Network Working Group H. Tschofenig
Request for Comments: 4081 D. Kroeselberg
Category: Informational Siemens
June 2005
Security Threats for Next Steps in Signaling (NSIS)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This threats document provides a detailed analysis of the security
threats relevant to the Next Steps in Signaling (NSIS) protocol
suite. It calls attention to, and helps with the understanding of,
various security considerations in the NSIS Requirements, Framework,
and Protocol proposals. This document does not describe
vulnerabilities of specific parts of the NSIS protocol suite.
Table of Contents
1. IntrodUCtion ....................................................2
2. Communications Models ...........................................3
3. Generic Threats .................................................7
3.1. Man-in-the-Middle Attacks ..................................8
3.2. Replay of Signaling Messages ..............................11
3.3. Injecting or Modifying Messages ...........................11
3.4. Insecure Parameter Exchange and Negotiation ...............12
4. NSIS-Specific Threat Scenarios .................................12
4.1. Threats during NSIS SA Usage ..............................13
4.2. Flooding ..................................................13
4.3. Eavesdropping and Traffic Analysis ........................15
4.4. Identity Spoofing .........................................15
4.5. Unprotected Authorization Information .....................17
4.6. Missing Non-Repudiation ...................................18
4.7. Malicious NSIS Entity .....................................19
4.8. Denial of Service Attacks .................................20
4.9. Disclosing the Network Topology ...........................21
4.10. Unprotected Session or Reservation Ownership .............21
4.11. Attacks against the NTLP .................................23
5. Security Considerations ........................................23
6. Contributors ...................................................24
7. Acknowledgements ...............................................24
8. References .....................................................25
8.1. Normative References ......................................25
8.2. Informative References ....................................25
1. Introduction
Whenever a new protocol is developed or existing protocols are
modified, threats to their security should be evaluated. To address
security in the NSIS working group, a number of steps have been
taken:
NSIS Analysis Activities (see [RSVP-SEC] and [SIG-ANAL])
Security Threats for NSIS
NSIS Requirements (see [RFC3726])
NSIS Framework (see [RFC4080])
NSIS Protocol Suite (see GIMPS [GIMPS], NAT/Firewall NSLP
[NATFW-NSLP] and QoS NSLP [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 [QOS-NSLP], NAT/Firewall
[NATFW-NSLP], and other NSLP documents need to provide a description
of their trust models and a threat assessment for their specific
application domain. This document aims to provide some help for the
subsequent design of the NSIS protocol suite. Investigations of
security threats in a specific architecture or context are outside
the scope of this document.
We use the NSIS terms defined in [RFC3726] and in [RFC4080].
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 that
are relevant to securing the NSIS protocol suite.
An abstract network topology with its administrative domains is shown
in Figure 1, and in Figure 2 the relationship between NSIS entities
along the path is shown. For illustrative reasons, only end-to-end
NSIS signaling is depicted, yet it might be used in other variations
as well. Signaling can start at any place and might terminate at any
other place within the network. Depending on the trust relationship
between NSIS entities and the traversed network parts, different
security problems arise.
The notion of trust and trust relationship used in this document is
informal and can best be captured by the definition provided in
Section 1.1 of [RFC3756]. For completeness we include the definition
of a trust relationship, which denotes a mutual a priori relationship
between the involved organizations or parties wherein the parties
believe that the other parties will behave correctly even in the
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 so that they
perform message processing and take the necessary actions. A
complete lack of trust between any of the participating entities will
cause NSIS signaling to fail.
Note that it is not possible to describe a trust model completely
without considering the details and behavior of the NTLP, the NSLP
(e.g., QoS NSLP), and the deployment environment. For example,
securing the communication between an end host (which acts as the
NSIS Initiator) and the first NSIS node (which might be in the
attached network or even a number of networks away) is impacted by
the trust relationships between these entities. In a corporate
network environment, a stronger degree of trust typically exists than
in an unmanaged network.
Figure 1 introduces convenient abbreviations for network parts with
similar properties: first-peer, last-peer, intra-domain, or
inter-domain.
+------------------+ +---------------+ +------------------+
Administrative Intermediate Administrative
Domain A Domains Domain B
(Inter-domain Communication)
+-------->+---+<------------->+---+<--------+
(Intra-domain (Intra-domain
Communication) Communication)
v v
+--------+---------+ +---------------+ +---------+--------+
^ ^
First Peer Communication Last Peer Communication
v v
+-----+-----+ +-----+-----+
NSIS NSIS
Initiator Responder
+-----------+ +-----------+
Figure 1: Communication patterns in NSIS
First-Peer/Last-Peer Communication:
The end-to-end communication scenario depicted in Figure 1
includes the communication between the end hosts and their nearest
NSIS hops. "First-peer communications" refers to the peer-to-peer
interaction between a signaling message originator, the NSIS
Initiator (NI), and the first NSIS-aware entity along the path.
