| Rfc | 6039 |
|---|
| Title | Issues with Existing Cryptographic Protection Methods for Routing
Protocols |
|---|
| Author | V. Manral, M. Bhatia, J. Jaeggli, R. White |
|---|
| Date | October 2010 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Engineering Task Force (IETF) V. Manral
Request for Comments: 6039 IP Infusion
Category: Informational M. Bhatia
ISSN: 2070-1721 Alcatel-Lucent
J. Jaeggli
Nokia Inc.
R. White
Cisco Systems
October 2010
Issues with Existing Cryptographic Protection Methods
for Routing Protocols
Abstract
Routing protocols have been extended over time to use cryptographic
mechanisms to ensure that data received from a neighboring router has
not been modified in transit and actually originated from an
authorized neighboring router.
The cryptographic mechanisms defined to date and described in this
document rely on a digest produced with a hash algorithm applied to
the payload encapsulated in the routing protocol packet.
This document outlines some of the limitations of the current
mechanism, problems with manual keying of these cryptographic
algorithms, and possible vectors for the exploitation of these
limitations.
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/rfc6039.
Copyright Notice
Copyright (c) 2010 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.
Table of Contents
1. Problem Statement ...............................................3
1.1. Pre-Image vs. Collision Attacks ............................4
1.2. Concerns about MD5 and the SHA-1 Algorithm .................4
2. Open Shortest Path First Version 2 (OSPFv2) .....................5
2.1. Management Issues with OSPFv2 ..............................5
2.2. Technical Issues with OSPFv2 ...............................6
3. Open Shortest Path First Version 3 (OSPFv3) .....................7
3.1. Management Issues with OSPFv3 ..............................7
3.2. Technical Issues with OSPFv3 ...............................8
4. Intermediate System to Intermediate System Routing
Protocol (IS-IS) ................................................9
4.1. Management Issues with IS-IS ...............................9
4.2. Technical Issues with IS-IS ...............................10
5. Border Gateway Protocol (BGP-4) ................................11
5.1. Management Issues with BGP-4 ..............................12
5.2. Technical Issues with BGP-4 ...............................13
6. The Routing Information Protocol (RIP) .........................13
6.1. Technical Issues with RIP .................................14
7. Bidirectional Forwarding Detection (BFD) .......................15
7.1. Technical Issues with BFD .................................15
8. Security Considerations ........................................17
9. Acknowledgements ...............................................17
10. References ....................................................17
10.1. Normative References .....................................17
10.2. Informative References ...................................18
11. Contributor's Address .........................................21
1. Problem Statement
Protocols, such as OSPF version 2 [RFC2328], version 3 [RFC5340],
IS-IS [RFC1195], BGP-4 [RFC4271], and BFD [RFC5880], employ various
mechanisms to create a cryptographic digest of each transmitted
protocol packet. Traditionally, these digests are the results of a
one-way hash algorithm, such as Message Digest 5 (MD5) [RFC1321],
across the contents of the packet being transmitted. A secret key is
used as the hash base (or seed). The digest is then recomputed by
the receiving router, using the same key as the original router used
to create the hash, then compared with the transmitted digest to
verify:
o That the router originating this packet is authorized via the
shared key mechanism to peer with the local router and exchange
routing data. The implicit trust of the routing protocol exchange
protected by a shared secret is intended to protect against the
injection of falsely generated routing data into the routing
system by unauthorized systems.
o That the data has not been altered in transit between the two
neighboring routers.
Digest verification schemes are not intended to protect the
confidentiality of information being exchanged between routers. The
information (entries in the routing table) is potentially available
through other mechanisms. Moreover, access to the physical media
between two routers exchanging routing data will confer the ability
to capture or otherwise discover the contents of the routing tables
in those routers.
Authentication mechanisms defined today have notable limitations:
o Manual configuration of shared secret keys, especially in large
networks and between networks, poses a major management problem.
In many cases, it is challenging to replace keys without
significant coordination or disruption.
o In some cases, when manual keys are configured, some forms of
replay protection are no longer possible, allowing the routing
protocol to be attacked through the replay of captured routing
messages.
This document outlines some of the problems with manual keying of
these cryptographic algorithms.
1.1. Pre-Image vs. Collision Attacks
A pre-image attack (an attempt to find new data with the same hash
value) would enable someone to find an input message that causes a
hash function to produce a particular output. In contrast, a
collision attack finds two messages with the same hash, but the
attacker can't pick what the message will be. Feasible collision
attacks against MD4, MD5, HAVAL-128, and RIPEMD have been documented
in [Crypto2004].
