Internet Engineering Task Force (IETF) U. Chunduri

Internet Engineering Task Force (IETF) U. Chunduri
Request for Comments: 7645 A. Tian
Category: Informational W. Lu
ISSN: 2070-1721 Ericsson Inc.
September 2015

The Keying and Authentication for Routing Protocol (KARP)
IS-IS Security Analysis

Abstract

This document analyzes the current state of the Intermediate System
to Intermediate System (IS-IS) protocol according to the requirements
set forth in "Keying and Authentication for Routing Protocols (KARP)
Design Guidelines" (RFC 6518) for both manual and automated key
management protocols.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7645.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Current State . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Key Usage . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Subnetwork Independent . . . . . . . . . . . . . . . 4
2.1.2. Subnetwork dependent . . . . . . . . . . . . . . . . 4
2.2. Key Agility . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Security Issues . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Replay Attacks . . . . . . . . . . . . . . . . . . . 5
2.3.1.1. Current Recovery Mechanism for LSPs . . . . . . . 6
2.3.2. Spoofing Attacks . . . . . . . . . . . . . . . . . . 7
2.3.3. DoS Attacks . . . . . . . . . . . . . . . . . . . . . 8
3. Gap Analysis and Security Requirements . . . . . . . . . . . 8
3.1. Manual Key Management . . . . . . . . . . . . . . . . . . 8
3.2. Key Management Protocols . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Normative References . . . . . . . . . . . . . . . . . . 10
5.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction

This document analyzes the current state of the Intermediate System
to Intermediate System (IS-IS) protocol according to the requirements
set forth in "Keying and Authentication for Routing Protocols (KARP)
Design Guidelines" [RFC6518] for both manual and automated key
management protocols.

With currently published work, IS-IS meets some of the requirements
expected from a manually keyed routing protocol. Integrity
protection is expanded by allowing more cryptographic algorithms to
be used [RFC5310]. However, even with this expanded protection, only
limited algorithm agility (HMAC-SHA family) is possible. [RFC5310]
makes possible a basic form of intra-connection rekeying, but with
some gaps as analyzed in Section 3 of this document.

This document summarizes the current state of cryptographic key usage
in the IS-IS protocol and several previous efforts that analyze IS-IS
security. This includes the base IS-IS specifications: [RFC1195],
[RFC5304], [RFC5310], and [RFC6039].

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This document also analyzes various threats to IS-IS (as described in
[RFC6862]), lists security gaps, and provides specific
recommendations to thwart the threats for both manual keying and
automated key management mechanisms.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Acronyms

DoS - Denial of Service

GDOI - Group Domain of Interpretation

IGP - Interior Gateway Protocol

IIH - IS-IS HELLO

IPv4 - Internet Protocol version 4

KMP - Key Management Protocol (automated key management)

LSP - Link State PDU

MKM - Manual Key Management

NONCE - Number Once

PDU - Protocol Data Unit

SA - Security Association

SNP - Sequence Number PDU

2. Current State

IS-IS is specified in International Standards Organization (ISO)
10589 [ISO10589], with extensions to support Internet Protocol
version 4 (IPv4) described in [RFC1195]. The specification includes
an authentication mechanism that allows for any authentication
algorithm and also specifies the algorithm for clear text passwords.
Further, [RFC5304] extends the authentication mechanism to work with
HMAC-MD5 and also modifies the base protocol for more effectiveness.
[RFC5310] provides algorithm agility, with a new generic
cryptographic authentication mechanism (CRYPTO_AUTH) for IS-IS.

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CRYPTO_AUTH also introduces a Key ID mechanism that maps to unique
IS-IS SAs.

The following sections describe the current authentication key usage
for various IS-IS messages, current key change methodologies, and the
various potential security threats.

2.1. Key Usage

IS-IS can be provisioned with a per-interface, peer-to-peer key for
IIH PDUs and a group key for LSPs and SNPs. If provisioned, IIH
packets can potentially use the same group key used for LSPs and
SNPs.

2.1.1. Subnetwork Independent

Link State PDUs, Complete and partial Sequence Number PDUs come under
Sub network Independent messages. For protecting Level-1 SNPs and
Level-1 LSPs, provisioned Area Authentication key is used. Level-2
SNPs as well as Level-2 LSPs use the provisioned domain
authentication key.

Because authentication is performed on the LSPs transmitted by an IS,
rather than on the LSP packets transmitted to a specific neighbor, it
is implied that all the ISes within a single flooding domain must be
configured with the same key in order for authentication to work
correctly. This is also true for SNP packets, though they are
limited to link-local scope in broadcast networks.

If multiple instances share the circuits as specified in [RFC6822],
instance-specific authentication credentials can be used to protect
the LSPs and SNPs within an area or domain. It is important to note
that [RFC6822] also allows usage of topology-specific authentication
credentials within an instance for the LSPs and SNPs.

