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Versions: 00 01 02

Routing Protocol Security                                       E. Jones
Requirements                                                O. Le Moigne
Internet-Draft                                                   Alcatel
Expires: December 18, 2006                                 June 16, 2006


                 OSPF Security Vulnerabilities Analysis
                   draft-ietf-rpsec-ospf-vuln-02.txt

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   This Internet-Draft will expire on December 18, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

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



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   secure a domain.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Attacker's Definition  . . . . . . . . . . . . . . . . . .  3
     1.2.  Attacker's Location  . . . . . . . . . . . . . . . . . . .  4
     1.3.  Vulnerabilities Damages and Consequences . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Generic Attack Techniques  . . . . . . . . . . . . . . . . . .  7
   4.  Vulnerabilities and Risks  . . . . . . . . . . . . . . . . . .  9
     4.1.  OSPF General Vulnerabilities . . . . . . . . . . . . . . .  9
       4.1.1.  Local Intrusion Global Impact  . . . . . . . . . . . .  9
       4.1.2.  Remote Attacker  . . . . . . . . . . . . . . . . . . .  9
       4.1.3.  Attacker Disabling Fight Back  . . . . . . . . . . . .  9
       4.1.4.  Attacker Leveraging Fight Back . . . . . . . . . . . . 11
       4.1.5.  Dealing with External Routes . . . . . . . . . . . . . 11
     4.2.  Protocol-specific Vulnerabilities  . . . . . . . . . . . . 11
       4.2.1.  Packet Header with Cryptographic Authentication
               Enabled  . . . . . . . . . . . . . . . . . . . . . . . 12
       4.2.2.  Hello Message  . . . . . . . . . . . . . . . . . . . . 13
       4.2.3.  DB Description, Link State Request and
               Acknowledgment . . . . . . . . . . . . . . . . . . . . 15
       4.2.4.  Link State Update  . . . . . . . . . . . . . . . . . . 15
     4.3.  Resource Consumption Vulnerabilities . . . . . . . . . . . 18
       4.3.1.  OSPF Cryptographic Authentication  . . . . . . . . . . 19
       4.3.2.  Hello Message  . . . . . . . . . . . . . . . . . . . . 19
       4.3.3.  Link State Request Message . . . . . . . . . . . . . . 20
       4.3.4.  Link State Acknowledgment Message  . . . . . . . . . . 20
       4.3.5.  Link State DB Overflow . . . . . . . . . . . . . . . . 20
       4.3.6.  Others . . . . . . . . . . . . . . . . . . . . . . . . 21
     4.4.  Vulnerabilities through Other Protocols  . . . . . . . . . 21
       4.4.1.  IP . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       4.4.2.  Other Supporting Protocols (Management)  . . . . . . . 22
     4.5.  Residual Risk  . . . . . . . . . . . . . . . . . . . . . . 22
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28








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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



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   attacker that is not in possession of the secret key.

   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,





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      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,

      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.












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2.  Terminology

   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 RFC 2119 [5].














































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3.  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 [6]).

      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 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



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      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.














































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4.  Vulnerabilities and Risks

4.1.  OSPF General Vulnerabilities

   The risks in OSPF arise from the following fundamental
   vulnerabilities:

4.1.1.  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 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.

4.1.2.  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.

4.1.3.  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 [7].  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



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   be disabled clearing the way to stealthy attacks.

4.1.3.1.  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] on 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
   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.

4.1.3.2.  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.





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4.1.3.3.  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.

4.1.4.  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.

4.1.5.  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 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.

4.2.  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



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   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.

4.2.1.  Packet Header with Cryptographic Authentication Enabled

4.2.1.1.  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".

4.2.1.2.  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 [8].  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
   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.






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4.2.2.  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.

4.2.2.1.  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.

4.2.2.2.  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.

   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



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   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.

4.2.2.3.  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.

4.2.2.4.  Hello Message Replay

   The Hello Replay attack cannot be perpetrated by an outsider as
   described by [8].  "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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4.2.3.  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.

4.2.4.  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.

4.2.4.1.  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.

