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Versions: 00 01 RFC 4272

none                                                       Sandra Murphy
INTERNET-DRAFT                                               Sparta, Inc
Expires: April 13, 2005                                     October 2004



                 BGP Security Vulnerabilities Analysis
                     draft-ietf-idr-bgp-vuln-01.txt



Status of this Memo


This document is an Internet-Draft and is subject to all provisions of
section 3 of RFC 3667.  By submitting this Internet-Draft, each author
represents that any applicable patent or other IPR claims of which he or
she is aware have been or will be disclosed, and any of which he or she
become aware will be disclosed, in accordance with RFC 3668.


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/ietf/1id-abstracts.txt.


The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.


This Internet-Draft will expire on April 13, 2005.


Copyright Notice


Copyright (C) The Internet Society (2004).


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 [RFC2119].








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Abstract


Border Gateway Protocol 4 (BGP-4), along with a host of other
infrastructure protocols designed before the Internet environment became
perilous, was originally designed with little consideration for
protection of the information it carries.  There are no mechanisms
internal to the BGP protocol to protect against attacks that modify,
delete, forge, or replay data, any of which has the potential to disrupt
overall network routing behavior.


This internet draft discusses some of the security issues with BGP
routing data dissemination.  This internet draft does not discuss
security issues with forwarding of packets.






































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Table of Contents



  Abstract ........................................................    2
1 Introduction ....................................................    4
2 Attacks .........................................................    6
3 Vulnerabilities and Risks .......................................    8
3.1 Vulnerabilities in BGP messages ...............................    9
3.1.1 Message Header ..............................................   10
3.1.2 OPEN ........................................................   10
3.1.3 KEEPALIVE ...................................................   12
3.1.4 NOTIFICATION ................................................   12
3.1.5 UPDATE ......................................................   12
3.1.5.1 Unfeasible Routes Length, Total Path Attribute Length .....   13
3.1.5.2 Withdrawn Routes ..........................................   14
3.1.5.3 Path Attributes ...........................................   14
 Attribute Flags, Attribute Type Codes, Attribute Length ..........   14
 ORIGIN ...........................................................   14
 AS_PATH ..........................................................   15
 Originating Routes ...............................................   15
 NEXT_HOP .........................................................   16
 MULTI_EXIT_DISC ..................................................   16
 LOCAL_PREF .......................................................   16
 ATOMIC_AGGREGATE .................................................   17
 AGGREGATOR .......................................................   17
3.1.5.4 NLRI ......................................................   17
3.2 Vulnerabilities through Other Protocols .......................   17
3.2.1 TCP messages ................................................   17
3.2.1.1 TCP SYN ...................................................   17
3.2.1.2 TCP SYN ACK ...............................................   18
3.2.1.3 TCP ACK ...................................................   18
3.2.1.4 TCP RST/FIN/FIN-ACK .......................................   18
3.2.1.5 DoS and DDos ..............................................   19
3.2.2 Other supporting protocols ..................................   19
3.2.2.1 Manual stop ...............................................   19
3.2.2.2 Open Collision Dump .......................................   19
3.2.2.3 Timer events ..............................................   19
4 Security Considerations .........................................   20
4.1 Residual Risk .................................................   20
4.2 Operational Protections .......................................   21
5 IANA Considerations .............................................   22
6 References ......................................................   22
6.1 Normative .....................................................   22
6.2 Informative ...................................................   22
7 Author's Address ................................................   23






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1.  Introduction


The inter-domain routing protocol BGP was created when the Internet
environment had not yet reached the present contentious state.
Consequently, the BGP protocol was not designed with protection against
deliberate or accidental errors causing disruptions of routing behavior.


We here discuss the vulnerabilities of BGP, based on the BGP
specification [BGP].  Readers are expected to be familiar with the BGP
RFC and the behavior of BGP.


It is clear that the Internet is vulnerable to attack through its
routing protocols and BGP is no exception.  Faulty, misconfigured or
deliberately malicious sources can disrupt overall Internet behavior by
injecting bogus routing information into the BGP distributed routing
database (by modifying, forging, or replaying BGP packets).  The same
methods can also be used to disrupt local and overall network behavior
by breaking the distributed communication of information between BGP
peers.  The sources of bogus information can be either outsiders or true
BGP peers.


Cryptographic authentication of the peer-peer communication is not an
integral part of the BGP protocol.  As a TCP/IP protocol, BGP is subject
to all the TCP/IP attacks, like IP spoofing, session stealing, etc.  Any
outsider can inject believable BGP messages into the communication
between BGP peers and thereby inject bogus routing information or break
the peer to peer connection.  Any break in the peer to peer
communication has a ripple effect on routing that can be widespread.
Furthermore, outsider sources can also disrupt communications between
BGP peers by breaking their TCP connection with spoofed packets.
Outsider sources of bogus BGP information can reside anywhere in the
world.


Consequently, the current BGP specification requires that a BGP
implementation must support the authentication mechanism specified in
[TCPMD5].  However, the requirement for support of that authentication
mechanism cannot ensure that the mechanism is configured for use.  The
mechanism of [TCPMD5] is based on a pre-installed shared secret; it does
not have the capability of IPsec [IPsec] to agree on a shared secret
dynamically.  Consequently, the use of [TCPMD5] must be a deliberate
decision, not an automatic feature or default.