2.1.2. Subnetwork Dependent

IIH PDUs use the Link Level Authentication key, which may be
different from that of LSPs and SNPs. This could be particularly
true for point-to-point links. In broadcast networks, it is possible
to provision the same common key used for LSPs and SNPs to protect
IIH messages. This allows neighbor discovery and adjacency formation
with more than one neighbor on the same physical interface. If
multiple instances share the circuits as specified in [RFC6822],
instance-specific authentication credentials can be used to protect
Hello messages.

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2.2. Key Agility

Key roll over without effecting the routing protocols operation in
general and IS-IS in particular is necessary for effective key
management protocol integration.

Current HMAC-MD5 cryptographic authentication as defined in
[RFC5304], suggests a transition mode so that ISes use a set of keys
when verifying the authentication value to allow key changes. This
approach will allow changing the authentication key manually without
bringing down the adjacency and without dropping any control packet.
But, this can increase the load on the control plane for the key
transition duration, as each control packet may have to be verified
by more than one key, and it also allows a potential DoS attack in
the transition duration.

The above situation is improved with the introduction of the Key ID
mechanism as defined in [RFC5310]. With this, the receiver
determines the active SA by looking at the Key ID field in the
incoming PDU and need not try with other keys when the integrity
check or digest verification fails. But, neither key coordination
across the group nor an exact key change mechanism is clearly
defined. [RFC5310] says:

Normally, an implementation would allow the network operator to
configure a set of keys in a key chain, with each key in the chain
having a fixed lifetime. The actual operation of these mechanisms
is outside the scope of this document.

2.3. Security Issues

The following section analyzes various possible security threats in
the current state of the IS-IS protocol.

2.3.1. Replay Attacks

Replaying a captured protocol packet to cause damage is a common
threat for any protocol. Securing the packet with cryptographic
authentication information alone cannot mitigate this threat
completely. Though this problem is more prevalent in broadcast
networks, it is important to note that most of the IGP deployments
use P2P-over-lan circuits [RFC5309], which makes it possible for an
adversary to replay an IS-IS PDU more easily than the traditional P2P
networks.

In intra-session replay attacks, a secured protocol packet of the
current session that is replayed can cause damage, if there is no
other mechanism to confirm this is a replay packet. In inter-session

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replay attacks, a captured packet from one of the previous sessions
can be replayed to cause damage. IS-IS packets are vulnerable to
both of these attacks, as there is no sequence number verification
for IIH and SNP packets. Also with current manual key management,
periodic key changes across the group are rarely done. Thus, the
intra-connection and inter-connection replay requirements are not
met.

IS-IS specifies the use of the HMAC-MD5 [RFC5304] and HMAC-SHA-1
family in [RFC5310] to protect IS-IS packets. An adversary could
replay old IIHs or replay old SNPs that would cause churn in the
network or bring down the adjacencies.

1. At the time of adjacency bring up an IS sends IIH packet with
empty neighbor list (TLV 6) and with the authentication
information as per the provisioned authentication mechanism. If
this packet is replayed later on the broadcast network, all ISes
in the broadcast network can bounce the adjacency to create a huge
churn in the network.

2. Today, LSPs have intra-session replay protection as the LSP header
contains a 32-bit sequence number, which is verified for every
received packet against the local LSP database. But, if a node in
the network is out of service (is undergoing some sort of high
availability condition or an upgrade) for more than LSP refresh
time and the rest of the network ages out the LSPs of the node
under consideration, an adversary can potentially plunge in inter-
session replay attacks in the network. If the key is not changed
in the above circumstances, attack can be launched by replaying an
old LSP with a higher sequence number and fewer prefixes or fewer
adjacencies. This may force the receiver to accept and remove the
routes from the routing table, which eventually causes traffic
disruption to those prefixes. However, as per the IS-IS
specification, there is a built-in recovery mechanism for LSPs
from inter-session replay attacks and it is further discussed in
Section 2.3.1.1.

3. In any IS-IS network (broadcast or otherwise), if an old and an
empty Complete Sequence Number Packet (CSNP) is replayed, this can
cause LSP flood in the network. Similarly, a replayed Partial
Sequence Number Packet (PSNP) can cause LSP flood in the broadcast
network.

2.3.1.1. Current Recovery Mechanism for LSPs

In the event of inter-session replay attack by an adversary, as an
LSP with a higher sequence number gets accepted, it also gets
propagated until it reaches the originating node of the LSP. The

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originator recognizes the LSP is "newer" than in the local database,
which prompts the originator to flood a newer version of the LSP with
a higher sequence number than that received. This newer version can
potentially replace any versions of the replayed LSP that may exist
in the network.

However, in the above process, depending on where in the network the
replay is initiated, how quickly the nodes in the network react to
the replayed LSP, and how different the content in the accepted LSP
is determines the damage caused by the replayed LSP.