The ability to produce a collision does not currently introduce any
obvious or known attacks on routing protocols. Pre-image attacks
have the potential to cause problems in the future; however, due to
the message length, there are serious limitations to the feasibility
of mounting such an attack.
Protocols themselves have some built-in protection against collision
attacks. This is because a lot of values for fields in a protocol
packet are invalid or will produce an unusable packet. For example,
in OSPF the Link State Advertisement (LSA) type can be from 1 to 11.
Any other value in the field will result in the packet being
discarded.
Assume two packets M and M' are generated and have the same hash.
The above condition will further reduce the ability to produce a
message that is also a correct message from the protocol perspective,
as a lot of potential values are themselves not valid.
1.2. Concerns about MD5 and the SHA-1 Algorithm
There are published concerns about the overall strength of the MD5
algorithm ([Dobb96a], [Dobb96b], [Wang04]). While those published
concerns apply to the use of MD5 in other modes (e.g., use of MD5
X.509v3/PKIX digital certificates), they are not an attack upon Keyed
MD5 and Hash-based Message Authentication Code MD5 (HMAC-MD5), which
is what the current routing protocols have specified. There are also
published concerns about the Secure Hash Algorithm (SHA) algorithm
([Wang05], [Philip01], [Prav01], [Prav02], [Arjen05]) and also
concerns about the MD5 and SHA algorithms in the HMAC [RFC2104] mode
([RR07], [RR08]). The National Institute of Standards and Technology
(NIST) will be supporting HMAC-SHA-1 even after 2010 [NISTHmacSHA],
whereas it will drop support for SHA-1 in digital signatures. NIST
also recommends application and protocol designers move to the SHA-2
family of hash functions (i.e., SHA-224, SHA-256, SHA-384 and
SHA-512) for all new applications and protocols.
However, as explained above, such attacks are currently not
applicable to the routing protocols. Separately, some organizations
(e.g., the US government) prefer NIST algorithms, such as the SHA
family, over other algorithms (like MD5) for local policy reasons.
2. Open Shortest Path First Version 2 (OSPFv2)
OSPF [RFC2328] describes the use of an MD5 digest with OSPF packets.
MD5 keys are manually configured. The OSPF packet header includes an
authentication type field as well as 64 bits of data for use by the
appropriate authentication scheme. OSPF also provides for a non-
decreasing sequence number to be included in each OSPF protocol
packet to protect against replay attacks.
"OSPF with Digital Signatures" [RFC2154] is an Experimental RFC that
describes extensions to OSPF to digitally sign its Link State
Advertisements (LSAs). It is believed that if stronger
authentication and security is required, then OSPF (and the other
routing protocols) must migrate to using full digital signatures.
Doing this would enable precise authentication of the OSPF router
originating each OSPF link-state advertisement, and thereby provide
much stronger integrity protection for the OSPF routing domain.
However, since there have been no deployments, there is precious
little operational experience with this specification, and hence it
is not covered in this document.
2.1. Management Issues with OSPFv2
According to the OSPF specification [RFC2328], digests are applied to
packets transmitted between adjacent neighbors, rather than being
applied to the routing information originated by a router (digests
are not applied at the LSA level, but rather at the packet level).
[RFC2328] states that any set of OSPF routers adjacent across a
single link may use a different key to build MD5 digests than the key
used to build MD5 digests on any other link. Thus, MD5 keys may be
configured, and changed, on a per-link basis in an OSPF network.
OSPF does not specify a mechanism to negotiate keys, nor does it
specify any mechanism to negotiate the hash algorithms to be used.
With the proliferation of the number of hash algorithms, as well as
the need to continuously upgrade the algorithms, manually configuring
the information becomes very tedious. It should also be noted that
rekeying OSPF requires coordination among the adjacent routers.
2.2. Technical Issues with OSPFv2
While OSPF provides relatively strong protection through the
inclusion of MD5 digests, with additional data and sequence numbers
in transmitted packets, there are still attacks against OSPF:
o The sequence number is initialized to zero when forming an
adjacency with a newly discovered neighbor. When an adjacency is
brought down, the sequence number is also set to zero. If the
cryptographically protected packets of a router that is brought
down (for administrative or other reasons) are replayed by a
malicious router, traffic could be forced through the malicious
router. A malicious router might then induce routing loops, or
intercept or blackhole the traffic.
o OSPF allows multiple packets with the same sequence number. This
means that it's possible to replay the same packet many times
before the next legitimate packet is sent. An attacker may resend
the same packet repeatedly until the next Hello packet is
transmitted and received. The Hello interval, which is unknown,
determines the attack window.
o OSPF does not require the use of any particular hash algorithm;
however, only the use of MD5 digests for authentication and replay
protection is specified in RFC 2328. Most OSPF implementations
only support MD5 in addition to Null and Simple Password
authentication.
