One document matched: draft-ietf-rpsec-ospf-vuln-01.txt
Differences from draft-ietf-rpsec-ospf-vuln-00.txt
Network Working Group Emanuele Jones
INTERNET DRAFT Alcatel
draft-ietf-rpsec-ospf-vuln-01.txt Olivier Le Moigne
Alcatel
1 December 2004
OSPF Security Vulnerabilities Analysis
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
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The list of current Internet-Drafts can be accessed at
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Abstract
Internet infrastructure protocols were designed at the very early
stages of computer networks when "cyberspace" was still perceived as
a benign environment. As a consequence, malicious attacks were not
considered to be a major risk when these protocols were designed,
leaving today's Internet vulnerable. This paper provides an analysis
of OSPF vulnerabilities that could be exploited to modify the normal
routing process across a single domain together with an assessment
of when internal OSPF mechanisms can or cannot be leveraged to
better secure a domain.
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Table of Contents
Status of this Memo ........................................... 1
Specification of Requirements ................................. 1
Abstract ...................................................... 1
1. Introduction ............................................... 3
1.1. Attacker's Definition .................................... 3
1.2. Attacker's Location ...................................... 4
1.3. Vulnerabilities Damages and Consequences ................. 4
2. Generic Attack Techniques .................................. 5
3. Vulnerabilities and Risks .................................. 6
3.1. OSPF General Vulnerabilities ............................. 6
3.1.1. Local Intrusion Global Impact .......................... 6
3.1.2. Remote Attacker ........................................ 7
3.1.3. Attacker Disabling Fight Back .......................... 7
3.1.4. Attacker Leveraging Fight Back ......................... 8
3.1.5. Dealing with External Routes ........................... 8
3.2. Protocol-specific Vulnerabilities ........................ 9
3.2.1. Packet Header with Cryptographic Authentication Enabled. 9
3.2.2. Hello Message .......................................... 10
3.2.3. DB Description, Link State Request and Acknowledgment .. 12
3.2.4. Link State Update ...................................... 12
3.3. Resource Consumption Vulnerabilities ..................... 15
3.3.1. OSPF Cryptographic Authentication ...................... 15
3.3.2. Hello Message .......................................... 16
3.3.3. Link State Request Message ............................. 16
3.3.4. Link State Acknowledgment Message ...................... 16
3.3.5. Link State DB Overflow ................................. 16
3.3.6. Others ................................................. 17
3.4. Vulnerabilities through Other Protocols .................. 18
3.4.1. IP ..................................................... 18
3.4.2. Other Supporting Protocols (Management) ................ 18
3.5. Residual Risk ............................................ 18
4. References ................................................. 19
5. Authors' Addresses ......................................... 20
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1. Introduction
Internet infrastructure protocols were designed at the very early
stages of computer networks when "cyberspace" was still perceived as
a benign environment. As a consequence, malicious attacks were not
considered to be a major risk when these protocols were designed,
leaving today's Internet infrastructure vulnerable.
Since routers work in a cooperatively manner based on forwarding
network information received from their peers, they are all
threatened by the possibility that the exchanged routing information
may have been contaminated or forged by a malicious or faulty
entity.
This paper provides an analysis of OSPF [1] vulnerabilities that
could be exploited to modify the normal routing process across a
single domain, together with an assessment of when internal OSPF
mechanisms can or cannot be leveraged to secure a domain.
1.1. Attacker's Definition
Throughout this paper the term attacker will be used to define any
entity capable of posing any threat to an OSPF routing domain.
Hence, this definition includes: 1) any subverted OSPF router, 2)
any malicious software capable of interacting with an OSPF routing
domain, 3) any faulty or misconfigured legitimate OSPF peer.
From a security standpoint, this paper is consolidating all possible
OSPF deployment situations into two opposite scenarios.
The first scenario requires OSPF Cryptographic Authentication or
Simple Password Authentication to be present on all links within a
routing domain. The second scenario takes place when Null
Authentication is adopted.
If one link is not protected then the whole routing domain becomes
potentially vulnerable; if the attacker is in the position to obtain
even a single copy of any OSPF message then the authentication
provided by Simple Password is compromised and the security for the
entire routing domain falls immediately in the second scenario.
In the first scenario, Cryptographic Authentication being deployed,
there are two kinds of entities capable of attacking or posing
threats: insiders and outsiders. An attacking entity is considered
an insider if it is in possession of the secret key for any OSPF
Cryptographic Authentication session either through: cryptanalysis,
social engineering, coercion or access to a compromised/subverted
routing resource. This also includes threats arising from
malfunctioning or faulty-configured OSPF routers. An outsider is an
attacker that is not in possession of the secret key.
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In the second scenario, when the routing domain is not protected by
OSPF Cryptographic or Simple Password authentication there is no
distinction between insider and outsider entities. Any attacker can
successfully forge OSPF messages on behalf of any OSPF peer,
legitimate or not.