This "first-peer communications" commonly comes with specific
security requirements that are especially important for addressing
security issues between the end host (and a user) and the network
it is attached to.
To illustrate this, in roaming environments, it is difficult to
assume the existence of a pre-established security association
directly available for NSIS peers involved in first-peer
communications, because these peers cannot be assumed to have any
pre-existing relationship with each other. In contrast, in
enterprise networks usually there is a fairly strong
(pre-established) trust relationship between the peers.
Enterprise network administrators usually have some degree of
freedom to select the appropriate security protection and to
enforce it. The choice of selecting a security mechanism is
therefore often influenced by the infrastructure already
available, and per-session negotiation of security mechanisms is
often not required (although, in contrast, it is required in a
roaming environment).
Last-Peer communication is a variation of First-Peer communication
in which the roles are reversed.
Intra-Domain Communication:
After verification of the NSIS signaling message at the border of
an administrative domain, an NSIS signaling message traverses the
network within the same administrative domain to which the first
peer belongs. It might not be necessary to repeat the
authorization procedure of the NSIS initiator again at every NSIS
node within this domain. Key management within the administrative
domain might also be simpler.
Security protection is still required to prevent threats by
non-NSIS nodes in this network.
Inter-Domain Communication:
Inter-Domain communication deals with the interaction between
administrative domains. For some NSLPs (for example, QoS NSLP),
this interaction is likely to take place between neighboring
domains, whereas in other NSLPs (such as the NAT/Firewall NSLP),
the core network is usually not involved.
If signaling messages are conveyed transparently in the core
network (i.e., if they are neither intercepted nor processed in
the core network), then the signaling message communications
effectively takes place between Access networks. This might place
a burden on authorization handling and on the key management
infrastructure required between these access networks, which might
not know of each other in advance.
To refine the above differentiation based on the network parts that
NSIS signaling may traverse, we subsequently consider relationships
between involved entities. Because a number of NSIS nodes might
actively participate in a specific protocol exchange, a larger number
of possible relationships need to be analyzed than in other
protocols. Figure 2 illustrates possible relationships between the
entities involved in the NSIS protocol suite.
****************************************
* *
+----+-----+ +----------+ +----+-----+
+-----+ 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 that are not allowed to eavesdrop or modify
certain objects.
Middle-to-Middle Communications:
Middle-to-middle communication refers to the exchange of
information between two non-neighboring NSIS nodes along the path.
Intermediate NSIS hops may have to deal with specific security
threats that do not involve the NSIS Initiator or the NSIS
Responder directly.
End-to-End Communications:
NSIS aims to signal information from an Initiator to some NSIS
nodes along the path to a data receiver. In the case of
end-to-end NSIS signaling, the last node is the NSIS Responder, as
it is the data receiver. The NSIS protocol suite is not an
end-to-end protocol used to exchange information purely between
end hosts.
Typically, it is not required to protect NSIS messages
cryptographically between the NSIS Initiator and the NSIS
Responder. Protecting the entire signaling message end-to-end
might not be feasible since intermediate NSIS nodes need to add,
inspect, modify, or delete objects from the signaling message.
3. Generic Threats
This section provides scenarios of threats that are applicable to
signaling protocols in general. Note that some of these scenarios
use the term "user" instead of "NSIS Initiator". This is mainly
because security protocols allow differentiation between entities
that are hosts and those that are users (based on the identifiers
used).
For the following subsections, we use the general distinction in two
cases in which attacks may occur. These are according to the
separate steps, or phases, normally encountered when applying
protocol security (with, e.g., IPsec, TLS, Kerberos, or SSH).
Therefore, this section starts by briefly describing a motivation for
this separation.
Security protection of protocols is often separated into two steps.
The first step primarily provides entity authentication and key
establishment (which result in a persistent state often called a
security association), whereas the second step provides message
protection (some combination of data origin authentication, data
integrity, confidentiality, and replay protection) using the
previously established security association. The first step tends to
be more expensive than the second, which is the main reason for the
separation. If messages are transmitted infrequently, then these two
steps may be collapsed into a single and usually rather costly one.
One such example is e-mail protection via S/MIME. The two steps may
be tightly bound into a single protocol, as in TLS, or defined in
separate protocols, as with IKE and IPsec. We use this separation to
cover the different threats in more detail.
3.1. Man-in-the-Middle Attacks
This section describes both security threats that exist if two peers
do not already share a security association or do not use security
mechanisms at all, and threats that are applicable when a security
association is already established.