Recently, limitations in collision-resistance properties of the
MD5 and SHA-1 hash functions have been discovered; [RFC4270]
summarizes the discoveries. There have been attacks against the
use of MD5 as a hash; while these attacks do not directly apply to
the use of MD5 in routing protocols, it is prudent to have other
options available. For this reason, the general use of these
algorithms should be discouraged, and [RFC5709] adds support for
using SHA-1 and SHA-2 with the HMAC construct for OSPF.
o OSPF on a broadcast network shares the same key between all
neighbors on that broadcast network. Some OSPF packets are sent
to a multicast address.
Spoofing by any malicious neighbor possessing credentials or
replayable packets is therefore very easy. Possession of the key
itself is used as an identity validation, and no other identity
check is used. A malicious neighbor could send a packet, forging
the identity of the sender as being from another neighbor. There
would be no way in which the victim could distinguish the identity
of the packet sender.
o In some OSPF implementations, neighbors on broadcast, non-
broadcast multi-access (NBMA), and point-to-multipoint networks
are identified by the IP address in the IP header. The IP header
is not covered by the MAC in the cryptographic authentication
scheme as described in RFC 2328, and an attack can be made to
exploit this omission.
Assume the following scenario.
R1 sends an authenticated HELLO to R2. This HELLO is captured and
replayed back to R1. The source IP in the IP header of the
replayed packet is changed to that of R2.
R1, not finding itself in the HELLO, would deduce that the
connection is not bidirectional and would bring down the
adjacency.
3. Open Shortest Path First Version 3 (OSPFv3)
OSPFv3 [RFC5340] relies on the IP Authentication Header (AH)
[RFC4302] and the IP Encapsulating Security Payload (ESP) [RFC4303]
to cryptographically sign routing information passed between routers.
When using ESP, the null encryption algorithm [RFC2410] is used, so
the data carried in the OSPFv3 packets is signed, but not encrypted.
This provides data origin authentication for adjacent routers, and
data integrity (which gives the assurance that the data transmitted
by a router has not changed in transit). However, it does not
provide confidentiality of the information transmitted; this is
acceptable because the privacy of the information being carried in
the routing protocols need not be kept secret.
"Authentication/Confidentiality for OSPFv3" [RFC4552] mandates the
use of ESP with null encryption for authentication and also does
encourage the use of confidentiality to protect the privacy of the
routing information transmitted, using ESP encryption. However, it
only specifies the use of manual keying of routing information as
discussed in the following section.
3.1. Management Issues with OSPFv3
The OSPFv3 security document ("Authentication/Confidentiality for
OSPFv3" [RFC4552]) discusses, at length, the reasoning behind using
manually configured keys, rather than some automated key management
protocol such as IKEv2 [RFC4306]. The primary problem is the lack of
a suitable key management mechanism, as OSPF adjacencies are formed
on a one-to-many basis and most key management mechanisms are
designed for a one-to-one communication model. This forces the
system administrator to use manually configured security associations
(SAs) and cryptographic keys to provide the authentication and, if
desired, confidentiality services.
Regarding replay protection, [RFC4552] states that:
Since it is not possible using the current standards to provide
complete replay protection while using manual keying, the proposed
solution will not provide protection against replay attacks.
In the OSPFv3 case, the primary administrative issue with manually
configured SAs and keys is the management issue -- maintaining shared
sets of keys on all routers within a network. As with OSPFv2,
rekeying is an infrequent event requiring coordination. [RFC4552]
does not require that all OSPFv3 routers have the same key configured
for every neighbor, so each set of neighbors connected to a given
link could have a different key configured. While this makes it
easier to change the keys (by forcing the system administrator to
only change the keys on the routers on a single link), the process of
manual configuration for all the routers in a network to change the
keys used for OSPFv3 digests and confidentiality on a periodic basis
can be difficult.