1.2. Attacker's Location
Since OSPF routers on broadcast, on Point-to-Multipoint, NBMA and on
virtual links will accept unicast packets that are destined directly
to them, no assumption is made on the location of the attacking
entity. This leads to a scenario where an attacker, in possession of
a secret key, if at all needed, can attack a router located in a
remote routing domain. The proper implementation of ingress
filtering and other mechanisms described by RFC2827 [2] and
recently by the Internet Draft [3] should mitigate this situation,
forcing insider and outsider attackers to at least have access to
one of the links in the routing domain target of their attack.
1.3. Vulnerabilities Damages and Consequences
Generally speaking attackers will be able to disrupt and manipulate
the routing domain, posing serious threats to the actual delivery of
data and control plane packets.
For instance, if the routing information creates loops in the
forwarding path some packets will never be delivered, denying
service to many destinations. Loops also create congestion by
leaving packets in the network longer than necessary and by
consuming resources without providing any useful service in the end.
The incorrect forwarding of large amounts of traffic over one link
may overwhelm the link and result in the delaying, or even
prevention, of traffic delivery. Moreover, incorrect routing
information could result in data traffic transiting networks that
otherwise would have never seen that data.
Finally, routing information that incorrectly reports OSPF Areas, or
any other portion of the domain, as unreachable will deny services
to all hosts connected to or exchanging traffic with said areas.
The damages [4] that might result from these attacks are:
starvation: data traffic destined for a node is forwarded to a part
of the network that cannot deliver it,
network congestion: more data traffic is forwarded through some
portion of the network than would otherwise need to carry the
traffic,
blackhole: large amounts of traffic are directed as to be forwarded
through one router that cannot handle the increased level of traffic
and drops many/most/all packets,
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delay: data traffic destined for a node is forwarded along a path
that is in some way inferior to the path it would otherwise take,
looping: data traffic is forwarded along a path that loops, so that
the data is never delivered,
eavesdrop: data traffic is forwarded through some router or network
that would otherwise not see the traffic, affording an opportunity
to see the data,
partition: some portion of the network believes that it is
partitioned from the rest of the network when it is not,
cut: some portion of the network believes that it has no route to
some network that is in fact connected,
churn: the forwarding in the network changes at a rapid pace,
resulting in large variations in the data delivery patterns (and
adversely affecting congestion control techniques),
instability: OSPF becomes unstable so that convergence on a global
forwarding state is not achieved,
overload: the OSPF messages themselves become a significant portion
of the traffic the network carries.
resource exhaustion: the OSPF messages themselves cause exhaustion
of critical router resources, such as table space and queues.
These consequences can fall exclusively on a single OSPF Area or may
effect the operation of the OSPF network domain as a whole.
2. Generic Attack Techniques
The OSPF protocol is subject to the following attacks (list taken
from the IAB Internet-Draft providing guideline for the security
considerations section of Internet-Drafts [5]).
Eavesdropping: The routing data carried in OSPF is carried in clear-
text, so eavesdropping is a possible attack against routing data
confidentiality.
Message Replay: In general, OSPF with Cryptographic Authentication
provides a sufficient mechanism for replay protection of its
messages. Nonetheless, there are still some scenarios in which an
outsider attacker can successfully replay OSPF messages; these are
illustrated over the next sections.
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Message Insertion: OSPF with Cryptographic Authentication enabled is
not vulnerable to message insertion from outsiders. In the case of
an insider or in the absence of Cryptographic Authentication,
message insertion becomes a trivial operation even for a remote
attacker.
Message Deletion: OSPF provides a certain degree of protection
against message deletion. The receiver itself cannot detect if a
message has been deleted or not, but the sender will detect a
deleted Link State Update (LSU) message since it will not receive
any OSPF Link State Acknowledgment message for it. There is no
acknowledging mechanism for Hello messages, but the deletion of
some, generally four or more, consecutive Hello messages belonging
to the same router will cause "adjacency breaking" and thus be
easily detected by all the parties involved.
Message Modification: OSPF with Cryptographic Authentication
provides protection against modification of messages. In the case of
an insider or in the absence of Cryptographic Authentication message
modification becomes possible.
Man-In-The-Middle: OSPF with Cryptographic Authentication provides
protection against man-in-the-middle attacks. In the case of an
insider or in the absence of Cryptographic Authentication, the
protocol becomes exposed to man-in-the-middle attacks through the
lower network layers - such as ARP spoofing - on all OSPF peers that
are one hop apart; while OSPF peers connected over virtual links are
exposed to Layer 3 man-in-the-middle attacks too.
Denial-of-Service: While bogus routing information data can
represent a Denial of Service attack on the end systems that are
trying to transmit data through the network and on the network
infrastructure itself, certain bogus information can represent a
more specific Denial of Service on the OSPF routing protocol itself.
For example, it is possible to reach the limits of the Link State
Database of a victim with External LSAs or with bogus LSA headers
during the Link State Database Exchange phase.