2.3.2. Spoofing Attacks

IS-IS shares the same key between all neighbors in an area or in a
domain to protect the LSP, SNP packets, and in broadcast networks
even IIH packets. False advertisement by a router is not within the
scope of the KARP work. However, given the wide sharing of keys as
described above, there is a significant risk that an attacker can
compromise a key from one device and use it to falsely participate in
the routing, possibly even in a very separate part of the network.

If the same underlying topology is shared across multiple instances
to transport routing/application information as defined in [RFC6822],
it is necessary to use different authentication credentials for
different instances. In this connection, based on the deployment
considerations, if certain topologies in a particular IS-IS instance
require more protection from spoofing attacks and less exposure,
topology-specific authentication credentials can be used for LSPs and
SNPs as facilitated in [RFC6822].

Currently, possession of the key itself is used as an authentication
check and there is no identity check done separately. Spoofing
occurs when an illegitimate device assumes the identity of a
legitimate one. An attacker can use spoofing to launch various types
of attacks, for example:

1. The attacker can send out unrealistic routing information that
might cause the disruption of network services, such as block
holes.

2. A rogue system that has access to the common key used to protect
the LSP can flood an LSP by setting the Remaining Lifetime field
to zero, thereby initiating a purge. Subsequently, this can cause
the sequence number of all the LSPs to increase quickly to max out
the sequence number space, which can cause an IS to shut down for
MaxAge + ZeroAgeLifetime period to allow the old LSPs to age out
in other ISes of the same flooding domain.

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2.3.3. DoS Attacks

DoS attacks using the authentication mechanism is possible and an
attacker can send packets that can overwhelm the security mechanism
itself. An example is initiating an overwhelming load of spoofed but
integrity-protected protocol packets, so that the receiver needs to
process the integrity check, only to discard the packet. This can
cause significant CPU usage. DoS attacks are not generally
preventable within the routing protocol. As the attackers are often
remote, the DoS attacks are more damaging to area-scoped or domain-
scoped packet receivers than link-local-scoped packet receivers.

3. Gap Analysis and Security Requirements

This section outlines the differences between the current state of
the IS-IS routing protocol and the desired state as specified in the
KARP Design Guidelines [RFC6518]. This section focuses on where the
IS-IS protocol fails to meet general requirements as specified in the
threats and requirements document [RFC6862].

This section also describes security requirements that should be met
by IS-IS implementations that are secured by manual as well as
automated key management protocols.

3.1. Manual Key Management

1. With CRYPTO_AUTH specification [RFC5310], IS-IS packets can be
protected with the HMAC-SHA family of cryptographic algorithms.
The specification provides limited algorithm agility (SHA family).
By using Key IDs, it also conceals the algorithm information from
the protected control messages.

2. Even though both intra- and inter-session replay attacks are best
prevented by deploying key management protocols with frequent key
change capability, basic constructs for the sequence number should
be in the protocol messages. So, some basic or extended sequence
number mechanism should be in place to protect IIH packets and SNP
packets. The sequence number should be increased for each
protocol packet. This allows mitigation of some of the replay
threats as mentioned in Section 2.3.1.

3. Any common key mechanism with keys shared across a group of
routers is susceptible to spoofing attacks caused by a malicious
router. A separate authentication check (apart from the integrity
check to verify the digest) with digital signatures as described
in [RFC2154] can effectively nullify this attack. But this
approach was never deployed, which we assume is due to operational
considerations at that time. The alternative approach to thwart

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this threat would be to use the keys from the group key management
protocol. As the group key(s) are generated by authenticating the
member ISes in the group first and are then periodically rekeyed,
per-packet identity or authentication checks may not be needed.

4. In general, DoS attacks may not be preventable with the mechanism
from the routing protocol itself. But some form of admin-
controlled lists at the forwarding plane can reduce the damage.
There are some other forms of DoS attacks common to any protocol
that are not in scope per Section 3.3 of [RFC6862].

As discussed in Section 2.2, though the Key ID mechanism described in
[RFC5310] helps, a better key coordination mechanism for key roll
over is desirable even with manual key management. But, [RFC5310]
does not specify the exact mechanism other than requiring use of key
chains. The specific requirements are as follows:

a. Keys SHOULD be able to change without effecting the established
adjacency, ideally without any control packet loss.

b. Keys SHOULD be able to change without effecting the protocol
operations; for example, LSP flooding should not be held for a
specific Key ID availability.

c. Any proposed mechanism SHOULD also be incrementally deployable
with key management protocols.

3.2. Key Management Protocols

In broadcast deployments, the keys used for protecting IS-IS
protocols messages can, in particular, be group keys. A mechanism is
needed to distribute group keys to a group of ISes in a Level-1 area
or Level-2 domain, using the Group Domain of Interpretation (GDOI)
protocol as specified in [RFC6407]. An example policy and payload
format is described in [GDOI].