3.2. Technical Issues with OSPFv3
The primary technical concern with the current specifications for
OSPFv3 is that when manual SA and key management is used as specified
in "Sequence Number Generation", Section 3.3.2 of [RFC4302]: "The
sender assumes anti-replay is enabled as a default, unless otherwise
notified by the receiver (see Section 3.4.3) or if the SA was
configured using manual key management". Replaying OSPFv3 packets
can induce several failures in a network, including:
o Replaying Hello packets with an empty neighbor list can cause all
the neighbor adjacencies with the sending router to be reset,
disrupting network communications.
o Replaying Hello packets from early in the designated router
election process on broadcast links can cause all the neighbor
adjacencies with the sending router to be reset, disrupting
network communications.
o Replaying database description (DB-Description) packets can cause
all FULL neighbor adjacencies with the sending router to be reset,
disrupting network communications.
o Replaying link state request (LS-Request) packets can cause all
FULL neighbor adjacencies with the sending router to be reset,
disrupting network communications.
o Capturing a full adjacency process (from two-way all the way to
FULL state), and then replaying this process when the router is no
longer attached can cause a false adjacency to be formed, allowing
an attacker to attract traffic.
o OSPFv3 on a broadcast network shares the same key between all
neighbors on that network. Some OSPF packets are sent to a
multicast address.
Spoofing by a malicious neighbor is very easy. Possession of the
key itself is used as an identity check. There is no other
identity check used. A neighbor could send a packet specifying
the packet came from some other neighbor and there would be no way
in which the attacked router could figure out the identity of the
packet sender.
4. Intermediate System to Intermediate System Routing Protocol (IS-IS)
Integrated IS-IS [RFC1195] uses HMAC-MD5 authentication with manual
keying, as described in [RFC5304], and has recently been extended to
provide support for using the HMAC construct along with the SHA
family of cryptographic hash functions [RFC5310]. There is no
provision within IS-IS to encrypt the body of a routing protocol
message.
4.1. Management Issues with IS-IS
[RFC5304] states that each Link State Protocol Data Unit (LSP)
generated by an intermediate system is signed with the HMAC-MD5
algorithm using a key manually defined by the network administrator.
Since authentication is performed on the LSPs transmitted by an
intermediate system, rather than on the packets transmitted to a
specific neighbor, it is implied that all the intermediate systems
within a single flooding domain must be configured with the same key
in order for authentication to work correctly.
The initial configuration of manual keys for authentication within an
IS-IS network is simplified by a state where LSPs containing
HMAC-MD5/HMAC-SHA authentication TLVs are accepted by intermediate
systems without the keys, but the digest is not validated. Once keys
are configured on all routers, changing those keys becomes much more
difficult.
IS-IS [RFC1195] does not specify a mechanism to negotiate keys, nor
does it specify any mechanism to negotiate the hash algorithms to be
used.
With the proliferation of available hash algorithms, as well as the
need to upgrade the algorithms, manual configuration requires
coordination among intermediate systems, which can become tedious.
4.2. Technical Issues with IS-IS
[RFC5304] states: "This mechanism does not prevent replay attacks;
however, in most cases, such attacks would trigger existing
mechanisms in the IS-IS protocol that would effectively reject old
information".
The few cases where existing mechanisms in the IS-IS protocol would
not effectively reject old information are:
- the Hello packets or the IS-IS Hellos (IIHs) that are used to
discover neighbors, and
- the Sequence Number Packets (SNPs).
As described in IS-IS [RFC1195], a list of known neighbors is
included in each Hello transmitted by an intermediate system to
ensure two-way communications with any specific neighbor before
exchanging link state databases.
IS-IS does not provide a sequence number. IS-IS packets are
vulnerable to replay attacks; any packet can be replayed at any point
of time. So long as the keys used are the same, protocol elements
that would not be rejected will affect existing sessions.
A Hello packet containing a digest within a TLV and an empty neighbor
list could be replayed, resulting in all adjacencies with the
original transmitting intermediate system to be restarted.
A replay of an old Complete Sequence Number Packet (CSNP) could cause
LSPs to be flooded, resulting in an LSP storm.
IS-IS specifies the use of the HMAC-MD5 and HMAC-SHA-1 to protect
IS-IS packets.
IS-IS does not have a notion of Key ID. During key rollover, each
message received has to be checked for integrity against all keys
that are valid. A denial-of-service (DoS) attack may be induced by
sending IS-IS packets with random hashes. This will cause the IS-IS
packet to be checked for authentication with all possible keys,
increasing the amount of processing required. This issue, however,
has been fixed in the recent [RFC5310], which introduces the concept
of Key IDs in IS-IS.
