Network Working Group Emanuele Jones
INTERNET DRAFT Alcatel
draft-jones-ospf-vuln-01.txt Olivier Le Moigne
Alcatel
October 2003
OSPF Security Vulnerabilities Analysis
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Specification of Requirements
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.
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 .. 11
3.2.4. Link State Update ...................................... 11
3.3. Resource Consumption Vulnerabilities ..................... 14
3.3.1. OSPF Cryptographic Authentication ...................... 15
3.3.2. Hello Message .......................................... 15
3.3.3. Link State Request Message ............................. 15
3.3.4. Link State Acknowledgment Message ...................... 15
3.3.5. Link State DB Overflow ................................. 15
3.4. Vulnerabilities through Other Protocols .................. 17
3.4.1. IP ..................................................... 17
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 session.
Hence, this definition includes: 1) any subverted OSPF router, 2)
any malicious software capable of interacting with an OSPF routing
session, 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
participating in the routing session. The second scenario takes
place when Null Authentication is adopted.
If one link is not protected then the whole 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 session 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 the 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.
In the second scenario, when the routing session is not protected by
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OSPF Cryptographic Authentication, there is no distinction between
insider and outsider entities. Any attacker can successfully forge
OSPF messages on behalf on 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
the secret cryptographic key, if at all needed, can attack a router
from another 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
participating in the routing session target of their attack.
1.3. Vulnerabilities Damages and Consequences
Generally speaking attackers will be able to disrupt and manipulate
the routing session, 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,
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,
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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.
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
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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 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. 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/access 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
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 session.
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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 edges of a
domain can start attacking the routing session in the core, if said
session 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 session, overriding the legitimate fight back:
According to RFC2328 [1], a router will never emit (or update) its
LSAs faster than MinLSInterval (5 seconds). This allows for
almost permanent changes in the routing session, if an attacker is
flooding this session 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], 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 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
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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 never fight back while the
flooding is in progress. This interpretation seems to be matched by
the behavior of the open source routing suite Zebra when receiving
bogus self-originated LSAs. Other implementations may not exhibit
this behavior due to a different interpretation of RFC2328 [1] or
due to design optimization choices.
So, if an attacker continuously injects malicious LSAs (at a rate
faster than MinLSInterval) the router, legitimate owner of the
information advertised by the malicious/faulty LSA, depending on its
implementation, may fight back with one correct message every
MinLSInterval or it may never have a chance to fight back at all,
while the attack is in progress.
3.1.3.2 Partitioned Networks
If the flooding mechanism does not have a chance/path to rely
malicious LSAs to the legitimate owner, for example when a subverted
router partitions a network, the router owning the information 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 session 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 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)
algorithms.
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 storm of LSA
updates that are trying to correct the malicious one. Even though such
a drastic 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
as stub-areas. Even though verification of routes outside the domain
is clearly beyond the scope of OSPF, the current flooding mechanism
of such information may be used as an efficient intrinsic vector for
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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 other ASBR 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 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 will 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
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
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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.
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.
DR and DBR
Tampering with these two fields can lead to many attack scenarios.
Next some examples are described in details.
In the Hello message, setting to null the DR and BDR fields, on
behalf of an existing peer on the link, will force the re-election
of the DR and BDR. If these are already the default ones - DR is the
router with the highest priority or the highest IP address - then
there is no impact, otherwise they will swap roles.
Bogus Hello messages from a non-existing (or malicious) router with
a Router Priority or an IP address - both spoofed or real - higher
than any legitimate router on the link while listing itself as DR,
will allow the new entity to effectively become the DR for the link.
The previous DR will immediately flush the Network LSA for the link
causing the link to disappear from the entire routing session for
RouterDeadInternal (40 seconds). After this, the legitimate BDR will
become DR and emit a new correct Network LSA for the link. There
will be no Link State Database resynchronization in this basic
scenario. If the attacker manages to elect also a non-existing (or
malicious) BDR identity, then the subverted BDR may not restore the
link and a new DR and BDR election will occur followed by a Link
State Database resynchronization between the DR (BDR) and all the
other routers on the network.
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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.
Finally 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.
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
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
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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 refreshment 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,
nonetheless it is listed in this paper since the current version of
the open source routing suite Zebra (0.92a) is 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
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 the 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 session. 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
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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 link, other OSPF routers will actually consider the router
owner of this LSA as a possible path to said remote link. 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
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 session, 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
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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 the 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
information by the other routers if the LSA is forged on behalf of a
non-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 AS 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 a
Stub Area, simply by mentioning one of the 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
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Every resource may be exploited in the attempt to interfere with the
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 legitimate 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 discarded afterward. This will impact OSPF by delaying or even
preventing legitimate messages to be authenticated and used.
3.3.2. Hello Message
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.
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 all 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
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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].
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.
Number of neighbors on multicast networks
OSPF routers keep a list containing Hello messages from new IP
addresses: these are considered new potential active neighbors. The
resources to store this information could be exhausted on a class B,
or bigger, network.
Moreover, 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.
As an example, the open source routing suite Zebra seems to be
vulnerable to this threat. In fact, on any Ethernet link, after
receiving Hello messages from more than 358 new neighbors the
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routing daemon is not capable of issuing Hello messages. This
is explained by the fact that an OSPF Hello message should be
listing all the router's current active neighbors, but since such a
message would exceed the Ethernet MTU, the message is delayed. The
router simply waits until enough Hello messages from the non-
existing neighbors pool start expiring (after RouterDeadInterval),
so that the total number of active neighbors drops below 358, before
emitting a new Hello message. Since this interval of silence is
longer than RouterDeadInterval (generally 40 seconds) all existing
adjacencies will be broken, forcing the victim router to re-
establish all of them. By "refreshing" the bogus Hello messages at
the appropriate rate it is possible to keep a router, running the
open source Zebra routing suite, silent for an indefinite interval
of time.
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.
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.
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.
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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 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).
This becomes crucial in all those cases where the secret defending
the routing session is poorly chosen and changed once every month,
or every year, or never. In all these scenarios an attacker that
somehow managed to obtain a copy of a single OSPF Hello message will
eventually be able to crack the secret key and attack the entire
routing session for a prolonged period of time.
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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] D. Beard, S. Murphy, Y. Yang. "Generic Threats to Routing
Protocols", work in progress, February 2003.
Available as <<draft-beard-rpsec-routing-threats-01.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 Comput. 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.
[10] J. Moy. "OSPF Database Overflow", Experimental, RFC1765,
March 1995.
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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
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