Attacks during NSIS SA Establishment:
While establishing a security association, an adversary fools the
signaling message Initiator with respect to the entity to which it
has to authenticate. The Initiator authenticates to the man-in-
the-middle adversary, who is then able to modify signaling
messages to mount DoS attacks or to steal services that get billed
to the Initiator. In addition, the adversary may be able to
terminate the Initiator's NSIS messages and to inject messages to
a peer itself, thereby acting as the peer to the Initiator and as
the Initiator to the peer. As a result, the Initiator wrongly
believes that it is talking to the "real" network, whereas it is
actually attached to an adversary. For this attack to be
successful, pre-conditions that are described in the following
three cases have to hold:
Missing Authentication:
In the first case, this threat can be carried out because of
missing authentication between neighboring peers: without
authentication, an NI, NR, or NF is unable to detect an
adversary. However, in some practical cases, authentication
might be difficult to accomplish, either because the next peer
is unknown, because there are misbelieved trust relationships
in parts of the network, or because of the inability to
establish proper security protection (inter-domain signaling
messages, dynamic establishment of a security association,
etc.). If one of the communicating endpoints is unknown, then
for some security mechanisms it is either impossible or
impractical to apply appropriate security protection.
Sometimes network administrators use intra-domain signaling
messages without proper security. This configuration allows an
adversary on a compromised non-NSIS-aware node to interfere
with nodes running an NSIS signaling protocol. Note that this
type of threat goes beyond those caused by malicious NSIS nodes
(described in Section 4.7).
Unilateral Authentication:
In the case of unilateral authentication, the NSIS entity that
does not authenticate its peer is unable to discover a man-in-
the-middle adversary. Although mutual authentication of
signaling messages should take place between each peer
participating in the protocol operation, special attention is
given here to first-peer communications. Unilateral
authentication between an end host and the first peer (just
authenticating the end host) is still common today, but it
opens up many possibilities for man-in-the-middle attackers
impersonating either the end host or the (administrative domain
represented by the) first peer.
Missing or unilateral authentication, as described above, is
part of a general problem of network access with inadequate
authentication, and it should not be considered something
unique to the NSIS signaling protocol. Obviously, there is a
strong need to address this correctly in a future NSIS protocol
suite. The signaling protocols addressed by NSIS are different
from other protocols in which only two entities are involved.
Note that first-peer authentication is especially important
because a security breach there could impact nodes beyond the
entities directly involved (or even beyond a local network).
Finally, note that the signaling protocol should be considered
a peer-to-peer protocol, wherein the roles of Initiator and
Responder can be reversed at any time. Thus, unilateral
authentication is not particularly useful for such a protocol.
However, some form of asymmetry might be needed in the
authentication process, whereby one entity uses an
authentication mechanism different from that of the other one.
As an example, the combination of symmetric and asymmetric
cryptography should be mentioned.
Weak Authentication:
In the case of weak authentication, the threat can be carried
out because information transmitted during the NSIS SA
establishment process may leak passWords or allow offline
dictionary attacks. This threat is applicable to NSIS for the
process of selecting certain security mechanisms.
Finally, we conclude with a description of a man-in-the-middle (MITM)
attack during the discovery phase. This attack benefits from the
fact that NSIS nodes are likely to be unaware of the network
topology. Furthermore, an authorization problem might arise if an
NSIS QoS NSLP node pretends to be an NSIS NAT/Firewall-specific node
or vice versa.
An adversary might inject a bogus reply message, forcing the
discovery message initiator to start a messaging association
establishment with either an adversary or with another NSIS node that
is not along the path. Figure 3 describes the attack in more detail
for peer-to-peer addressed messages with a discovery mechanism. For
end-to-end addressed messages, the attack is also applicable,
particularly if the adversary is located along the path and able to
intercept the discovery message that traverses the adversary. The
man-in-the-middle adversary might redirect to another legitimate NSIS
node. A malicious NSIS node can be detected with the corresponding
security mechanisms, but a legitimate NSIS node that is not the next
NSIS node along the path cannot be detected without topology
knowledge.
+-----------+ 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 on the
initial discovery message sent by the sender of the discovery
message. Furthermore, we assume that the discovery reply message by
the adversary returns to the discovery message initiator faster than
the real response. This represents some race condition
characteristics if the next NSIS node is very close (in IP-hop terms)
to the initiator. Note that the problem is self-healing since the
discovery process is periodically repeated. If an adversary is
unable to mount this attack with every discovery message, then the
correct next NSIS node along the path will be discovered again. A
ping-pong behavior might be the consequence.
As shown in message step (2) in Figure 3, the adversary returns a
discovery reply message with its own IP address as the next NSIS-
aware node along the path. Without any additional information, the
discovery message initiator has to trust this information. Then a
messaging association is established with an entity at a given IP
address IPx (i.e., with the adversary) in step (3). The adversary
then establishes a messaging association with a further NSIS node and
forwards the signaling message. Note that the adversary might just
modify the Discovery Reply message to force NSIS Node A to establish
a messaging association with another NSIS node that is not along the
path. This can then be exploited by the adversary. The interworking
with NSIS-unaware NATs in particular might cause additional
unexpected problems.
As a variant of this attack, an adversary not able to eavesdrop on
transmitted discovery requests could flood a node with bogus
discovery reply messages. If the discovery message sender
accidentally accepts one of those bogus messages, then a MITM attack
as described in Figure 3 is possible.