Recently, limitations in collision-resistance properties of the MD5
and SHA-1 hash functions have been discovered; [RFC4270] summarizes
the discoveries. There have been attacks against the use of MD5 as a
hash; while these attacks do not directly apply to the use of
HMAC-MD5 in IS-IS, it is prudent to have other options available.
For this reason, the general use of these algorithms should be
discouraged, and [RFC5310] adds support for using HMAC-SHA with
IS-IS.
IS-IS on a broadcast network shares the same key between all
neighbors on that network.
This makes spoofing by a malicious neighbor easy since IS-IS packets
are sent to a link-layer multicast address. Possession of the key
itself is used as an authorization check. A neighbor could send a
packet spoofing the identity of a neighbor, and there would be no way
in which the attacked router could discern the identity of the
malicious packet sender.
The Remaining Lifetime field in the LSP is not covered by the
authentication. An IS-IS router can receive its own self-generated
LSP segment with zero lifetime remaining. In that case, if it has a
copy with non-zero lifetime, it purges that LSP, i.e., it increments
the current sequence number and floods all the segments again. This
is much worse in IS-IS than in OSPF because there is only one LSP
other than the pseudonode LSPs for the LANs on which the IS-IS router
is the Designated Intermediate System (DIS).
In this way, an attacker can force the router to flood all segments
-- potentially a large number if the number of routes is large. It
also causes the sequence number of all the LSPs to increase fast. If
the sequence number increases to the maximum (0xFFFFFFFF), the IS-IS
process must shut down for around 20 minutes (the product of MaxAge +
ZeroAgeLifetime) to allow the old LSPs to age out of all the router
databases.
5. Border Gateway Protocol (BGP-4)
BGP-4 [RFC4271] uses TCP [RFC0793] for transporting routing
information between BGP speakers that have formed an adjacency.
[RFC2385] describes the use of the TCP MD5 digest option for
providing packet origin authentication and data integrity protection
of BGP packets. [RFC3562] provides suggestions for choosing the key
length of the ad hoc Keyed MD5 mechanism specified in [RFC2385].
There is no provision for confidentiality for any of these BGP
messages.
TCP MD5 [RFC2385] has recently been obsoleted by a new TCP
Authentication Option (TCP-AO) [RFC5925]. [RFC5925] specifies the
use of stronger Message Authentication Codes (MACs), protects against
replays even for long-lived TCP connections, and provides more
details than TCP-MD5 on the association of security with TCP
connections. It allows rekeying during a TCP connection, assuming
that an out-of-band protocol or manual mechanism provides the new
keys. Note that TCP MD5 does not preclude rekeying during a
connection, but does not require its support either. Further, TCP-AO
supports key changes with zero segment loss, whereas key changes in
TCP MD5 can lose segments in transit during the changeover or require
trying multiple keys on each received segment during key use overlap
because TCP MD5 lacks an explicit Key ID. Although TCP recovers lost
segments through retransmission, loss can have a substantial impact
on performance.
However, this document covers only TCP MD5, as all current
deployments are still using BGP with TCP MD5 and have not upgraded to
[RFC5925]. There isn't enough operational experience present to
evaluate the technical and management issues with this proposal yet.
Compared to previously described IGP protocols, BGP has additional
exposure due to the nature of the environment where it is typically
used -- namely, between autonomous networks (under different
administrative control). While routers running interior gateway
protocols may all be configured with the same administrative
authority, two BGP peers may be in different administrative domains,
having different policies for key strength, rollover frequency, etc.
An autonomous system must often support a large number of keys at
different BGP boundaries, as each connecting AS represents a
different administrative entity. In practice, once set, shared
secrets between BGP peers are rarely, if ever, changed.
5.1. Management Issues with BGP-4
Each pair of BGP speakers forming a peering may have a different MD5
shared key that facilitates the independent configuration and
changing of keys across a large-scale network. Manual configuration
and maintenance of cryptographic keys across all BGP sessions is a
challenge in any large-scale environment.
Most BGP implementations will accept BGP packets with a bad digest up
to the hold interval negotiated between BGP peers at peering startup,
in order to allow for MD5 keys to be changed with minimal impact on
operation of the network. This technique does, however, allow some
short period of time during which an attacker may inject BGP packets
with false MD5 digests into the network and can expect those packets
to be accepted, even though the MD5 digests are not valid.