3. Vulnerabilities and Risks
3.1. OSPF General Vulnerabilities
The risks in OSPF arise from the following fundamental
vulnerabilities:
3.1.1. Local Intrusion Global Impact
Compromising a single network equipment (router) or a link's
security has an obvious and immediate local impact (ability to
disable local links, to change properties, to stop routers etc...).
Unfortunately, due to the lack of end-to-end authentication
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mechanisms - such as a Public Key Infrastructure (PKI) - a breach in
a single link has also a global impact since the attacker is now in
the position to tamper with information regarding any other remote
network equipment belonging to the same routing domain.
3.1.2. Remote Attacker
Even though OSPF is designed and deployed to be used as an intra-
domain routing protocol, in most scenarios and situations an OSPF
router will still accept unicast IP packets directly addressed to
itself as described in paragraph 8.1 of RFC2328 [1]. "On physical
point-to-point networks, the IP destination is always set to the
address AllOSPFRouters. On all other network types (including
virtual links), the majority of OSPF packets are sent as unicasts,
i.e., sent directly to the other end of the adjacency." This opens
the door to attacks that may be originating from outside the OSPF
domain. Timing the stream of different packets needed for a given
attack poses a certain degree of difficulty if executed from a
remote AS, but it may not be enough to stop a skilled and motivated
attacker. This means that, for example, customers on the access
edges of a network can start attacking the routing domain in the
core, if said domain were not to be protected by Cryptographic
Authentication or if the malicious subscribers were to obtain the
secret key.
3.1.3. Attacker Disabling Fight Back
It is often the case while reading papers, or other literature
material, about OSPF to come across the concept of an OSPF "natural"
fight back mechanism, for example [6]. OSPF fight back can be
defined as follows: any router receiving an LSA that lists itself as
the advertising router and noticing that the content of this LSA is
not coherent with its status of resources will try to correct the
situation either by flushing or updating the erroneous LSA. The
following three scenarios show how the OSPF fight back mechanism can
be disabled clearing the way to stealthy attacks.
3.1.3.1 Periodic Injection
This is a brief explanation on how a malicious LSA will succeed in
attacking a routing domain, overriding any fight back:
According to RFC2328 [1], a router will never emit (or update) its
LSAs faster than once every MinLSInterval (5 seconds). This allows
for almost permanent changes in the routing domain, if an attacker
is flooding the OSPF domain with malicious LSAs at a rate higher
than one every MinLSInterval.
On top of this, if an OSPF implementation behaves as described by
RFC2328 [1, paragraph 13], the router owner of the LSA may never
fight back and it will collaborate in the flooding of malicious
routing information on its behalf. The flooding happens because the
malicious LSA is considered newer than the copy already present in
the legitimate owner's Link State Database - the malicious LSA will
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have a higher sequence number - (check performed on Step 5) and
because the legitimate copy of the LSA already present in the Link
State Database was not received via flooding but installed by the
router itself (check performed in step 5.a). When step 5.f is
finally executed - after the malicious LSA has been already flooded
- a simple test reveals that the LSA was owned by the router and
that it contained erroneous information. Only at this stage action
is taken to correct it; but since any router must wait MinLSInterval
before updating any of its LSAs, the owner will fight back every
MinLSInterval while the flooding is in progress. We have also
observed a complete lack of fight back in implementations that
erroneously reset MinLSInterval when flooding LSAs.
3.1.3.2 Partitioned Networks
If the flooding mechanism does not have a path to rely malicious
LSAs to the legitimate owner, said owner will never initiate a fight
back. An example of this could be a subverted router conveniently
located on a partitioning link. If said router is removed, the
entire network domain would be partitioned into two disconnected
portions. This subverted router could choose to inject a given
malicious LSA only into one part of the routing domain, claiming
that this LSA is coming from a legitimate router located on the
opposite portion of the network. The legitimate router will never be
made aware of the forged information on its behalf and thus will
never initiate a fight back. This will create fatal inconsistencies
between the Link State Databases of the various OSPF routers.
3.1.3.3 Phantom Routers
All information injected in the routing domain on behalf of non-
existing (phantom) OSPF routers will never trigger a fight back
reaction. Thus, this information will remain in the Link State
Databases of the legitimate routers for MaxAge (1 hour, by default).
It is important to underline that even if Link State Advertisements
(LSAs) crafted on behalf of phantom routers are kept in the Link
State Database, these are not taken into account by the Shortest
Path First (SPF) algorithm.
3.1.4. Attacker Leveraging Fight Back
The fight back mechanism can contribute to amplify certain Denial of
Service attacks. One single false LSA may unleash a significant
number of LSA updates that are trying to correct it. Even though
such a reaction is both efficient and desirable, it may be leveraged
to amplify the effects of certain Denial of Service attacks, if
continuously triggered.