3.2. Replay of Signaling Messages
This threat scenario covers the case in which an adversary
eavesdrops, collects signaling messages, and replays them at a later
time (or at a different place, or uses parts of them at a different
place or in a different way; e.g., cut-and-paste attacks). Without
proper replay protection, an adversary might mount man-in-the-middle,
denial of service, and theft of service attacks.
A more difficult attack (that may cause problems even if there is
replay protection) requires that the adversary crash an NSIS-aware
node, causing it to lose state information (sequence numbers,
security associations, etc.), and then replay old signaling messages.
This attack takes advantage of re-synchronization deficiencies.
3.3. Injecting or Modifying Messages
This type of threat involves integrity violations, whereby an
adversary modifies signaling messages (e.g., by acting as a
man-in-the-middle) in order to cause unexpected network behavior.
Possible actions an adversary might consider for its attack are
reordering, delaying, dropping, injecting, truncating, and otherwise
modifying messages.
An adversary may inject a signaling message requesting a large amount
of resources (possibly using a different user's identity). Other
resource requests may then be rejected. In combination with identity
spoofing, it is 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 Sections 4.4,
4.7, and 4.8.
3.4. Insecure Parameter Exchange and Negotiation
First, protocols may be useful in a variety of scenarios with
different security requirements. Second, different users (e.g., a
university, a hospital, a commercial enterprise, or a government
ministry) have inherently different security requirements. Third,
different parts of a network (e.g., within a building, across a
public carrier's network, or over a private microwave link) may need
different levels of protection. It is often difficult to meet these
(sometimes conflicting) requirements with a single security mechanism
or fixed set of security parameters, so often a selection of
mechanisms and parameters is offered. Therefore, a protocol is
required to agree on certain security mechanisms and parameters. An
insecure parameter exchange or security negotiation protocol can help
an adversary to mount a downgrading attack to force selection of
mechanisms weaker than those mutually desired. Thus, without binding
the negotiation process to the legitimate parties and protecting it,
an NSIS protocol suite might only be as secure as the weakest
mechanism provided (e.g., weak authentication), and the benefits of
defining configuration parameters and a negotiation protocol are
lost.
4. NSIS-Specific Threat Scenarios
This section describes eleven threat scenarios in terms of attacks on
and security deficiencies in the NSIS signaling protocol. A number
of security deficiencies might enable an attack. Fraud is an example
of an attack that might be enabled by missing replay protection,
missing protection of authorization tokens, identity spoofing,
missing authentication, and other deficiencies that help an adversary
steal resources. Different threat scenarios based on deficiencies
that could enable an attack are addressed in this section.
The threat scenarios are not independent. Some of them (e.g., denial
of service) are well-established security terms and, as such, need to
be addressed, but they are often enabled by one or more deficiencies
described under other scenarios.
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 that a router
do some additional processing by intercepting packets with IP
options, which might lead to additional delay for legitimate
requests, or even rejection of some of them. A router being
flooded with a large number of bogus messages requires resources
before finding out that these messages have to be dropped.
If the protocol is based on using interception for message
delivery, this threat cannot be completely eliminated, but the
protocol design should attempt to limit the processing that has to
be done on the RAO-bearing packet so that it is as similar as
possible to that for an arbitrary packet addressed directly to one
of the router interfaces.
Attacks against the Transport Layer Protocol:
Certain attacks can be mounted against transport protocols by
flooding a node with bogus requests, or even to finish the
handshake phase to establish a transport layer association. These
types of threats are also addressed in Section 4.11.
Force NTLP to Do More Processing:
Some protocol fields might allow an adversary to force an NTLP
node to perform more processing. Additionally it might be
possible to interfere with the flow control or the congestion
control procedure. These types of threats are also addressed in
Section 4.11.
Furthermore, it might be possible to force the NTLP node to
perform some computations or signaling message exchanges by
injecting "trigger" events (which are unprotected).
Force NSLP to Do More Processing:
An adversary might benefit from flooding an NSLP node with
messages that must be stored (e.g., due to fragmentation handling)
before verifying the correctness of signaling messages.
Furthermore, causing memory allocation and computational efforts
might allow an adversary to harm NSIS entities. If a signaling
message contains, for example, a digital signature, then some
additional processing is required for the cryptographic
verification. An adversary can easily create a random bit
sequence instead of a digital signature to force an NSIS node into
heavy computation.
Idempotent signaling messages are particularly vulnerable to this
type of attack. The term "idempotent" refers to messages that
contain the same amount of information as the original message.
An example would be a refresh message that is equivalent to a
create message. This property allows a refresh message to create
state along a new path, where no previous state is available. For
this to work, specific classes of cryptographic mechanisms
supporting this behavior are needed. An example is a scheme based
on digital signatures, which, however, should be used with care
due to possible denial of service attacks.
Problems with the usage of public-key-based cryptosystems in
protocols are described in [AN97] and in [ALN00].