5.2. Technical Issues with BGP-4
BGP relies on TCP [RFC0793] for transporting data between BGP
speakers. BGP can rely on TCP's protection against data corruption
and replay to preclude replay attacks against BGP sessions. A great
deal of research has gone into the feasibility of an attacker
overcoming these protections, including [TcpWindow] and [Conv01].
Most router and operating system (OS) vendors have modified their TCP
implementations to resolve the security vulnerabilities described in
these references, where possible.
As mentioned earlier, MD5 is vulnerable to collision attacks and can
be attacked through several means, such as those explored in
[Wang04].
Though it can be argued that the collision attacks do not have a
practical application in this scenario, the use of MD5 should be
discouraged.
Routers performing cryptographic processing of packets in software
may be exposed to additional opportunities for DoS attacks. An
attacker may be able to transmit enough spoofed traffic with false
digests that the router's processor and memory resources are
consumed, causing the router to be unable to perform normal
processing. This exposure is particularly problematic between
routers not under unified administrative control.
6. The Routing Information Protocol (RIP)
The initial version of RIP was specified in STD 34 [RFC1058]. This
version did not provide for any authentication or authorization of
routing data, and thus was vulnerable to any of a number of attacks
against routing protocols. This limitation was one reason why this
protocol was moved to Historic status [RFC1923].
RIPv2, originally specified in [RFC1388], then [RFC1723], was
finalized in STD 56 [RFC2453]. This version of the protocol provides
for authenticating packets with a digest. The details thereof have
initially been provided in "RIP-2 MD5 Authentication" [RFC2082];
"RIPv2 Cryptographic Authentication" [RFC4822] obsoletes [RFC2082]
and adds details of how the SHA family of hash algorithms can be used
to protect RIPv2. [RFC2082] only specified the use of Keyed MD5.
6.1. Technical Issues with RIP
o The sequence number used by a router is initialized to zero at
startup, and is also set to zero whenever the neighbor is brought
down. If the cryptographically protected packets of a router that
is brought down (for administrative or other reasons) are stored
by a malicious router, the new router could replay the packets
from the previous session, thus forcing traffic through the
malicious router. Dropping of such packets by the router could
result in blackholes. Also, forwarding wrong packets could result
in routing loops.
o RIPv2 allows multiple packets with the same sequence number. This
could mean the same packet may be replayed many times before the
next legitimate packet is sent. An attacker may resend the same
packet repeatedly until the next Hello packet is transmitted and
received, which means the Hello interval therefore determines the
attack window.
o RIPv2 [RFC2453] did not specify the use of any particular hash
algorithm. RFC 4822 introduced HMAC-SHA1 as mandatory to
implement, along with Keyed MD5 as specified in [RFC2082].
Support for Keyed MD5 was mandated to ensure compatability with
legacy implementations.
o "RIPv2 Cryptographic Authentication" [RFC4822] does not cover the
UDP and the IP headers. It is therefore possible for an attacker
to modify some fields in the above headers without routers
becoming aware of it.
There is limited exposure to modification of the UDP header, as
the RIP protocol uses only it to compute the length of the RIP
packet. Changes introduced in the UDP header would cause RIP
authentication to fail the RIP authentication, thereby limiting
exposure.
RIP uses the source IP address from the IP header to determine
which RIP neighbor it has learnt the RIP Update from. Changing
the source IP address can be used by an attacker to disrupt the
RIP routing sessions between two routers R1 and R2, as shown in
the following examples.
Scenario 1:
R1 sends an authenticated RIP message to R2 with a cryptographic
sequence number X.
The attacker then needs a packet with a higher sequence number
originated by R2 either, from this session or from some earlier
session.
The attacker can then replay this packet to R2 by changing the
source IP to that of R1.
R2 would then no longer accept any more RIP Updates from R1, as
those would have a lower cryptographic sequence number. After 180
seconds (or less), R2 would consider R1 timed out and bring down
the RIP session.
Scenario 2:
R1 announces a route with cost C1 to R2. This packet can be
captured by an attacker. Later, if this cost changes and R1
announces this with a different cost C2, the attacker can replay
the captured packet, modifying the source IP to a new arbitrary IP
address, thereby masquerading as a different router.
R2 will accept this route and the router as a new gateway, and R2
would then use the non-existent router as a next hop for that
network. This would only be effective if the cost C1 is less than
C2.
7. Bidirectional Forwarding Detection (BFD)
BFD is specified in [RFC5880]. Extensions to BFD for multihop
[RFC5883] and single hop [RFC5881] are defined for IPv4 and IPv6. It
is designed to detect failure with the forwarding plane next hop.
