Internet DRAFT - draft-ietf-karp-ospf-analysis
draft-ietf-karp-ospf-analysis
KARP S. Hartman
Internet-Draft Painless Security
Intended status: Informational D. Zhang
Expires: May 30, 2013 Huawei Technologies co. ltd
November 26, 2012
Analysis of OSPF Security According to KARP Design Guide
draft-ietf-karp-ospf-analysis-06.txt
Abstract
This document analyzes OSPFv2 and OSPFv3 according to the guidelines
set forth in section 4.2 of RFC6518. Key components of solutions to
gaps identified in this draft are already underway.
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].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on May 30, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements to Meet . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements notation . . . . . . . . . . . . . . . . . . 4
2. Current State . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Impacts of OSPF Replays . . . . . . . . . . . . . . . . . . . 6
4. Gap Analysis and Specific Requirements . . . . . . . . . . . . 8
5. Solution Work . . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This document analyzes the current state of OSPFv2 and OSPFv3
according to the requirements of [RFC6518]. This draft builds on
several previous analysis efforts into routing security. The OPSEC
working group put together [RFC6039] an analysis of cryptographic
issues with routing protocols. Earlier, the RPSEC working group put
together [I-D.ietf-rpsec-ospf-vuln] a detailed analysis of OSPF
vulnerabilities. Solution work to address gaps identified in this
analysis is underway [I-D.ietf-ospf-security-extension-manual-keying]
[RFC6506]
OSPF meets many of the requirements expected from a manually keyed
routing protocol. Integrity protection is provided with modern
cryptographic algorithms. Algorithm agility is provided: the
algorithm can be changed as part of re-keying an interface or peer.
Intra-connection re-keying is provided by the specifications,
although apparently some implementations have trouble with this in
practice. OSPFv2 security does not interfere with prioritization of
packets.
However, some gaps remain between the current state and the
requirements for manually keyed routing security expressed in
[I-D.ietf-karp-threats-reqs]. This document explores these gaps and
proposes directions for addressing the gaps.
1.1. Requirements to Meet
There are a number of requirements described in section 3 of
[I-D.ietf-karp-threats-reqs] that OSPF does not currently meet. The
gaps are as follows:
o Secure Simple PSKs: Today, OSPF directly uses the key as
specified. Related key attacks such as those described in section
4.1 of [I-D.ietf-karp-ops-model] are possible.
o Replay Protection: The requirements document addresses
requirements for both inter-connection replay protection and
intra-connection replay protection. OSPFv3 has no replay
protection at all. OSPFv2 has most of the mechanisms necessary
for intra-connection replay protection. Unfortunately, OSPFv2
does not securely identify the neighbor with whom replay
protection state is associated in all cases. This weakness can be
used to create significant denial-of- service issues using intra-
connection replays. OSPFv2 has no inter-connection replay
protection; this creates significant denial-of-service
opportunities.
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o Packet Prioritization: OSPFv3 uses IPsec [RFC4301]to process
packets. This complicates implementations that wish to process
some packets such as hellos and acknowledgements above others. In
addition, if IPsec replay mechanisms were used, packets would need
to be processed at least by IPsec even if they were low priority.
o Neighbor Identification: In some cases, OSPF identifies a neighbor
based on the IP address. This is never protected with OSPFv2 and
is not typically protected with OSPFv3.
The remainder of this document explains the details of how these
requirements fail to be met and proposes mechanisms for addressing
them.
1.2. Requirements notation
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 [RFC2119].
2. Current State
This section describes the security mechanisms built into OSPFv2 and
OSPFv3. There are two goals to this section. First, this section
gives a brief explanation of the OSPF security mechanisms to those
familiar with connectionless integrity mechanisms but not with OSPF.
Second, this section explains the background necessary to understand
how OSPF fails to meet some of the requirements proposed for routing
security.
2.1. OSPFv2
Appendix D of [RFC2328] describes the basic procedure for
cryptographic authentication in OSPFv2. An authentication data field
in the OSPF packet header contains a key ID, the length of the
authentication data and a sequence number. A message authentication
code (MAC) is appended to the OSPF packet. This code protects all
fields of the packet including the sequence number but not the IP
header.
RFC 2328 defined the use of a keyed-MD5 MAC. While MD5 has not been
broken as a MAC, it is not the algorithm of choice for new MACs.
However, RFC 5709 [RFC5709] adds support for the SHA [FIPS180] family
of hashes to OSPFv2. The cryptographic authentication described in
RFC 5709 meets modern standards for per-packet integrity protection.