In addition to the threat scenario described above, an incoming
signaling message might trigger communication with third-party
nodes such as policy servers, LDAP servers, or AAA servers. If an
adversary is able to transmit a large number of signaling messages
(for example, with QoS reservation requests) with invalid
credentials, then the verifying node may not be able to process
other reservation messages from legitimate users.
4.3. Eavesdropping and Traffic Analysis
This section covers threats whereby an adversary is able to eavesdrop
on signaling messages. The signaling packets collected may allow
traffic analysis or be used later to mount replay attacks, as
described in Section 3.2. The eavesdropper might learn QoS
parameters, communication patterns, policy rules for firewall
traversal, policy information, application identifiers, user
identities, NAT bindings, authorization objects, network
configuration and performance information, and more.
An adversary's capability to eavesdrop on signaling messages might
violate a user's preference for privacy, particularly if unprotected
authentication or authorization information (including policies and
profile information) is exchanged.
Because the NSIS protocol signals messages through a number of nodes,
it is possible to differentiate between nodes actively participating
in the NSIS protocol and those that do not. For certain objects or
messages, it might be desirable to permit actively participating
intermediate NSIS nodes to eavesdrop. On the other hand, it might be
desirable that only the intended end points (NSIS Initiator and NSIS
Responder) be able to read certain other objects.
4.4. Identity Spoofing
Identity spoofing relevant for NSIS occurs in three forms: First,
identity spoofing can happen during the establishment of a security
association based on a weak authentication mechanism. Second, an
adversary can modify the flow identifier carried within a signaling
message. Third, it can spoof data traffic.
In the first case, Eve, acting as an adversary, may claim to be the
registered user Alice by spoofing Alice's identity. Eve thereby
causes the network to charge Alice for the network resources
consumed. This type of attack is possible if authentication is based
on a simple username identifier (i.e., in absence of cryptographic
authentication), or if authentication is provided for hosts, and
multiple users have access to a single host. This attack could also
be classified as theft of service.
In the second case, an adversary may be able to exploit the
established flow identifiers (required for QoS and NAT/FW NSLP).
These identifiers are, among others, IP addresses, transport protocol
type (UDP, TCP), port numbers, and flow labels (see [RFC1809] and
[RFC3697]). Modification of these flow identifiers allows
adversaries to exploit or to render ineffective quality of service
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 that is
associated with a specified behavior (e.g., a particular flow
experiences QoS treatment or allows packets to traverse a firewall).
An adversary might, therefore, use IP spoofing and inject data
packets to benefit from previously installed flow identifiers.
We will provide an example of the latter threat. After NSIS nodes
along the path between the NSIS initiator and the NSIS receiver
processes a properly protected reservation request, transmitted by
the legitimate user Alice, a QoS reservation is installed at the
corresponding NSIS nodes (for example, the edge router). The flow
identifier is used for flow identification and allows data traffic
originated from a given source to be assigned to this QoS
reservation. The adversary Eve now spoofs Alice's IP address. In
addition, Alice's host may be crashed by the adversary with a denial
of service attack or may lose connectivity (for example, because of
mobility). If Eve is able to perform address spoofing, then she is
able to receive and transmit data (for example, RTP data traffic)
that receives preferential QoS treatment based on the previous
reservation. Depending on the installed flow identifier granularity,
Eve might have more possibilities to exploit the QoS reservation or a
pin-holed firewall. Assuming the soft state paradigm, whereby
periodic refresh messages are required, Alice's absence will not be
detected until a refresh message is required, forcing Eve to respond
with a protected signaling message. Again, this attack is applicable
not only to QoS traffic, but also to a Firewall control protocol,
with a different consequence.
The ability for an adversary to inject data traffic that matches a
certain flow identifier established by a legitimate user and to get
some benefit from injecting that traffic often also requires the
ability to receive the data traffic or to have one's correspondent
receive it. For example, an adversary in an unmanaged network
observes a NAT/Firewall signaling message towards a corporate
network. After the signaling message exchange was successful, the
user Alice is allowed to traverse the company firewall based on the
establish packet filter in order to contact her internal mail server.
Now, the adversary Eve, who was monitoring the signaling exchange, is
able to build a data packet towards this mail server that will pass
the company firewall. The packet will hit the mail server and cause
some actions, and the mail server will reply with some response
messages. Depending on the exact location of the adversary and the
degree of routing asymmetry, the adversary might even see the
response messages. Note that for this attack to work, Alice does not
need to participate in the exchange of signaling messages.
We could imagine using attributes of a flow identifier that is not
related to source and destination addresses. For example, we could
think of a flow identifier for which only the 21-bit Flow ID is used
(without source and destination IP address). Identity spoofing and
injecting traffic is much easier since a packet only needs to be
marked and an adversary can use a nearly arbitrary endpoint
identifier to achieve the desired result. Obviously, though, the
endpoint identifiers are not irrelevant, because the messages have to
hit some nodes in the network where NSIS signaling messages installed
state (in the above example, they would have to hit the same
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, various attacks are possible. These
problems have been known in the DiffServ community for a long time
and have been documented in various DiffServ-related documents. The
IPsec protection of DiffServ Code Points is described in Section 6.2
of [RFC2745]. Related security issues (for example denial of service
attacks) are described in Section 6.1 of the same document.