The BFD base specification specifies an optional authentication
mechanism that can be used by the receiver of a packet to be able to
authenticate the source of the packet. It relies on the facts that
the keys are shared between the peers and no mechanism is defined for
the actual key generation.
7.1. Technical Issues with BFD
o The level of security provided is based on the Authentication Type
used. However, the authentication algorithms defined are MD5 or
SHA-1 based. As mentioned earlier, MD5 and SHA-1 are both known
to be vulnerable to collision attacks.
o The BFD specification mentions mechanisms to allow for the change
of authentication state based on the state of a received packet.
This can cause a denial-of-service attack where a malicious
authenticated packet (stored from a past session) can be relayed
over a session that does not use authentication. This causes one
end to assume that authentication is enabled at the other end, and
hence the BFD adjacency is dropped. This would be a harder attack
to put forth when meticulously keyed authentication is in use.
o BFD works on microsecond timers. When malicious packets are sent
at short intervals, with the authentication bit set, it can cause
a DoS attack.
o BFD allows a mode called the echo mode. Echo packets are not
defined in the BFD specification, though they can keep the BFD
session up. There are no guidelines on the properties of the echo
packets beyond the choice of the source and destination addresses.
While the BFD specification recommends applying security
mechanisms to prevent spoofing of these packets, there are no
guidelines on what type of mechanisms are appropriate.
Any security issues in the echo mode will directly affect the BFD
protocol and session states, and hence the network stability. The
potential effects and remedies as understood today are somewhat
limited, however. For instance, any replay attacks would be
indistinguishable from normal forwarding of the tested router. An
attack would still cause a faulty link to be believed to be up,
but there is little that can be done about it. However, if the
echo packets are guessable, it may be possible to spoof from an
external source and cause BFD to believe that a one-way link is
really bidirectional. As a result, it is important that the echo
packets contain random material that is also checked upon
reception.
o BFD packets can be sent at millisecond intervals (the protocol
uses timers at microsecond intervals). When using authentication,
this can cause frequent sequence number wrap-around as a 32-bit
sequence number is used, thus considerably reducing the security
of the authentication algorithms.
o Recently, limitations in collision-resistance properties of the
MD5 and SHA-1 hash functions have been discovered; [RFC4270]
summarizes the discoveries. There have been attacks against the
use of MD5 as a hash; while these attacks do not directly apply to
the use of HMAC-MD5 and keyed SHA-1 in BFD, it is prudent to have
other options available. Such attacks do not mean that BFD using
SHA-1 for authentication is at risk. However, it does mean that
SHA-1 should be replaced as soon as possible and should not be
used for new applications.
It should be noted that if SHA-1 is used in the Hashed Message
Authentication Code (HMAC) [RFC2104] construction, then collision
attacks currently known against SHA-1 do not apply. The new
attacks on SHA-1 have no impact on the security of HMAC-SHA-1.
There are already proposals [GenBFDAuth] that add support for HMAC
with the SHA-1 and SHA-2 family of hash functions for BFD.
8. Security Considerations
This document outlines security issues arising from the current
methodology for manual keying of various routing protocols. No
specific changes to routing protocols are proposed in this document;
likewise, no new security requirements result.
9. Acknowledgements
We would like to acknowledge Sam Hartman, Ran Atkinson, Stephen Kent
and Brian Weis for their initial comments on this document. Thanks
to Merike Kaeo and Alfred Hoenes for reviewing many sections of the
document and providing lot of useful comments.
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP
and dual environments", RFC 1195, December 1990.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
1998.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the
TCP MD5 Signature Option", RFC 2385, August 1998.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
November 1998.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
2006.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005. Kent, S., "IP Authentication Header",
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4552] Gupta, M. and N. Melam,
"Authentication/Confidentiality for OSPFv3", RFC
4552, June 2006.
[RFC4822] Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
Authentication", RFC 4822, February 2007.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
"OSPF for IPv6", RFC 5340, July 2008.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White,
R., and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, February 2009.
10.2. Informative References
[Arjen05] Arjen K. Lenstra, "Further progress in Hashing
cryptanalysis", Lucent Bell Laboratories, February
26, 2005.
[Conv01] Convery, et al., "BGP Vulnerability Testing:
Separating Fact from FUD v1.00", NANOG 28, pp. 1-61,
June 2003.