3.1.5. Dealing with External Routes
Every piece of routing information that is dealing with outside
routes, forged or real, that is introduced in the domain - by means
of route redistribution via BGP, RIP or any other routing protocol
including statically configured - cannot be verified and it is
propagated to all OSPF Areas of the domain that are not configured
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as stub-areas or NSSA. Even though verification of routes that are
outside the routing domain is clearly beyond the scope of OSPF, the
current flooding mechanism of such information may be used as an
efficient intrinsic vector for conveying malicious/bogus messages.
Moreover, if an attacker manages to subvert an ASBR node, or
successfully masquerades as one, there will be no fight back from
any of the other ASBRs regarding ownership, validity and metric
advertisement for the External routes claimed by the subverted ASBR;
thus, the attacker could easily attract to itself big portions of
the traffic destined outside the AS.
3.2. Protocol-specific Vulnerabilities
There are two types of authentication mechanisms in OSPF: Simple
Password and Cryptographic. Simple Password authentication consists
of a plain text password carried in the header of each OSPF message;
the vulnerability of this Authentication method is obvious and will
not be discussed further. There are five different OSPF message
types: Hello, Database Description, Link State Request, Link State
Update, Link State Acknowledgement. The next sections discuss
general vulnerabilities for every field in the five OSPF messages as
well as the ones arising from Cryptographic Authentication. Each
section also defines the ability for outsiders, insiders or faulty
OSPF peers to exploit these weaknesses.
3.2.1. Packet Header with Cryptographic Authentication Enabled
IP Header
No field of the IP header is protected by the Message Authentication
Code (MAC) available when Cryptographic Authentication is enabled.
This poses a threat to OSPF any time the protocol relies on any IP
field. For example RFC2328 [1] states on paragraph 10.5: "When
receiving an Hello on a point-to-point network (but not on a virtual
link) set the neighbor structure's Neighbor IP address to the
packet's IP source address".
OSPF Header
Neighbor OSPF routers may reset their Cryptographic Sequence Number
states when a peer reboots (if the "resetting" peer is not capable
of storing Cryptographic Sequence Numbers across reboots) or when
the peer's Cryptographic Sequence Number rolls over. At this point,
any previously logged packet can be maliciously replayed and will
look legitimate if the secret key has not changed in the mean time.
Moreover, if the replayed packet is chosen with a high enough
sequence number, it will block the communication between the
recently rebooted router and its peer(s) for RouterDeadInterval plus
the time needed to establish a new adjacency [7]. This vulnerability
is exploitable by any outsider that is able to log OSPF packets. It
is important to underline that this vulnerability could be used to
break adjacencies between OSPF peers.
Breaking an adjacency will cause an OSPF router to update its own
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Router LSA which in turn will force a new SPF calculation, this may
lead to changes in the routing table due to the loss of one peer. If
the router is also the Designated Router (DR) for the link, breaking
an adjacency also entails modifying the corresponding link's Network
LSA, potentially resulting in transit links being declared as stub
connections and/or partitioning of the domain.
Finally, even for an insider attacker (with or without the ability
to log packets) forging a single Hello message, with a high enough
sequence number, is an excellent and quick option to break any
established adjacency. In conclusion this vulnerability may be
appealing to both outsider and insider attackers.
3.2.2. Hello Message
In general errors in Hello message parameters such as incorrect
AreaID, RouterDeadInterval, HelloInterval and so on will cause the
Hello to be silently discarded with no further impact.
Other Hello parameters are analyzed next, and in order to modify the
following parameters, the attacker must be an insider, i.e. in
possession of the secret for the link to be attacked or the link
must be configured with the Null Authentication security option.
3.2.2.1. Neighbor
Omission of one or more adjacent neighbors in the neighbor list will
immediately break the adjacency and force a synchronization process
between the legitimate owner of the Hello message and all the
omitted neighbors.
Breaking an adjacency will cause an OSPF router to update its own
Router LSA which in turn will force a new SPF calculation, this may
lead to changes in the routing table due to the loss of one peer. If
the router is also the Designated Router (DR) for the link, breaking
an adjacency also entails modifying the corresponding link's Network
LSA, potentially resulting in transit links being declared as stub
connections and/or partitioning of the domain.
3.2.2.2. DR and BDR
Tampering with these two fields can lead to several problematic
scenarios,(concerning broadcast and NBMA networks) each leading to
different consequences for the routing domain.
In order to be taken into account by the DR election process on a
victim router, the attacker must list the victim router ID into the
active neighbor list of its malicious Hello. Next some examples of
attacks are described.
In the Hello message, setting to null the DR and BDR fields, on
behalf of a legitimate router on the network, and listing all
neighbors in the malicious Hello, will force a full re-election of
the DR and BDR.