If a group key is used, the authentication granularity becomes group
membership of devices, not peer authentication between devices. The
deployed group key management protocol SHOULD support rekeying.

In some deployments, where IS-IS point-to-point (P2P) mode is used
for adjacency bring-up, subnetwork-dependent messages (e.g., IIHs)
can use a different key shared between the two P2P peers, while all
other messages use a group key. When a group keying mechanism is
deployed, even the P2P IIHs can be protected with the common group
keys. This approach facilitates one key management mechanism instead
of both pair-wise keying and group keying protocols being deployed
together. If the same circuits are shared across multiple instances,

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the granularity of the group can become per instance for IIHs and per
instance/topology for LSPs and SNPs as specified in [RFC6822].

Effective key change capability within the routing protocol that
allows key roll over without impacting the routing protocol operation
is one of the requirements for deploying any group key mechanism.
Once such mechanism is in place with the deployment of group key
management protocol; IS-IS can be protected from various threats and
is not limited to intra- and inter-session replay attacks and
spoofing attacks.

Specific use of cryptographic tables [RFC7210] should be defined for
the IS-IS protocol.

4. Security Considerations

This document is mostly about security considerations of the IS-IS
protocol, and it lists potential threats and security requirements
for mitigating these threats. This document does not introduce any
new security threats for the IS-IS protocol. In view of openly
published attack vectors, as noted in Section 1 of [RFC5310] on HMAC-
MD5 cryptographic authentication mechanism, IS-IS deployments SHOULD
use the HMAC-SHA family [RFC5310] instead of HMAC-MD5 [RFC5304] to
protect IS-IS PDUs. For more detailed security considerations,
please refer the Security Considerations section of the IS-IS Generic
Cryptographic Authentication [RFC5310], the KARP Design Guide
[RFC6518] document, as well as the KARP threat document [RFC6862].

5. References

5.1. Normative References

[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <http://www.rfc-editor.org/info/rfc1195>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <http://www.rfc-editor.org/info/rfc5304>.

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[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <http://www.rfc-editor.org/info/rfc5310>.

5.2. Informative References

[GDOI] Weis, B. and S. Rowles, "GDOI Generic Message
Authentication Code Policy", Work in Progress,
draft-weis-gdoi-mac-tek-03, September 2011.

[ISO10589] International Organization for Standardization,
"Intermediate System to Intermediate System intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)", ISO/IEC
10589:2002, Second Edition, November 2002.

[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, DOI 10.17487/RFC2154, June
1997, <http://www.rfc-editor.org/info/rfc2154>.

[RFC5309] Shen, N., Ed., and A. Zinin, Ed., "Point-to-Point
Operation over LAN in Link State Routing Protocols",
RFC 5309, DOI 10.17487/RFC5309, October 2008,
<http://www.rfc-editor.org/info/rfc5309>.

[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, DOI 10.17487/RFC6039, October 2010,
<http://www.rfc-editor.org/info/rfc6039>.

[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
October 2011, <http://www.rfc-editor.org/info/rfc6407>.

[RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines", RFC 6518,
DOI 10.17487/RFC6518, February 2012,
<http://www.rfc-editor.org/info/rfc6518>.

[RFC6822] Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and
D. Ward, "IS-IS Multi-Instance", RFC 6822,
DOI 10.17487/RFC6822, December 2012,
<http://www.rfc-editor.org/info/rfc6822>.

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[RFC6862] Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
Authentication for Routing Protocols (KARP) Overview,
Threats, and Requirements", RFC 6862,
DOI 10.17487/RFC6862, March 2013,
<http://www.rfc-editor.org/info/rfc6862>.

[RFC7210] Housley, R., Polk, T., Hartman, S., and D. Zhang,
"Database of Long-Lived Symmetric Cryptographic Keys",
RFC 7210, DOI 10.17487/RFC7210, April 2014,
<http://www.rfc-editor.org/info/rfc7210>.

Acknowledgements

Authors would like to thank Joel Halpern for initial discussions on
this document and for giving valuable review comments. The authors
would like to acknowledge Naiming Shen for reviewing and providing
feedback on this document. Thanks to Russ White, Brian Carpenter,
and Amanda Barber for reviewing the document during the IESG review
process.

Authors' Addresses

Uma Chunduri
Ericsson Inc.
300 Holger Way,
San Jose, California 95134
United States
Phone: 408 750-5678
Email: [email protected]

Albert Tian
Ericsson Inc.
300 Holger Way,
San Jose, California 95134
United States
Phone: 408 750-5210
Email: [email protected]

Wenhu Lu
Ericsson Inc.
300 Holger Way,
San Jose, California 95134
United States
Email: [email protected]

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