4.5. Unprotected Authorization Information
Authorization is an important criterion for providing resources such
as QoS reservations, NAT bindings, and pinholes through firewalls.
Authorization information might be delivered to the NSIS-
participating entities in a number of ways.
Typically, the authenticated identity is used to assist during the
authorization procedure (as described in [RFC3182], for example).
Depending on the chosen authentication protocol, certain threats may
exist. Section 3 discusses a number of issues related to this
approach when the authentication and key exchange protocol is used to
establish session keys for signaling message protection.
Another approach is to use some sort of authorization token. The
functionality and structure of such an authorization token for RSVP
is described in [RFC3520] and [RFC3521].
Achieving secure interaction between different protocols based on
authorization tokens, however, requires some care. By using such an
authorization token, it is possible to link state information between
different protocols. Returning an unprotected authorization token to
the end host might allow an adversary (for example, an eavesdropper)
to steal resources. An adversary might also use the token to monitor
communication patterns. Finally, an untrustworthy end host might
also modify the token content.
The Session/Reservation Ownership problem can also be regarded as an
authorization problem. Details are described in Section 4.10. In
enterprise networks, authorization is often coupled with membership
in a particular class of users or groups. This type of information
either can be delivered as part of the authentication and key
agreement procedure or has to be retrieved via separate protocols
from other entities. If an adversary manages to modify information
relevant to determining authorization or the outcome of the
authorization process itself, then theft of service might be
possible.
4.6. Missing Non-Repudiation
Signaling for QoS often involves three parties: the user, a network
that offers QoS reservations (referred to as "service provider") and
a third party that guarantees that the party making the reservation
actually receives a financial compensation (referred to as "trusted
third party").
In this context,"repudiation" refers to a problem where either the
user or the service provider later deny the existence or some
parameters (e.g., volume or price) of a QoS reservation towards the
trusted third party. Problems stemming from a lack of non-
repudiation appear in two forms:
Service provider's point-of-view:
A user may deny having issued a reservation request for which it
was charged. The service provider may then want to be able to
prove that a particular user issued the reservation request in
question.
User's point-of-view:
A service provider may claim to have received a number of
reservation requests from a particular user. The user in question
may want to show that such reservation requests have never been
issued and may want to see correct service usage records for a
given set of QoS parameters.
In today's networks, non-repudiation is not provided. Therefore, it
might be difficult to introduce with NSIS signaling. The user has to
trust the network operator to meter the traffic correctly, to collect
and merge accounting data, and to ensure that no unforeseen problems
occur. If a signaling protocol with the non-repudiation property is
desired for establishing QoS reservations, then it certainly impacts
the protocol design.
Non-repudiation functionality places additional requirements on the
security mechanisms. Thus, a solution would normally increase the
overhead of a security solution. Threats related to missing non-
repudiation are only considered relevant in certain specific
scenarios and for specific NSLPs.
4.7. Malicious NSIS Entity
Network elements within a domain (intra-domain) experience a
different trust relationship with regard to the security protection
of signaling messages from that of edge NSIS entities. It is assumed
that edge NSIS entities are responsible for performing cryptographic
processing (authentication, integrity and replay protection,
authorization, and accounting) for signaling messages arriving from
the outside. This prevents unprotected signaling messages from
appearing within the internal network. If, however, an adversary
manages to take over an edge router, then the security of the entire
network is compromised. An adversary is then able to launch a number
of attacks, including denial of service; integrity violations; replay
and reordering of objects and messages; bundling of messages;
deletion of data packets; and various others. A rogue firewall can
harm other firewalls by modifying policy rules. The chain-of-trust
principle applied in peer-to-peer security protection cannot protect
against a malicious NSIS node. An adversary with access to an NSIS
router is also able to get access to security associations and to
transmit secured signaling messages. Note that even non-peer-to-peer
security protection might not be able to prevent this problem fully.
Because an NSIS node might issue signaling messages on behalf of
someone else (by acting as a proxy), additional problems need to be
considered.
An NSIS-aware edge router is a critical component that requires
strong security protection. A strong security policy applied at the
edge does not imply that other routers within an intra-domain network
do not need to verify signaling messages cryptographically. If the
chain-of-trust principle is deployed, then the security protection of
the entire path (in this case, within the network of a single
administrative domain) is only as strong as the weakest link. In the
case under consideration, the edge router is the most critical
component of this network, and it may also act as a security gateway
or firewall for incoming and outgoing traffic. For outgoing traffic,
this device has to implement the security policy of the local domain
and to apply the appropriate security protection.
For an adversary to mount this attack, either an existing NSIS-aware
node along the path has to be attacked successfully, or an adversary
must succeed in convincing another NSIS node to make it the next NSIS
peer (man-in-the-middle attack).