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Bogus Hello messages from a non-existing router, with a Router
Priority and an IP address higher than any legitimate router on a
network, listing itself as DR will allow the attacker to
successfully convince all the routers present in the neighbor list
(of the malicious Hello) that the DR has changed. Any router
believing in the non-existing DR will update its Router LSA by
listing a link to a stub network instead of the transit network
(because it is not fully adjacent to the non-existing DR). Thus,
this router will not use this network anymore as a transit network;
this will lead to connectivity loss.
If the attacker is listing the current DR and BDR in the active
neighbors, then the current DR and BDR will also be deceived into
thinking that the non-existing router is the new DR. This will have
an impact on all the routers connected to the network at once.
3.2.2.3. Deletion of Hello Messages
If no Hello message is received from a given neighbor for a period
of time longer than RouterDeadInterval, then the adjacency with this
router is considered to be broken.
Breaking an adjacency will cause an OSPF router to update its own
Router LSA which in turn will force a new SPF calculation, this may
lead to changes in the routing table due to the loss of one peer. If
the router is also the Designated Router (DR) for the link, breaking
an adjacency also entails modifying the corresponding link's Network
LSA, potentially resulting in transit links being declared as stub
connections and/or partitioning of the domain.
3.2.2.4. Hello Message Replay
The Hello Replay attack cannot be perpetrated by an outsider as
described by [7]. "The HELLO packet lists the recently seen routers,
so if an attacker replays a HELLO packet back to its source, the
source won't see itself in the list and will deduce the connection
isn't bidirectional. [...] On broadcast, NBMA or Point to Multipoint
networks, the neighbor is identified by its IP address, so both
attacks can be used." [7, paragraphs 3.2.2 and 3.2.3] This clashes
with what is stated by RFC2328 [1, paragraph 10.5]: "When receiving
a Hello Packet from a neighbor on a broadcast, Point-to-MultiPoint
or NBMA network, set the neighbor structure's Neighbor ID equal to
the Router ID found in the packet's OSPF header." Zebra seems to be
in agreement with the RFC's interpretation provided above and is not
vulnerable to the Hello Replay attack.
In conclusion, the RouterID field is covered by Cryptographic
Authentication and therefore it cannot be modified by an outsider
without infringing on the MAC (Message Authentication Code), and if
the Hello message is replayed to its owner without modifying
anything the RouterID will match the one of the owner and the
message will be ignored.
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3.2.3. DB Description, Link State Request and Acknowledgment
There is no clear threat except for an insider attacker, or a faulty
router, that behaves as described in the resource consumption
section.
3.2.4. Link State Update
In order to modify the parameters described in the following
subsections, the attacker must be able to successfully inject
malicious LSUs. Hence, the attacker must either subvert, impersonate
or fake a router which is at least in the exchange state or higher.
In the two latter cases, the attacker must be an insider, i.e. in
possession of the secret key for a link or a link must be configured
with the Null Authentication security option.
3.2.4.1 Link State Update Header
The Link State Update (LSU) Header does not appear to present any
vulnerability in and for itself. In the case of attacks involving
bogus LSAs, some fields of the LSU header may need to be maliciously
modified to be consistent with the bogus information carried by the
LSAs.
In general, errors in some LSU Header parameters such as incorrect
RouterID, AreaID and AuType will cause the LSU to be silently
discarded with no further impact.
3.2.4.2. Link State Advertisement Header
LS age (MaxAge Attack)
Setting the age field of an LSA to MaxAge will cause the LSA to be
flushed from all the routers reached by the flooding mechanism. The
owner of the LSA will fight back by issuing a new LSA with age set
to 0 and a higher sequence number. Any attack exploiting this
vulnerability could cause unnecessary flooding and refreshing of the
Link State Database, hence making the routing information
inconsistent. Routers that do not have a copy of the LSA in their
Link State Databases will not contribute to the flushing of it, this
can help the owner of the LSA in its fight back [8].
LS sequence number (Max Sequence Number Attack)
This is an implementation bug that has been published long ago [9]
and not a protocol vulnerability. Nonetheless it is listed in this
memo for historical reasons and because at least one recent
implementation of OSPF was still affected by it.
The bug concerns sequence numbers roll-over. When an LSA reaches its
maximum (0x7FFFFFFF) value it is not flushed by flooding it with its
age set to MaxAge; instead, the erroneous implementation will simply
re-issue the LSA with a rolled-over sequence number. Any newer
instance will always be considered outdated when compared to the old
one having the LS sequence number set to the maximum value. Thus, an
insider attacker could install a bogus LSA on all routers for a
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MaxAge-long interval without any effective fight back from the owner
of the LSA [9].
3.2.4.3. Router Link State Advertisement
Remove, add routers to the domain
It is possible to tamper with the topology of a domain by
introducing phantom OSPF routers through bogus Router LSAs.
Depending on how said phantom OSPF nodes are claiming to be
interconnected with each other and with real OSPF peers, they may or
may not be utilized by the SPF algorithms present in other OSPF
peers. A similar situation applies when a Router LSA is maliciously
flushed impacting routes across the domain. Adding or deleting OSPF
routers through bogus existing router LSAs will trigger a fight back
reaction by the owner of the LSA, except under the circumstances
stated in paragraph 3.1.3.
