig | ||
| Last updated | 2013-03-02 (Latest revision 2004-10-26) | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | RFC 4081 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Allison J. Mankin | ||
| Send notices to | john.loughney@nokia.com |
draft-ietf-nsis-threats-06
NSIS Working Group H. Tschofenig
Internet-Draft D. Kroeselberg
Expires: April 24, 2005 Siemens
October 24, 2004
Security Threats for NSIS
draft-ietf-nsis-threats-06.txt
Status of this Memo
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of section 3 of RFC 3667. By submitting this Internet-Draft, each
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This Internet-Draft will expire on April 24, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This threats document provides a detailed analysis of the security
threats relevant to the 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Communications Models . . . . . . . . . . . . . . . . . . . 4
3. Generic Threats . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 9
3.2 Replay of Signaling Messages . . . . . . . . . . . . . . . 13
3.3 Injecting or Modifying Messages . . . . . . . . . . . . . 13
3.4 Insecure Parameter Exchange and Negotiation . . . . . . . 13
4. NSIS-Specific Threat Scenarios . . . . . . . . . . . . . . . 15
4.1 Threats during NSIS SA Usage . . . . . . . . . . . . . . . 15
4.2 Flooding . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3 Eavesdropping and Traffic Analysis . . . . . . . . . . . . 17
4.4 Identity Spoofing . . . . . . . . . . . . . . . . . . . . 17
4.5 Unprotected Authorization Information . . . . . . . . . . 19
4.6 Missing Non-Repudiation . . . . . . . . . . . . . . . . . 20
4.7 Malicious NSIS Entity . . . . . . . . . . . . . . . . . . 21
4.8 Denial of Service Attacks . . . . . . . . . . . . . . . . 22
4.9 Disclosing the Network Topology . . . . . . . . . . . . . 23
4.10 Unprotected Session or Reservation Ownership . . . . . . 23
4.11 Attacks against the NTLP . . . . . . . . . . . . . . . . 25
5. Security Considerations . . . . . . . . . . . . . . . . . . 26
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 29
8.2 Informative References . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . 32
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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
taken:
NSIS Analysis Activities (see [I-D.ietf-nsis-rsvp-sec-properties]
and [I-D.ietf-nsis-signalling-analysis])
Security Threats for NSIS
NSIS Requirements (see [RFC3726])
NSIS Framework (see [I-D.ietf-nsis-fw])
NSIS Protocol Suite (see GIMPS [I-D.ietf-nsis-ntlp], NAT/Firewall
NSLP [I-D.ietf-nsis-nslp-natfw] and QoS NSLP
[I-D.ietf-nsis-qos-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 [I-D.ietf-nsis-qos-nslp],
NAT/Firewall 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 specifc architecture or context are outside the
scope of this document.
We use the NSIS terms defined in [RFC3726] and in [I-D.ietf-nsis-fw].
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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
entities.
This section offers terminology for common communication models,
which 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 illustrated. For illustrative reasons only
end-to-end NSIS signaling is depicted but 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 be best 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 where the parties
believe that the other parties will behave correctly even in the
future.
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 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.
Please note that it is not possible to completely describe a trust
model 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
inter-domain.
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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. For enterprise
networks, in contrast, the situation is different. Usually there
is a fairly strong (pre-established) trust relationship between
the peers. Enterprise network administrators usually have some
degree of freedom to select the appropriate security protection
and to enforce it. The choice of selecting a security mechanism
is therefore often influenced by the already available
infrastructure, and per-session negotiation of security mechanisms
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is often not required (which, in contrast, is required in a
roaming environment).
Last-Peer communication is a variation of First-Peer communication
where 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., 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. Since 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 |
+-----------+ +-----------+
Legend:
-----: 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
[RFC3261].
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 which 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, which do not directly involve the NSIS Initiator or the
NSIS Responder.
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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 case of end-to-end
NSIS signaling the last node is the NSIS Responder as 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 cryptographically protect NSIS
messages between the NSIS Initiator and the NSIS Responder.
Protecting the entire signaling message end-to-end is not feasible
since intermediate NSIS nodes need to add, inspect, modify or
delete objects from the signaling message.
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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 as hosts
and as users (based on the identifiers used).
For the following subsections, we use the general distinction into
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 with a brief motivation for this
separation.
Security protection of protocols is often separated into two steps.