Thus, OSPFv2 meets the requirement for strong algorithms. Since
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multiple algorithms are defined and a new algorithm can be selected
with each key, OSPFv2 meets the requirement for algorithm agility.
In order to provide cryptographic algorithms believed to have a
relatively long useful life, RFC 5709 mandates support for SHA-2
rather than SHA-1.
These security services provide integrity protection on each packet.
In addition, limited replay detection is provided. The sequence
number is non-decreasing. So, once a router has increased its
sequence number, an attacker cannot replay an old packet.
Unfortunately, sequence numbers are not required to increase for each
packet. For instance, because existing OSPF security solutions do
not specify how to set the sequence number, it is possible that some
implementations use, e.g., "seconds since reboot" as their sequence
numbers. The sequence numbers are thus only increased by every
second, permitting an opportunity for intra-connection replay. Also,
no mechanism is provided to deal with the loss of anti-replay state;
if sequence numbers are reused when a router reboots, then inter-
connection replays are straight forward. In
[I-D.ietf-ospf-security-extension-manual-keying], the OSPFv2 sequence
number is expanded to 64-bits with the least significant 32-bit value
containing a strictly increasing sequence number and the most
significant 32-bit value containing the boot count. The boot count
is retained in non-volatile storage for the deployment life of a OSPF
router. Therefore, the sequence number will never decrease even
after a cold reboot.
Also, because the IP header is not protected, the sequence number may
not be associated with the right neighbor; this opens up
opportunities for outsiders to perform replay attacks. See Section 3
for analysis of these attacks. In
[I-D.ietf-ospf-security-extension-manual-keying], this issue is
addressed by changing the definition of Apad from a constant defined
in [RFC5709] to the source address from the IP header of the OSPFv2
protocol packet. In this way, the source address from the IP header
is incorporated in the cryptographic authentication computation, and
any change of the IP source address will be detected.
The mechanism provides good support for key rollover. There is a key
ID; in addition mechanisms are described for managing key lifetimes
and starting the use of a new key in an orderly manner. Performing
orderly key rollover requires that implementations support accepting
a new key for received packets before using that key to generate
packets. Section D.3 of RFC 2328 requires this support in the form
of four configurable lifetimes for each key: two lifetimes control
the beginning and ending period for acceptance while two lifetimes
control the beginning and ending period for generation. This
provides a superset of the functionality in the key table
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[I-D.ietf-karp-crypto-key-table] regarding lifetime.
The OSPFv2 replay mechanism does not handle prioritized transmission
of OSPF Hello and Link State Acknowledgement packets as recommended
in [RFC4222]. When OSPF packets are transmitted with varied
prioritization, they can arrive out-of-order resulting in packets
with lower prioritization being discarded.
2.2. OSPFv3
RFC 4552 describes how the IPsec authentication header and
encapsulating security payload mechanism can be used to protect
OSPFv3 packets. This mechanism provides per-packet integrity and
optional confidentiality using a wide variety of cryptographic
algorithms. Because OSPF uses multicast traffic, only manual key
management is supported. This mechanism meets requirements related
to algorithm selection and agility.
The Security Parameter Index (SPI) [RFC4301] provides an identifier
for the security association. This along with other IPsec facilities
provides a mechanism for moving from one key to another, meeting the
key rollover requirements.
Because manual keying is used, no replay protection is provided for
OSPFv3. Thus the intra-connection and inter-connection replay
requirements are not met.
There is another serious problem with the OSPFv3 security: rather
than being integrated into OSPF, it is based on IPsec. In practice,
this has lead to deployment problems.
OSPF implementations generally prioritize packets in order to
minimize disruption when router resources such as CPU or memory
experience contention. When IPsec is used with OSPFv3, the offset of
the packet type, which is used to prioritize packets, depends on what
integrity transform is used. For this reason, prioritizing packets
may be more complex for OSPFv3. One approach is to establish per-SPI
filters to find the packet type and act accordingly.
3. Impacts of OSPF Replays
As discussed, neither version of OSPF meets the requirements of
inter-connection or intra-connection replay protection. In order to
mount a replay, an attacker needs some mechanism to inject a packet;
physical security can limit a particular deployment's vulnerability
to replay attacks. This section discusses the impacts of OSPF
replays.