E Bit
A Router LSA carrying the E bit set to 1 automatically allows a
router to introduce External LSAs in the routing domain. This could
be exploited to escalate a normal router into an ASBR.
Setting the E bit to 1 on Router LSAs will trigger a fight back
reaction by the owner of the LSA, except under the circumstances
stated in paragraph 3.1.3.
Link ID, Link data
Adding links (stub or transit) to any Router LSA will result in
adversely impacting the normal flow of data-traffic through the
domain. The same applies in the case of a Router LSA omitting any
link previously present. More specifically: advertised stub networks
are not verifiable by the Shortest Path First algorithms running on
other routers present in the same Area. So, if a bogus Router LSA
lists a stub network matching the network address of any existing
remote network, other OSPF routers will actually consider the router
owner of this LSA as a possible path to said remote network. This
implies that a malicious or faulty entity advertising bogus stub
networks could attract traffic towards itself and/or deviate normal
routing across the domain.
Adding any kind of link to a Router LSA will trigger fight back by
the owner of the LSA, except under the circumstances stated in
paragraph 3.1.3.
Metric
The metric fields of an LSA can be modified in the attempt to affect
the SPF algorithm. Such operation could serve the purpose of
attracting traffic to a node for eavesdropping or overloading; on
the other hand, it could also be used for starving a given node.
Modifying the fields of a Router LSA regarding a link's metric will
trigger a fight back reaction by the owner of the LSA, except under
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the circumstances stated in paragraph 3.1.3.
3.2.4.4. Network Link State Advertisement
Remove or add links to a domain
It is possible to tamper with the topology of a domain by
introducing phantom transit links through bogus Network LSAs.
Depending on how said phantom transit links are connected to real or
phantom OSPF routers, the bogus nodes may or may not be utilized by
the SPF algorithms present in other OSPF peers. A similar situation
applies where an existing transit link is maliciously flushed
impacting routes across the domain.
Adding or subtracting transit links through bogus Network LSAs will
trigger a fight back reaction by the owner of the LSA, except under
the circumstances stated in paragraph 3.1.3.
Attached Router
It is possible to add or eliminate nodes from a transit link by
tampering with the list of attached routers. If a legitimate node is
removed from this list, that router will be considered disconnected
by all the remaining OSPF peers in the domain, even though its
Router LSA will state the opposite. There must be consistency
between Network and Router LSAs for a router to be considered part
of a link.
Subtracting a router from the list of attached routers through a
bogus Network LSA will trigger a fight back reaction by the owner of
the LSA, the DR for the network link, except under the circumstances
stated in paragraph 3.1.3.
3.2.4.5. Summary Link State Advertisement
It is possible to add or eliminate information contained in both
types of Summary Link State LSA affecting routes across different
Areas.
Forging bogus Summary Link State LSAs will trigger a fight back
reaction by the owner of the LSA, except under the circumstances
stated in paragraph 3.1.3.
3.2.4.6. AS External Link State Advertisement
Every external route introduced by an ASBR is advertised by a single
External LSA. There is no way for OSPF routers to verify the
information carried by External LSA messages. Introduction of bogus
External LSAs will affect the domain's knowledge of the outside
world. Bogus External LSAs can be used to attract a portion of the
data traffic destined outside the domain to a specific node for
eavesdropping or overloading purposes. The same considerations apply
to any attempt to starve one or more nodes.
Introducing false External LSAs will trigger a fight back reaction
by the owner of the LSA and/or will not be recognized as legitimate
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information by other routers if the LSA is forged on behalf of anon-
ASBR router, except under the circumstances stated in paragraph
3.1.3.
Forward
The Forward field of an External LSA specifies the host (OSPF router
or not) meant to be used as gateway for that external route; said
host can be located everywhere in the domain including Stub Areas.
If this field is forged and the forward host is not an OSPF router
then there will be no OSPF fight back from the host itself, but
there may be a fight back reaction from the ASBR owner of the LSA.
By exploiting this feature, an attacker could redirect traffic
destined outside the routing domain to any given host in the domain
which may, or may not, be under its control. For example, this can
be used to generate loops between an ABR and any of its neighbors
located in its Stub Area, simply by mentioning one of these
neighbors in the forward field of an External LSA advertisement for
traffic destined outside the domain.
Forging bogus AS External LSAs with modified Forward field
information will trigger a fight back reaction by the owner of the
LSA, except under the circumstances stated in paragraph 3.1.3.