4.2.4.2.  Link State Advertisement Header

4.2.4.2.1.  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 [9].

4.2.4.2.2.  LS sequence number (Max Sequence Number Attack)

   This is an implementation bug that has been published long ago [10]
   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.



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   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 [10].

4.2.4.3.  Router Link State Advertisement

4.2.4.3.1.  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 Section 4.1.3.

4.2.4.3.2.  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 Section 4.1.3.

4.2.4.3.3.  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.



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   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
   Section 4.1.3.

4.2.4.3.4.  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 Section 4.1.3.

4.2.4.4.  Network Link State Advertisement

4.2.4.4.1.  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 Section 4.1.3.

4.2.4.4.2.  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 Section 4.1.3.

4.2.4.5.  Summary Link State Advertisement

   It is possible to add or eliminate information contained in both



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   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 Section 4.1.3.

4.2.4.6.  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 other routers if the LSA is forged on behalf of anon-
   ASBR router, except under the circumstances stated in Section 4.1.3.

4.2.4.6.1.  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 Section 4.1.3.

4.3.  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



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   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.

4.3.1.  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.

4.3.2.  Hello Message

4.3.2.1.  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.

4.3.2.2.  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.

4.3.2.3.  Message Size

   Since a router must list all its current active neighbors in each of



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   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.

4.3.3.  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.

4.3.4.  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.

4.3.5.  Link State DB Overflow

4.3.5.1.  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
   [11].

4.3.5.2.  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.





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4.3.5.3.  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.

4.3.5.4.  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.

4.3.6.  Others

4.3.6.1.  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.

4.3.6.2.  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.

4.4.  Vulnerabilities through Other Protocols





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4.4.1.  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.

4.4.2.  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.

4.4.2.1.  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.

4.4.2.2.  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.

4.5.  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



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   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 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.








































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5.  IANA Considerations

   This document has no actions for IANA
















































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6.  Security Considerations

   This document is security related and discusses threats against the
   OSPF routing protocol.















































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7.  References

7.1.  Normative References

   [1]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

7.2.  Informative References

   [2]   Ferguson, P. and D. Senie, "Network Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Source
         Address Spoofing", BCP 38, RFC 2827, May 2000.

   [3]   Zinin, A., "Protecting Internet Routing Infrastructure from
         Outsider DoS Attacks", draft-zinin-rtg-dos-02 (work in
         progress), May 2005.

   [4]   Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
         Routing Protocols", draft-ietf-rpsec-routing-threats-07 (work
         in progress), October 2004.

   [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [6]   Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on
         Security Considerations", draft-iab-sec-cons-00 (work in
         progress), August 2002.

   [7]   Wang, F. and S. Wu, "On the Vulnerabilities and Protection of
         OSPF Routing Protocols", IEEE Comp. Soc. Proceedings 7th
         International Conference on Computer  Communications and
         Networks: 148-152.  Los Alamitos, CA, 1998.

   [8]   Etienne, J., "Flaws in Packet's Authentication of OSPFv2",
         draft-etienne-ospv2-auth-flaws-00 (work in progress),
         November 2001.

   [9]   Murphy, S., "Retrofitting Security into Internet Infrastructure
         Protocols.", DISCEX '00 Proceedings of DARPA Information
         Survivability Conference and Exposition, 2000.

   [10]  Vetter, B., Wang, F., and S. Wu, "An Experimental Study of
         Insider Attacks for the OSPF Routing Protocol", IEEE 5th IEEE
         International Conference on  Network Protocols, Atlanta, GA,
         Oct 28-31, 1997.

   [11]  Moy, J., "OSPF Database Overflow", RFC 1765, March 1995.





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Authors' Addresses

   Emanuele Jones
   Alcatel
   3400 W. Plano Pkwy.
   Plano, TX  75075
   USA

   Email: emanuele.jones@alcatel.com
   URI:   http://www.alcatel.com/


   Olivier Le Moigne
   Alcatel
   600 March Road
   Ottawa, ON  K2K 2E6
   Canada

   Email: olivier.le_moigne@alcatel.com
   URI:   http://www.alcatel.com/































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