[Crypto2004] Xiaoyun Wang, Xuejia Lai, Dengguo Feng, and Hongbo
Yu, "Collisions for hash functions MD4, MD5,
HAVAL-128, and RIPEMD", Crypto 2004 Rump Session.
[Dobb96a] Dobbertin, H., "Cryptanalysis of MD5 Compress",
Technical Report, 2 May 1996. (Presented at the Rump
Session of EuroCrypt 1996.)
[Dobb96b] Dobbertin, H., "The Status of MD5 After a Recent
Attack", CryptoBytes, Vol. 2, No. 2, Summer 1996.
[GenBFDAuth] Bhatia, M. and V. Manral, "BFD Generic Cryptographic
Authentication", Work in Progress, June 2010.
[NISTHmacSHA] "NIST's Policy on Hash Functions", 2006,
http://csrc.nist.gov/groups/ST/hash/policy.html.
[Philip01] Philip Hawkes, Michael Paddon, and Gregory G. Rose,
"On Corrective Patterns for the SHA-2 Family", IACR
ePrint Archive, 2004,
http://eprint.iacr.org/2004/207.
[Prav01] Praveen Gauravaram, et al., "Collision Attacks on MD5
and SHA-1: Is this the 'Sword of Domocles' for
Electronic Commerce?", Information Security Institue
(ISI), Queensland University of Technology (QUT),
Australia.
[Prav02] Praveen Gauravaram, et al., "Some thoughts on
Collision Attacks in the Hash Functions Md5, SHA-0
and SHA-1", Information Security Institue (ISI),
Queensland University of Technology (QUT), Australia.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC
1058, June 1988.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
[RFC1388] Malkin, G., "RIP Version 2 Carrying Additional
Information", RFC 1388, January 1993.
[RFC1723] Malkin, G., "RIP Version 2 - Carrying Additional
Information", RFC 1723, November 1994.
[RFC1923] Halpern, J. and S. Bradner, "RIPv1 Applicability
Statement for Historic Status", RFC 1923, March 1996.
[RFC2082] Baker, F. and R. Atkinson, "RIP-2 MD5
Authentication", RFC 2082, January 1997.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2154] Murphy, S., Badger, M., and B. Wellington, "OSPF with
Digital Signatures", RFC 2154, June 1997.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm
and Its Use With IPsec", RFC 2410, November 1998.
[RFC3562] Leech, M., "Key Management Considerations for the TCP
MD5 Signature Option", RFC 3562, July 2003.
[RFC4270] Hoffman, P. and B. Schneier, "Attacks on
Cryptographic Hashes in Internet Protocols", RFC
4270, November 2005.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[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.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection (BFD)", RFC 5880, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC
5881, June 2010.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection (BFD) for Multihop Paths", RFC 5883, June
2010.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RR07] Rechberger, C. and V. Rijmen, "On Authentication with
HMAC and Non-random Properties", Financial
Cryptography and Data Security, Lecture Notes in
Computer Science, Volume 4886/2008, Springer-Verlag,
Berlin, December 2007.
[RR08] Rechberger, C. and V. Rijmen, "New Results on
NMAC/HMAC when Instantiated with Popular Hash
Functions", Journal of Universal Computer Science,
Volume 14, Number 3, pp. 347-376, 1 February 2008.
[TcpWindow] Watson, P., "Slipping in the Window: TCP Reset
attacks", Presentation at 2004 CanSecWest,
http://cansecwest.com/csw04archive.html.
[Wang04] Wang, X., et al., "Collisions for Hash Functions MD4,
MD5, HAVAL-128 and RIPEMD", August 2004, IACR ePrint
Archive, http://eprint.iacr.org/2004/199.
[Wang05] Wang, X., et al., "Finding Collisions in the Full
SHA-1", Proceedings of Crypto 2005, Lecture Notes in
Computer Science, Volume 3621, pp. 17-36, Springer-
Verlag, Berlin, August 31, 2005.
11. Contributor's Address
Sue Hares
NextHop
USA
EMail: shares@nexthop.com
Authors' Addresses
Vishwas Manral
IP Infusion, Inc.
1188 E. Arques Ave.
Sunnyvale, CA 94085
EMail: vishwas@ipinfusion.com
Manav Bhatia
Alcatel-Lucent
Bangalore
India
EMail: manav.bhatia@alcatel-lucent.com
Joel P. Jaeggli
Nokia Inc.
EMail: joel.jaeggli@nokia.com
Russ White
Cisco Systems
RTP North Carolina
USA
EMail: riw@cisco.com