3.3. Resource Consumption Vulnerabilities
Every resource may be exploited in the attempt to interfere with
traffic flows from legitimate users. In some cases the resource may
be so overwhelmed by malicious/illegitimate packets that legitimate
users will not only experience a drop in the performance of the
service, but they may be even prevented from accessing the service
itself. If one, or more, critical resource of a router is busy
serving bogus traffic, or dropping malicious routing messages, then
the whole router will be impacted and enter a delicate and more
vulnerable state. Next is a list of possible weaknesses that can be
exploited to produce a resource consumption attack.
3.3.1. OSPF Cryptographic Authentication
With Cryptographic Authentication disabled both outsider and insider
entities - including attackers and faulty routers - can successfully
forge malicious/erroneous OSPF messages that will be in the position
to attack a router or exhaust its control plane resources, such as
queues and CPU cycles.
On the other hand, when Cryptographic Authentication is enabled,
only insiders may successfully force malicious OSPF messages to be
accepted by the victim's control plane. Unfortunately though,
outsider entities are still in the position to generate a powerful
resource consumption attack by intentionally exploiting the
Cryptographic Authentication mechanism itself as described in [3].
These entities may inject OSPF packets with bogus cryptographic
information that will consume critical resources only to be
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discarded afterward. This will impact OSPF by delaying or even
preventing legitimate messages to be authenticated and used.
3.3.2. Hello Message
DR and BDR Election
Hello messages are used by OSPF also to carry out the DR and BDR
election process. The DR election process itself presents a possible
resource consumption vulnerability since it may be fooled into
electing a new DR at every run. When a new DR is elected all routers
on the network will have to use resources to establish adjacency
with this new DR; the same applies in the case of the BDR.
Number of Neighbors
OSPF routers create a neighbor data structure for each neighbor
discovered through the Hello protocol. The resources to store this
information could be exhausted on a broadcast or NBMA network with a
large host address range.
Message Size
Since a router must list all its current active neighbors in each of
its Hello messages, it may have to issue a Hello message bigger than
the Layer 2 media's MTU, e.g. bigger than the Ethernet frame's size.
Since this is usually a delicate area in implementation and design
all the necessary care should be exerted.
3.3.3. Link State Request Message
Any Link State Request message forces the destination router to
reply with a Link State Update message containing the requested LSA.
An insider attacker, or a faulty router, could mount a resource
consumption attack by continuously requesting Link State information
from its neighbors at any desired rate.
3.3.4. Link State Acknowledgment Message
Not acknowledging Link State Update messages forces the originating
peer to keep a copy of the LSU on the retransmission list; this
leads to re-transmission loops wasting resources on both sides.
3.3.5. Link State DB Overflow
Router/Network LSA
Router/Network LSAs received from non-existing OSPF peers will not
be used by the SPF algorithm and will not directly adverse the
routes nor the topology. Nonetheless, these LSAs will consume
resources in the Link State Database and will not be removed from
this database until they "naturally" expire after MaxAge (1 hour).
If the purpose of an attacker is to simply consume database
resources, then crafting LSAs on behalf of non-existing OSPF routers
is a good option since it makes the effects of the attack last
longer and triggers no fight back reaction at all. Finally, it is
important to highlight that Link State Database overflows produced
by Router and Network LSAs will not be limited by the mitigation
mechanism detailed in RFC1765 [10].
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External LSA
External LSAs may also be successfully exploited in the attempt to
fill Link State Database resources. If these LSAs are crafted on
behalf of non-existing ASBRs, their information will not be used by
any SPF algorithm; however they will be successfully installed in
the Link State Databases. Moreover, External LSAs are forwarded to
all routers in the domain (except routers located in Stub Areas),
expire only after MaxAge if no fight back is place, and are never
consolidated by OSPF.
Link State Database Description Messages
The Database Exchange process poses a resource consumption threat on
the slave router participating to the process. An insider attacker -
or a faulty router - capable of leading a victim into the Database
Exchange process could advertise a huge list of non-existing links
through Database Description messages. The victim will keep updating
this list and start asking for details via Link State Request
messages. The number of bogus links that the victim router will have
to store poses an immediate resource consumption threat, while the
prolonged request for details about the bogus LSAs will keep the
victim's retransmission list full and busy.
Retransmission List Exhaustion
Any LSU that is not acknowledged is put on a re-transmission list.
OSPF messages present in this list are sent over regular intervals
until they are acknowledged by the receivers. Failing to acknowledge
LSUs, accidentally or voluntarily, will trigger resource consumption
on the remote peer's retransmission mechanisms.
3.3.6 Others
Routing table size/performance issue
Increasing the size of the routing table could potentially move a
router into a very delicate state and eventually reach the limits
assigned to some resources. This could be achieved by using Router,
Network or External LSAs from existing peers and somehow disabling
the fight back from the legitimate owners.
Fragmentation
Fragmentation of OSPF messages due to Layer 2 MTU is a crucial
factor for any given implementation; any situation involving such
process should be carefully tested. For example in the case of a
router running the open source routing suite Zebra over Ethernet
links, receiving a forged Router LSA that claims to have more than
118 links will adversely impact the routing daemon. Even though the
LSA does not violate RFC2328, which states that a Router LSA must be
entirely contained into one single IP packet, a Router LSA listing
more than 118 links does exceed the Ethernet MTU and will be
fragmented over multiple Ethernet frames: this seems to have a
serious impact on the behavior of Zebra.