4.8. Denial of Service Attacks
A number of denial of service (DoS) attacks can cause NSIS nodes to
malfunction. Other attacks that could lead to DoS, such as man-in-
the-middle attacks, replay attacks, and injection or modification of
signaling messages, etc., are mentioned throughout this document.
Path Finding:
Some signaling protocols establish state (e.g., routing state) and
perform some actions (e.g., querying resources) at a number of
NSIS nodes without requiring authorization (or even proper
authentication) based on a single message (e.g., PATH message in
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 that is not along the data path, or to cause the discovery
process to fail. In the first case, the signaling protocol could
appear to continue correctly, except that policy rules are
installed at the incorrect firewalls or QoS resource reservations
take place at the wrong entities. For an end host, this means
that the protocol failed for unknown reasons.
Faked Error or Response Messages:
An adversary may be able to inject false error or response
messages as part of a DoS attack. This could be at the signaling
message protocol layer (NTLP), the layer of each client layer
protocol (e.g., QoS NSLP or NAT/Firewall NSLP), or the transport
protocol layer. An adversary might cause unexpected protocol
behavior or might succeed with a DoS attack. The discovery
protocol, especially, exhibits vulnerabilities with regard to this
threat scenario (see the above discussion on discovery). If no
separate discovery protocol is used and signaling messages are
addressed to end hosts only (with a Router Alert Option to
intercept message as NSIS aware nodes), an error message might be
used to indicate a path change. Such a design combines a
discovery protocol with a signaling message exchange protocol.
4.9. Disclosing the Network Topology
In some organizations or enterprises there is a desire not to reveal
internal network structure (or other related information) outside of
a closed community. An adversary might be able to use NSIS messages
for network mapping (e.g., discovering which nodes exist, which use
NSIS, what version, what resources are allocated, what capabilities
nodes along a path have, etc.). Discovery messages, traceroute,
diagnostic messages (see [RFC2745] for a description of diagnostic
message functionality for RSVP), and query messages, in addition to
record route and route objects, provide potential assistance to an
adversary. Thus, the requirement of not disclosing a network
topology might conflict with other requirements to provide means for
discovering NSIS-aware nodes automatically or to provide diagnostic
facilities (used for network monitoring and administration).
4.10. Unprotected Session or Reservation Ownership
Figure 4 shows an NSIS Initiator that has established state
information at NSIS nodes along a path as part of the signaling
procedure. As a result, Access Router 1, Router 3, and Router 4 (and
other nodes) have stored session-state information, including the
Session Identifier SID-x.
Session ID(SID-x)
+--------+
+-----------------+ Router +------------>
Session ID(SID-x) 4
+---+----+ +--------+
Router
+------+ 3 +*******
+---+----+ *
*
Session ID(SID-x) * Session ID(SID-x)
+---+----+ +---+----+
Access Access
Router Router
1 2
+---+----+ +---+----+
*
Session ID(SID-x) * Session ID(SID-x)
+----+------+ +----+------+
NSIS Adversary
Initiator
+-----------+ +-----------+
Figure 4: Session or Reservation Ownership
The Session Identifier is included in signaling messages to reference
to the established state.
If an adversary were able to oBTain the Session Identifier (for
example, by eavesdropping on signaling messages), it would be able to
add the same Session Identifier SID-x to a new signaling message.
When the new signaling message hits Router 3 (as shown in Figure 4),
existing state information can be modified. The adversary can then
modify or delete the established reservation and cause unexpected
behavior for the legitimate user.
The source of the problem is that Router 3 (a cross-over router) is
unable to decide whether the new signaling message was initiated from
the owner of the session or reservation.
In addition, nodes other than the initial signaling message
originator are allowed to signal information during the lifetime of
an established session. As part of the protocol, any NSIS-aware node
along the path (and the path might change over time) could initiate a
signaling message exchange. It might, for example, be necessary to
provide mobility support or to trigger a local repair procedure. If
only the initial signaling message originator were allowed to trigger
signaling message exchanges, some protocol behavior would not be
possible.
If this threat scenario is not addressed, an adversary can launch
DoS, theft of service, and various other attacks.
4.11. Attacks against the NTLP
In [2LEVEL], a two-level architecture is proposed, that would split
an NSIS protocol into layers: a signaling message transport-specific
layer and an application-specific layer. This is further developed
in the NSIS Framework [RFC4080]. Most of the threats described in
this threat analysis are applicable to the NSLP application-specific
part (e.g., QoS NSLP). There are, however, some threats that are
applicable to the NTLP.
Network and transport layer protocols lacking protection mechanisms
are vulnerable to certain attacks, such as header manipulation, DoS,
spoofing of identities, session hijacking, unexpected aborts, etc.
Malicious nodes can attack the congestion control mechanism to force
NSIS nodes into a congestion avoidance state.
Threats that address parts of the NTLP that are not related to
attacks against the use of transport layer protocols are covered in
various sections throughout this document, such as Section 4.2.