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In OSPFv2, two facilities limit the scope of replay attacks. First,
when cryptographic authentication is used, each packet includes a
sequence number that is non-decreasing. In the current
specifications, the sequence number is remembered as part of an
adjacency: if an attacker can cause an adjacency to go down, then
replay state is lost. Database Description packets also include a
per-LSA sequence number that is part of the information that is
flooded. Even if a packet is replayed, the per-LSA sequence number
will prevent an old LSA from being installed. Unlike the per-packet
sequence number, the per-LSA sequence number must increase when an
LSA is changed. As a result, replays cannot be used to install old
routing information.
While the LSA sequence number provides some defense, the RPSEC
analysis [I-D.ietf-rpsec-ospf-vuln] describes a number of attacks
that are possible because of per-packet replays. The most serious
appear to be attacks against Hello packets, which may cause an
adjacency to fail. Other attacks may cause excessive flooding or
excessive use of CPU.
Another serious attack concerns Database Description packets. In
addition to the per-packet sequence number that is part of
cryptographic authentication for OSPFv2 and the per-LSA sequence
numbers, Database Description packets also include a Database
Description sequence number. If a Database Description packet with
the incorrect sequence number is received, then the database exchange
process will be restarted.
The per-packet OSPFv2 sequence number can be used to reduce the
window in which a replay is valid. A receiver will harmlessly reject
a packet whose per-packet sequence number is older than the one most
recently received from a neighbor. Replaying the most recent packet
from a neighbor does not appear to create problems. So, if the per-
packet sequence number is incremented on every packet sent, then
replay attacks should not disrupt OSPFv2. Unfortunately, OSPFv2 does
not have a procedure for dealing with sequence numbers reaching the
maximum value. It may be possible to figure out a set of rules
sufficient to disrupt the damage of packet replays while minimizing
the use of the sequence number space.
As mentioned previously, when an adjacency is dropped, replay state
is lost. So, after rebooting or when all adjacencies are lost, a
router may allow its sequence number to decrease. An attacker can
cause significant damage by replaying a packet captured before the
sequence number decrease at a time after the sequence number
decrease. If this happens, then the replayed packet will be accepted
and the sequence number will be updated. However, the legitimate
sender will be using a lower sequence number, so legitimate packets
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will be rejected. A similar attack is possible in cases where OSPF
identifies a neighbor based on source address. An attacker can
change the source address of a captured packet and replay it. If the
attacker causes a replay from a neighbor with a high sequence number
to appear to be from a low sequence number neighbor, then
connectivity with that neighbor will be disrupted until the adjacency
fails.
OSPFv3 lacks the per-packet sequence number but has the per-LSA
sequence number. As such, OSPFv3 has no defense against denial of
service attacks that exploit replay.
4. Gap Analysis and Specific Requirements
The design guide requires each design team to enumerate a set of
requirements for the routing protocol. The only concerns identified
with OSPF are areas where it fails to meet general requirements
outlined in the threats and requirements document. This section
explains how some of these general requirements map specifically onto
the OSPF protocol and enumerates the specific gaps that need to be
addressed.
There is a general requirement for inter-connection replay
protection. In the context of OSPF, this means that if an adjacency
goes down between two neighbors and later is re-established,
replaying packets from before the adjacency went down cannot disrupt
the adjacency. In the context of OSPF, intra-connection replay
protection means that replaying a packet cannot prevent an adjacency
from forming or disrupt an adjacency. Meeting the requirements for
intra-connection and inter-connection replay protection is a
significant gap between the optimal state and where OSPF is today.
Since OSPF uses fields in the IP header, the general requirement to
protect the IP header and handle neighbor identification applies.
This is another gap that needs to be addressed. Because the replay
protection will depend on neighbor identification, the replay
protection cannot be adequately addressed without handling this issue
as well.
In order to encourage deployment of OSPFv3 security, an
authentication option is required that does not have the deployment
challenges of IPsec.
In order to support the requirement for simple preshared keys, OSPF
needs to make sure that when the same key is used for two different
purposes, no problems result.
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In order to support packet prioritization, it is desirable for the
information needed to prioritize OSPF packets (the packet type) to be
at a constant location in the packet.
5. Solution Work
A security solution will be developed for OSPFv2 and OSPFv3 based on
the OSPFv2 cryptographic authentication option. This solution will
have the following improvements over the existing OSPFv2 option:
Address most inter-connection replay attacks by splitting the
sequence number and requiring preservation of state so that the
sequence number increases on every packet.
Add a form of simple key derivation so that if the same preshared
key is used for OSPF and other purposes, cross-protocol attacks do
not result
Support OSPFv3 authentication without use of IPsec
Specify processing rules sufficient to permit replay detection and
packet prioritization
Emphasize requirements already present in the OSPF specification
sufficient to permit key migration without disrupting adjacencies
Specify the proper use of the key table for OSPF
Protect the source IP address
Require that sequence numbers be incremented on each packet
The key components of this solution work are already underway.