The first step provides primarily 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.
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 steal services that get billed to
the Initiator. In addition, it may be able to terminate the
Initiator's NSIS messages 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
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it is talking 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 in the following
three cases:
Missing Authentication:
In the first case, this threat can be carried out because of
missing authentication between neighboring peers: without
authentication a 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 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
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. 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 those caused by malicious NSIS nodes
(described in Section 4.7).
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 correctly address this in a future NSIS protocol
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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 here could impact nodes beyond the
entities directly involved (or even beyond a local network).
Finally it should be noted that the signaling protocol should
be considered as a peer-to-peer protocol, where the roles of
Initiator and Responder can be reversed at any time. Hence,
unilateral authentication is not particularly 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:
In this case, the threat can be carried out because of weak
authentication mechanisms whereby 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 a description of a man-in-the-middle attack
during the discovery phase. This attack benefits from the fact that
NSIS nodes are likely to be unaware of the network topology.
Furthermore, an authorization problem might arise if an NSIS QoS NSLP
node pretends to be a NSIS NAT/Firewall specific node or vice versa.
An adversary might want to 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
which 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 which traverses the
adversary. The man-in-the-middle adversary might redirect to another
legimimate NSIS node. A malicious NSIS node can be detected with the
corresponding security mechanisms but a legitimate NSIS node which is
not the next NSIS node along the path cannot be detected without
having topology knowledge.
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+-----------+ 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)
IPi
Figure 3: MITM Attack during the Discovery Exchange
This attack assumes that the adversary is able to eavesdrop 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. It should be noted that the process is
self-healing since the discovery process is periodically
retransmitted. 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.
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 Disovery Reply message to force NSIS Node A to establish a
messaging association with another NSIS node which is not along the
path. This can then be exploited by the adversary. Particularly the
interworking with NSIS unaware NATs might cause additional unexpected
problems.
As a variant of this attack an adversary not able to eavesdrop
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.
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3.2 Replay of Signaling Messages
This threat scenario covers the case in which an adversary eavesdrops
and 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 in case of
replay protection requires the adversary to crash an NSIS-aware node,
causing it to lose state information (sequence numbers, security
associations, etc.), and then be able to 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) 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
spoofing, it is also possible to carry out fraud. This attack is
only feasible in the absence of authentication and signaling message
protection.
Some threats directly related to these are described in Section 4.4,
Section 4.7, and Section 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
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insecure parameter exchange or security negotiation protocol can help
an adversary mount a downgrading attack to force selection of weaker
mechanisms than mutually desired. Hence, without binding the
negotiation process to the legitimate parties and protecting it, an
NSIS protocol suite might be only as secure as the weakest mechanism
provided (e.g., weak authentication), and the benefits of defining
configuration parameters and a negotiation protocol are lost.
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4. NSIS-Specific Threat Scenarios
This section describes 11 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 are often enabled by one or more deficiencies
described under other scenarios.
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
attack.
We will discuss this threat at different layers in the NSIS protocol
suite:
Processing of Router Alert Options:
The processing of Router Alert Option (RAO) requires a router to
do some additional processing by intercepting packets with IP
options, which might lead to additional delay for legitimate
requests, or even to reject 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
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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.
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 which 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 do harm to 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. Idempotent refers to messages which 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
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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.
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 others that 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. On the other hand, it might be desirable that only the
intended end points (NSIS Initiator and NSIS Responder) are 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 and third, it can spoof data traffic.
In the first case, Eve, acting as an adversary, may claim to be the
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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
[I-D.ietf-ipv6-flow-label]). Modification of these flow identifiers
allows adversaries to exploit or to render ineffective quality of
service reservations or policy rules at middleboxes. An adversary
could mount an attack by modifying the flow identifier of a signaling
message.
In the third case, an adversary may spoof data traffic. 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).
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 the IP address of Alice.
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, the absence
of Alice will not be detected until a refresh message is required and
forces Eve to respond with a protected signaling message. Again,
this attack is applicable not just to QoS traffic and the same attack
is also applicable to a Firewall control protocol, with a different
consequence.
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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 requires the ability
also 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 user
Alice is allowed to traverse the company firewall based on the
establish packet filter to contact her internal mail server. Now,
adversary Eve, which was monitoring the signaling exchange is able to
build a data packet towards this mail server which 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 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.
