RFC7645
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 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Contents
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
Security Issues
The following section analyzes various possible security threats in the current state of the IS-IS protocol.
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
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.
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
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.
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.
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.
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.
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
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.
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,
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.
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.
References
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>.
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>.
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>.
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]