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3.4. Vulnerabilities through Other Protocols
3.4.1. IP
OSPF runs directly over IP. Therefore, OSPF is subject to attack
through attacks on IP. Direct attacks against the IP stack of a
router, such as integrity and fragmentation attacks, will impact
OSPF but are clearly beyond the scope of this document.
3.4.2. Other Supporting Protocols (Management)
The security of OSPF is inherently dependent on the security of the
managing procedures. Critical examples are the configuration of the
state of any interface, the Manual Stop procedure and the Timer
Values.
Manual stop
A manual stop event causes the OSPF router to bring down all its
adjacencies, release all associated OSPF resources, and delete all
associated routes. If the mechanisms by which an OSPF router was
informed of a manual stop is not carefully protected, OSPF
connections could be destroyed by an attacker. Consequently, OSPF
security is secondarily dependent on the security of whatever
protocols are used to operate the platform.
Timer events
The RxmtInterval, InfTransDelay, RouterDeadInterval, HelloInterval
timers together with the RouterPriority parameter are critical to
OSPF operation. For example, if the HelloInterval timer value is
changed, all remote peers will refuse Hello messages from that
router and after RouterDeadInterval bring the adjacency down.
Consequently, OSPF security is secondarily dependent on the security
of the protocols by which the platform is managed and configured.
3.5. Residual Risk
OSPF Cryptographic Authentication assumes that the cryptographic
algorithms are secure, that the secrets used are protected from
exposure and are chosen well so as not to be guessable, that the
platforms are securely managed and operated to prevent break-ins,
etc.
Information theory states that the English language has about 1.3
bits of entropy for each 8-bit character. If an administrator were
to choose the secret key for the Cryptographic Authentication to be
any English word, the entropy associated to the secret key
protecting the session would be drastically reduced from 128 bits to
the point where it could be guessed in a matter of minutes or days.
On top of that, Common Line Interfaces (CLI) will generally limit
the key input to a specific subset of ASCII characters - letters and
number plus a few symbols - and will not accept a 128-bits number
value (for example in hexadecimal format).
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This becomes crucial in all those cases where the secret defending
an OSPF adjacency is poorly chosen and changed once every three
months, or every year, or never. In all these scenarios an attacker
that somehow managed to obtain a copy of a single OSPF Hello message
may eventually be able to crack the secret key and attack the entire
routing domain for a prolonged period of time.
4. References
[1] J. Moy. "OSPF Version 2", STD 54, RFC2328, April 1998.
[2] P. Ferguson, D. Senie. "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC2827, May 2000.
[3] A. Zinin. "Protecting Internet Routing Infrastructure from
Outsider CPU Attacks", work in progress, February 2003.
Available as <draft-zinin-rtg-dos-00.txt> at Internet-Draft
shadow sites.
[4] A. Babir, S. Murphy, Y. Yang. "Generic Threats to Routing
Protocols", work in progress, April 2004. Available as
<draft-ietf-rpsec-routing-threats-06.txt> at Internet-Draft
shadow sites.
[5] E. Rescorla, B. Korver. "Guidelines for Writing RFC Text on
Security Considerations", work in progress, January 2003.
Available as <draft-iab-sec-cons-03.txt> at Internet-Draft
shadow sites.
[6] F. Wang, S. Felix Wu. "On the Vulnerabilities and Protection of
OSPF Routing Protocols" In Proceedings 7th International
Conference on Computer Communications and Networks: 148-152. Los
Alamitos, CA: IEEE Comp. Soc., 1998.
[7] J. Etienne. "Flaws in Packet's Authentication of OSPFv2", work
in progress, November 2001. Available as
<draft-etienne-ospv2-auth-flaws-00.txt> at Internet-Draft shadow
sites.
[8] S. Murphy, et al. "Retrofitting Security into Internet
Infrastructure Protocols." Proceedings of DARPA Information
Survivability Conference and Exposition (DISCEX'00), 2000.
[9] B. Vetter, F. Wang and S. F. Wu. "An Experimental Study of
Insider Attacks for the OSPF Routing Protocol", in 5th IEEE
International Conference on Network Protocols, Atlanta, GA,
Oct 28-31, 1997.
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[10] J. Moy. "OSPF Database Overflow", Experimental, RFC1765, March
1995.
Authors' Addresses
Emanuele Jones
Alcatel
600 March Road - Kanata, ON, Canada K2K 2E6
EMail: emanuele.jones@alcatel.com
Olivier Le Moigne
Alcatel
600 March Road - Kanata, ON, Canada K2K 2E6
EMail: olivier.le_moigne@alcatel.com
Full Copyright Statement
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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