If existing transport layer protocols are used for exchanging NSIS
signaling messages, security vulnerabilities known for these
protocols need to be considered. A detailed threat description of
these protocols is outside the scope of this document.
5. Security Considerations
This entire memo discusses security issues relevant for NSIS protocol
design. It begins by identifying the components of a network running
NSIS (Initiator, Responder, and different Administrative Domains
between them). It then considers five cases in which communications
take place between these components, and it examines the trust
relationships presumed to exist in each case: First-Peer
Communications, End-to-Middle Communications, Intra-Domain
Communications, Inter-Domain Communications, and End-to-End
Communications. This analysis helps determine the security needs and
the relative seriousness of different threats in the different cases.
The document points out the need for different protocol security
measures: authentication, key exchange, message integrity, replay
protection, confidentiality, authorization, and some precautions
against denial of service. The threats are subdivided into generic
ones (e.g., man-in-the-middle attacks, replay attacks, tampering and
forgery, and attacks on security negotiation protocols) and eleven
threat scenarios that are particularly applicable to the NSIS
protocol. Denial of service, for example, is covered in the
NSIS-specific section, not because it cannot be carried out against
other protocols, but because the methods used to carry out denial of
service attacks tend to be protocol specific. Numerous illustrative
examples provide insight into what can happen if these threats are
not mitigated.
This document repeatedly points out that not all of the threats are
equally serious in every context. It does attempt to identify the
scenarios in which security failures may have the highest impact.
However, it is difficult for the protocol designer to foresee all the
ways in which NSIS protocols will be used or to anticipate the
security concerns of a wide variety of likely users. Therefore, the
protocol designer needs to offer a full range of security
capabilities and ways for users to negotiate and select what they
need, on a case-by-case basis. To counter these threats, security
requirements have been listed in [RFC3726].
6. Contributors
We especially thank Richard Graveman, who provided text for the
security considerations section, as well as a detailed review of the
document.
7. Acknowledgements
We would like to thank (in alphabetical order) Marcus Brunner, Jorge
Cuellar, Mehmet Ersue, Xiaoming Fu, and Robert Hancock for their
comments on an initial version of this document. Jorge and Robert
gave us an extensive list of comments and provided information on
additional threats.
Jukka Manner, Martin Buechli, Roland Bless, Marcus Brunner, Michael
Thomas, Cedric Aoun, John Loughney, Rene Soltwisch, Cornelia Kappler,
Ted Wiederhold, Vishal Sankhla, Mohan Parthasarathy, and Andrew
McDonald provided comments on more recent versions of this document.
Their input helped improve the content of this document. Roland
Bless, Michael Thomas, Joachim Kross, and Cornelia Kappler, in
particular, provided good proposals for regrouping and restructuring
the material.
A final review was given by Michael Richardson. We thank him for his
detailed comments.
8. References
8.1. Normative References
[RFC4080] Hancock, R., Karagiannis, G., Loughney, J., and S. van
den Bosch, "Next Steps in Signaling (NSIS): Framework",
RFC 4080, June 2005.
[RFC3726] Brunner, M., "Requirements for Signaling Protocols",
RFC 3726, April 2004.
8.2. Informative References
[ALN00] Aura, T., Leiwo, J., and P. Nikander, "Towards Network
Denial of Service Resistant Protocols, In Proceedings
of the 15th International Information Security
Conference (IFIP/SEC 2000), Beijing, China",
August 2000.
[AN97] Aura, T. and P. Nikander, "Stateless Connections", In
Proceedings of the International Conference on
Information and Communications Security (ICICS'97),
Lecture Notes in Computer Science 1334, Springer",
1997.
[2LEVEL] Braden, R. and B. Lindell, "A Two-Level Architecture
for Internet Signaling", Work in Progress,
November 2002.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S.
Deering, "IPv6 Flow Label Specification", RFC 3697,
March 2004.
[NATFW-NSLP] Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer
Protocol (NSLP)", Work in Progress, February 2005.
[GIMPS] Schulzrinne, H., "GIMPS: General Internet Messaging
Protocol for Signaling", Work in Progress,
February 2005.
[QOS-NSLP] Bosch, S., Karagiannis, G., and A. McDonald, "NSLP for
Quality-of-Service signaling", Work in Progress,
February 2005.
[RSVP-SEC] Tschofenig, H., "RSVP Security Properties", Work in
Progress, February 2005.
[SIG-ANAL] Manner, J. and X. Fu, "Analysis of Existing Quality-
of-Service Signaling Protocols", RFC 4094, May 2005.
[RFC1809] Partridge, C., "Using the Flow Label Field in IPv6",
RFC 1809, June 1995.
[RFC2745] Terzis, A., Braden, B., Vincent, S., and L. Zhang,
"RSVP Diagnostic Messages", RFC 2745, January 2000.
[RFC3182] Yadav, S., Yavatkar, R., Pabbati, R., Ford, P., Moore,
