Analysis of OSPF Security According to KARP Design Guide
draft-hartman-ospf-analysis-01.txt

Abstract

This document analyzes OSPFv2 and OSPFv3 according to the guidelines set forth in section 4.2 of draft-ietf-karp-design-guide.

Status of this Memo

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Table of Contents

1.  Introduction
2.  Requirements notation
3.  Current State
    3.1.  OSPFv2
    3.2.  OSPFv3
4.  Replay Detection
5.  IP Layer Issues
6.  Packet Prioritization
7.  OSPF Security Requirements
8.  Gap Analysis
    8.1.  OSPFv2 Replay Mechanism
    8.2.  OSPFv3 Replay
    8.3.  IP Header
9.  Security Considerations
10.  Acknowledgments
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Authors' Addresses

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] (Lebovitz, G. and M. Bhatia, “Keying and Authentication for Routing Protocols (KARP) Design Guidelines,” February 2010.). This draft builds on several previous analysis efforts into routing security. The OPSEC working group put together [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,” June 2010.) an analysis of cryptographic issues with routing protocols. Earlier, the RPSEC working group put together [I‑D.ietf‑rpsec‑ospf‑vuln] (Jones, E. and O. Moigne, “OSPF Security Vulnerabilities Analysis,” June 2006.) a detailed analysis of OSPF vulnerabilities.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Current State

This section describes the security mechanisms built into OSPFv2 and OSPFv3.

3.1.  OSPFv2

Appendix D of [RFC2328] (Moy, J., “OSPF Version 2,” April 1998.) 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.

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.

RFC 5709 [RFC5709] (Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M., Li, T., and R. Atkinson, “OSPFv2 HMAC-SHA Cryptographic Authentication,” October 2009.) adds support for the SHA [FIPS180] (US National Institute of Standards and Technology, “Secure Hash Standard (SHS),” August 2002.) family of hashes to OSPFv2. The cryptographic authentication described in RFC 5709 meets modern standards for per-packet integrity protection. 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. See Section 4 (Replay Detection) for discussion of the impacts of these replay 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.

3.2.  OSPFv3

RFC 4552 [RFC4552] (Gupta, M. and N. Melam, “Authentication/Confidentiality for OSPFv3,” June 2006.) 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.

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.

Because manual keying is used, no replay protection is provided for OSPFv3.

4.  Replay Detection

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.

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] (Jones, E. and O. Moigne, “OSPF Security Vulnerabilities Analysis,” June 2006.) 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 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 ligitimate 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.

5.  IP Layer Issues

The OSPFv2 cryptographic authentication does not include the IP header. So, fields like the source address are not protected. The OSPF specification uses the source address in some locations: for some network types, the source address is used to match which neighbor a packet is from. Changing the source address of a packet can produce a number of denial of service attacks. If the packet is interpreted as coming from a different neighbor, the sequence number received from the neighbor may be updated. This may disrupt communication with the legitimate neighbor. Hello packets may be reflected to cause a neighbor to appear to have one-way communication. Old Database descriptions may be reflected in cases where the per-packet sequence numbers are sufficiently divergent in order to disrupt an adjacency.

RFC 4552 requires the support of ESP and permits the use of AH. ESP does not provide any protection of the IP source address . Ah does provide such protection.Similarly, ESP does not provide protection against the insertion, deletion, or modification of any optional IPv6 headers in the OSPFv3 packet, while AH does provide that protection.

6.  Packet Prioritization

OSPF implementations generally prioritize packets in order to minimize disruption when router resources such as CPU or memory experience contention. This can cause complications for the OSPFv2 packet replay mechanism. As specified, if packets are processed out-of-order, then the skipped packets may be discarded if the sequence number increased. Refinements to the replay mechanism need to keep this usage in mind.

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.

7.  OSPF Security Requirements

This section describes requirements for OSPF security that should be met within the routing protocol.

As with all routing protocols, per-packet protection with a cryptographic MAC is required.

Mechanisms are required in order to support key rollover. Multiple approaches could be used. However since the existing mechanisms provide a protocol field to identify the key as well as management mechanisms to introduce and retire new keys, focusing on the existing mechanism as a starting point is prudent.

Replay protection is required. The replay mechanism needs to be sufficient to prevent an attacker from creating a denial of service or disrupting the integrity of the routing protocol by replaying packets. It is important that an attacker not be able to disrupt service by capturing packets and waiting for replay state to be lost.

8.  Gap Analysis

8.1.  OSPFv2 Replay Mechanism

The OSPFv2 mechanism allows an attacker to replay packets until the sequence number increases. Implementations are expected to increase their sequence number once a second or so. As discussed in Section 4 (Replay Detection), this strategy creates several opportunities for attack.

In order to avoid significant denial of service issues, the replay mechanism needs to be improved. Part of this improvement may come simply from rules about when the sequence number should be increased. Other improvements are required to prevent attacks where an old replay is made either when an adjacency is down or from a time when a sequence number is significantly greater than the current sequence number to disrupt an association.

8.2.  OSPFv3 Replay

The replay vulnerabilities for OSPFv3 are greater than for OSPFv2. There is no mechanism providing replay protection at all. Even for manual keying, some replay mechanism is required.

8.3.  IP Header

OSPF depends on fields in the IP header, particularly the source address. These fields are not integrity protected. This interacts particularly badly with the OSPFv2 replay mechanism, although other attacks are also possible.

9.  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] (Murphy, S., Badger, M., and B. Wellington, “OSPF with Digital Signatures,” June 1997.) 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.

10.  Acknowledgments

Funding for Sam Hartman's work on this memo is provided by Huawei.

The authors would like to thank Ran Atkinson and Manav Bhatia for valuable comments.

11.  References

11.1. Normative References

11.2. Informative References

Authors' Addresses