If we imagine using attributes of a flow identifier that is not
related to source and destination addresses. As an example, we could
think of a flow identifier where 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 (e.g., in the above example they would have to hit the same
firewall.)
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 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, e.g., described in [RFC3182]. Depending
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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)
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 for determining authorization or the outcome of the
authorization process itself, then theft of service might be
possible.
4.6 Missing Non-Repudiation
Signaling for QoS often involves three parties: the user, a network
that offer QoS reservations (referred as service provider) and a
third party which guarantees that the party making the reservation
actually receives a financial compensation (referred as trusted third
party).
Repudiation in this context refers to a problem where either the user
or the service provider later deny about 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 providers 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
question.
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Users 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 a 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. As such, it
might be difficult to introduce with NSIS signaling. The user has to
trust the network operator to meter the traffic correctly, collect
and merge accounting data, and ensure that no unforeseen problems
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 additional places requirements on the
security mechanisms. Hence, 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 compared with 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, reordering of objects and messages, bundling of
messages, and 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 a NSIS router is also able to get access to security
associations and 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 considered.
An NSIS-aware edge router is a critical component that requires
strong security protection. A strong security policy applied at the
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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 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 apply the appropriate security protection.
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, 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
RSVP).
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 not along the data path or to cause the discovery process
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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.
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 either at the
signaling message protocol layer (NTLP), at the layer of each
client layer protocol (e.g., QoS NSLP or NAT/Firewall NSLP), or at
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). In the case in which 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 together 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. Hence, the requirement of not disclosing a network
topology might conflict with other requirements to provide means for
automatically discovering NSIS-aware nodes 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 3),
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
possible.
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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 [I-D.braden-2level-signal-arch] a two-level architecture is
proposed, which suggests splitting an NSIS protocol into layers: a
signaling message transport-specific layer and an
application-specific layer. This is further developed in the NSIS
Framework [I-D.ietf-nsis-fw]. 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 which address parts of the NTLP which are not related to
attacks against the use of transport layer protocols are covered in
various sections throughout this document, such as in Section 4.2.
In the case in which 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.
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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 11 threat
scenarios particularly applicable to the NSIS 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 points out repeatedly 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].
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6. Contributors
We especially thank Richard Graveman, who provided text for the
security considerations section, besides a detailed review of the
document.
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7. Acknowledgments
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 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, 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 draft.
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
[I-D.ietf-nsis-fw]
Hancock, R., "Next Steps in Signaling: Framework",
draft-ietf-nsis-fw-06 (work in progress), July 2004.
[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, <ALN00>.
[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", 1997, <AN97>.
[I-D.braden-2level-signal-arch]
Braden, R. and B. Lindell, "A Two-Level Architecture for
Internet Signaling", draft-braden-2level-signal-arch-01
(work in progress), November 2002.
[I-D.ietf-ipv6-flow-label]
Rajahalme, J., Conta, A., Carpenter, B. and S. Deering,
"IPv6 Flow Label Specification",
draft-ietf-ipv6-flow-label-09 (work in progress), December
2003.
[I-D.ietf-nsis-nslp-natfw]
Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer
Protocol (NSLP)", draft-ietf-nsis-nslp-natfw-03 (work in
progress), July 2004.
[I-D.ietf-nsis-ntlp]
Schulzrinne, H., "GIMPS: General Internet Messaging
Protocol for Signaling", draft-ietf-nsis-ntlp-03 (work in
progress), July 2004.
[I-D.ietf-nsis-qos-nslp]
Bosch, S., Karagiannis, G. and A. McDonald, "NSLP for
Quality-of-Service signaling", draft-ietf-nsis-qos-nslp-04
(work in progress), July 2004.
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[I-D.ietf-nsis-rsvp-sec-properties]
Tschofenig, H., "RSVP Security Properties",
draft-ietf-nsis-rsvp-sec-properties-05 (work in progress),
September 2004.
[I-D.ietf-nsis-signalling-analysis]
Manner, J., "Analysis of Existing Quality of Service
Signaling Protocols",
draft-ietf-nsis-signalling-analysis-04 (work in progress),
May 2004.
[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.
Authors' Addresses
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
EMail: Hannes.Tschofenig@siemens.com
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Dirk Kroeselberg
Siemens
Otto-Hahn-Ring 6
Munich, Bayern 81739
Germany
EMail: Dirk.Kroeselberg@siemens.com
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