Network Working Group S. Hartman
Internet-Draft Painless Security
Intended status: Informational D. Zhang
Expires: September 4, 2011 Huawei
March 3, 2011
Analysis of OSPF Security According to KARP Design Guide
draft-ietf-karp-ospf-analysis-00.txt
Abstract
This document analyzes OSPFv2 and OSPFv3 according to the guidelines
set forth in section 4.2 of draft-ietf-karp-design-guide.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements to Meet . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements notation . . . . . . . . . . . . . . . . . . 4
2. Current State . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. OSPFv2 . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. OSPFv3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Impacts of OSPF Replays . . . . . . . . . . . . . . . . . . . 7
4. Gap Analysis and Specific Requirements . . . . . . . . . . . . 9
5. Solution Work . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
This document performs the initial analysis of the current state of
OSPFv2 and OSPFv3 according to the requirements of
[I-D.ietf-karp-design-guide]. 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.
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 interfeer 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] the requirements. 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] thatt OSPF does not currently meet:
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.hartman-karp-ops-model] are possible.
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.
Packet Prioritization: OSPFv3 uses IPsec 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.
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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].
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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
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 beleived 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
implementation use, e.g., "seconds since reboot" as their sequence
numbers. The sequence numbers is thus only increased by every
second. 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 streight forward. 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
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of these attacks.
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
[I-D.ietf-karp-crypto-key-table] regarding lifetime.
The OSPFv2 replay mechanism does not handle packet priorities as
described. If packets are processed out-of-order, then if the
sequence number increases, packets processed later will be discarded.
2.2. OSPFv3
RFC 4552 [RFC4552] describes how the 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) 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.
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3. Impacts of OSPF Replays
As discussed, neither version of OSPF meets the requirements of
inter-connection or intra-connection replay protection. This section
discusses the impacts of OSPF replays.
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, there are a
number of attacks that are possible because of a per-packet replay.
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 age. 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
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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
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.
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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 adressed 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.
In order to support packet prioritization, the information needed to
prioritize OSPF packets (the packet type) MUST be at a constant
location in the packet.
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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:
Detect liveness of neighbors by adding additional information to
the Hello exchanges in order to detect inter-connection replay
Add a form of simple key derivation so that if the same preshared
key is used for OSPF and other purposes, related key 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
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6. 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. RFC 2154
[RFC2154] provides an experimental solution for 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.
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7. Acknowledgments
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.
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8. References
8.1. Normative References
[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.
[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.
8.2. Informative References
[FIPS180] US National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", August 2002.
[I-D.hartman-karp-ops-model]
Hartman, S. and D. Zhang, "Operations Model for Router
Keying", draft-hartman-karp-ops-model-01 (work in
progress), October 2010.
[I-D.ietf-karp-crypto-key-table]
Housley, R. and T. Polk, "Database of Long-Lived Symmetric
Cryptographic Keys", draft-ietf-karp-crypto-key-table-00
(work in progress), November 2010.
[I-D.ietf-karp-design-guide]
Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines",
draft-ietf-karp-design-guide-00 (work in progress),
February 2010.
[I-D.ietf-karp-threats-reqs]
Lebovitz, G., Bhatia, M., and R. White, "The Threat
Analysis and Requirements for Cryptographic Authentication
of Routing Protocols' Transports",
draft-ietf-karp-threats-reqs-01 (work in progress),
October 2010.
[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",
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draft-ietf-opsec-routing-protocols-crypto-issues-06 (work
in progress), June 2010.
[I-D.ietf-rpsec-ospf-vuln]
Jones, E. and O. Moigne, "OSPF Security Vulnerabilities
Analysis", draft-ietf-rpsec-ospf-vuln-02 (work in
progress), June 2006.
[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, October 2010.
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Authors' Addresses
Sam Hartman
Painless Security
Email: hartmans-ietf@mit.edu
Dacheng Zhang
Huawei
Email: zhangdacheng@huawei.com
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