OSPFv3 now supports an authentication option [RFC6506] that meets the
requirements of this section, except that it does not describe how
the key tables are used for OSPF. OSPFv2 is being enhanced
[I-D.ietf-ospf-security-extension-manual-keying] to protect the
source address, provide inter-connection replay and describe how to
use the key table.
6. IANA Considerations
This document makes no request of IANA.
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7. Security Considerations
This memo discusses and compiles vulnerabilities in the existing OSPF
cryptographic handling.
In analyzing proposed improvements to OSPF per-packet security, it is
desirable to consider how these improvements interact with potential
improvements in overall routing security. For example, the impact of
replay attacks currently depends on the LSA sequence number
mechanism. If cryptographic protections against insider attackers
are considered by future work, then that work will need to provide a
solution that meets the needs of the per-packet replay defense as
well as protection of routing data from insider attack. An
experimental solution is discussed in [RFC2154] that explores end-to-
end protection of routing data in OSPF. It may be beneficial to
consider how improvements to the per-packet protections would
interact with such a mechanism to future-proof these mechanisms.
Implementations have a number of options in minimizing the potential
denial of service impact of OSPF cryptographic authentication. The
Generalized TTL Security Mechanism (GTSM) [RFC5082] might be
appropriate for OSPF packets other than those traversing virtual
links. Using this mechanism requires support of the sender; new OSPF
cryptographic authentication could specify this behavior if desired.
Alternatively implementations can limit the source addresses from
which they accept packets. Non-hello packets need only be accepted
from existing neighbors. If a system is under attack hello packets
from existing neighbors could be prioritized over hellos from new
neighbors. These mechanisms can be considered to limit the potential
impact of denial of service attacks on the cryptographic
authentication mechanism itself.
8. Acknowledgements
Funding for Sam Hartman's work on this memo is provided by Huawei.
The authors would like to thank Ran Atkinson, Michael Barnes, and
Manav Bhatia for valuable comments.
9. References
9.1. Normative References
[I-D.ietf-karp-threats-reqs]
Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Overview, Threats, and
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Requirements", draft-ietf-karp-threats-reqs-06 (work in
progress), September 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, June 2006.
[RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines", RFC 6518,
February 2012.
9.2. Informative References
[FIPS180] US National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", August 2002.
[I-D.ietf-karp-crypto-key-table]
Housley, R., Polk, T., Hartman, S., and D. Zhang,
"Database of Long-Lived Symmetric Cryptographic Keys",
draft-ietf-karp-crypto-key-table-04 (work in progress),
October 2012.
[I-D.ietf-karp-ops-model]
Hartman, S. and D. Zhang, "Operations Model for Router
Keying", draft-ietf-karp-ops-model-04 (work in progress),
October 2012.
[I-D.ietf-opsec-routing-protocols-crypto-issues]
Jaeggli, J., Hares, S., Bhatia, M., Manral, V., and R.
White, "Issues with existing Cryptographic Protection
Methods for Routing Protocols",
draft-ietf-opsec-routing-protocols-crypto-issues-07 (work
in progress), August 2010.
[I-D.ietf-ospf-security-extension-manual-keying]
Bhatia, M., Hartman, S., Zhang, D., and A. Lindem,
"Security Extension for OSPFv2 when using Manual Key
Management",
draft-ietf-ospf-security-extension-manual-keying-03 (work
in progress), October 2012.
[I-D.ietf-rpsec-ospf-vuln]
Jones, E. and O. Moigne, "OSPF Security Vulnerabilities
Analysis", draft-ietf-rpsec-ospf-vuln-02 (work in
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progress), June 2006.
[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
[RFC4222] Choudhury, G., "Prioritized Treatment of Specific OSPF
Version 2 Packets and Congestion Avoidance", BCP 112,
RFC 4222, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, October 2009.
[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, October 2010.
[RFC6506] Bhatia, M., Manral, V., and A. Lindem, "Supporting
Authentication Trailer for OSPFv3", RFC 6506,
February 2012.
Authors' Addresses
Sam Hartman
Painless Security
Email: hartmans-ietf@mit.edu
URI: http://www.painless-security.com/
Dacheng Zhang
Huawei Technologies co. ltd
Huawei Building No.3 Xinxi Rd., Shang-Di Information Industrial Base Hai-Dian District, Beijing
China
Email: zhangdacheng@huawei.com
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