The current BGP specification also allows for implementations that would
accept connections from "unconfigured peers" ([BGP] Section 8).
However, the specification is not clear as to what an unconfigured peer






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might be or how the protections of [TCPMD5] would apply in such a case.
It is therefore not possible to include an analysis of the security
issues of this feature.  When a specification is released that describes
this feature more fully, a security analysis should be part of that same
specification.


BGP speakers themselves can inject bogus routing information, either by
masquerading as any other legitimate BGP speaker, or by distributing
unauthorized routing information as themselves.  Historically,
misconfigured and faulty routers have been responsible for widespread
disruptions in the Internet.  The legitimate BGP  peers have the context
and information to produce believable bogus routing information and
therefore have the opportunity to cause great damage.  The cryptographic
protections of [TCPMD5] and operational protections cannot exclude the
bogus information arising from a legitimate peer.  The risk of
disruptions caused by legitimate BGP speakers is real and cannot be
ignored.


Bogus routing information can have many different effects on routing
behavior.  If the bogus information removes routing information for a
particular network, that network can become unreachable for the portion
of the Internet that accepts the bogus information.  If the bogus
information changes the route to a network, then packets destined for
that network may be forwarded by a sub-optimal path, or a path that does
not follow the expected policy, or a path that will not forward the
traffic.  As a consequence, traffic to that network could be delayed by
a longer than necessary path.  The network could become unreachable from
areas where the bogus information is accepted.  Traffic might also be
forwarded along a path that permits some adversary a view of the data or
a chance to modify the data.  If the bogus information makes it appear
that an autonomous system originates a network when it does not, then
packets for that network may not be deliverable for the portion of the
Internet that accepts the bogus information.  A false announcement that
an autonomous systems originates a network may also fragment aggregated
address blocks in other parts of the Internet and cause routing problems
for other networks.


The damages that might result from these attacks include:


     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.






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     blackhole: Large amounts of traffic are directed 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: BGP become unstable so that convergence on a global
     forwarding state is not achieved.


     overload: The BGP messages themselves become a significant portion
     of the traffic the network carries.


     resource exhaustion: The BGP messages themselves cause exhaustion
     of critical router resources, such as table space.


     address-spoofing: Data traffic is forwarded through some router or
     network that is spoofing the legitimate address, enabling an active
     attack by affording the opportunity to modify the data.


These consequences can fall exclusively on one end system prefix or may
effect the operation of the network as a whole.


2.  Attacks


The BGP protocol, in and of itself, is subject to the following attacks
(list taken from the IAB RFC providing guidelines for the security
considerations section of Internet-Drafts and RFCs [SecCons]):






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     confidentiality violations:  The routing data carried in BGP is
     carried in cleartext, so eavesdropping is a possible attack against
     routing data confidentiality.  (Routing data confidentiality is not
     a common requirement.)


     replay:  BGP does not provide for replay protection of its
     messages.


     message insertion:  BGP does not provide protection against
     insertion of messages.  However, because BGP uses TCP, when the
     connection is fully established, message insertion by an outsider
     would require accurate sequence number prediction (not entirely out
     of the question, but more difficult with mature TCP
     implementations) or session stealing attacks.


     message deletion:  BGP does not provide protection against deletion
     of messages.  Again, this attack is more difficult against a mature
     TCP implementation but is not entirely out of the question.


     message modification:  BGP does not provide protection against
     modification of messages.  A modification that was syntactically
     correct and did not change the length of the TCP payload would in
     general not be detectable.


     man-in-the-middle:  BGP does not provide protection against
     man-in-the-middle attacks.  As BGP does no peer entity
     authentication, a man-in-the-middle attack is childs-play.


     denial of service:  While bogus routing data can present 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 denial of service on the
     BGP routing protocol.  For example, advertising large numbers of
     more specific routes (longer prefixes) can cause BGP traffic and
     router table size to increase, even explode.


The mandatory-to-support mechanism of [TCPMD5] will counter the message
insertion, deletion, and modification, man-in-the-middle attacks and the
denial of service attacks from outsiders.  The use of [TCPMD5] does not
protect against eavesdropping attacks, but routing data confidentiality
is not a goal of BGP.  The mechanism of [TCPMD5] does not protect
against replay attacks, so the only protection against replay is
provided by the TCP sequence number processing.  Therefore, a replay
attack could be mounted against a BGP connection protected with [TCPMD5]
but only in very carefully timed circumstances.  The mechanism of






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[TCPMD5] cannot protect against bogus routing information originating
with an insider.


3.  Vulnerabilities and Risks


The risks in BGP arise from three fundamental vulnerabilities:


     BGP has no internal mechanism that provides strong protection of
     the integrity, freshness and peer entity authenticity of the
     messages in peer-peer BGP communications.


     no mechanism has been specified within BGP to validate the
     authority of an AS to announce NLRI information.


     no mechanism has been specified within BGP to ensure the
     authenticity of the path attributes announced by an AS.


The first fundamental vulnerability motivated the mandated support of
[TCPMD5] in the BGP specification.  When that is employed, message
integrity and peer entity authentication is provided.  The mechanism of
[TCPMD5] assumes that the MD5 algorithm is secure and that the shared
secret is protected and chosen to be difficult to guess.


In the discussion that follows, the vulnerabilities are described in
terms of the BGP Finite State Machine events where the message
processing occurs.  The events are defined and discussed in section 8 of
[BGP].  The events mentioned here are:


[Administrative Events]


     Event 2: ManualStop


     Event 8: AutomaticStop


[Timer Events]


     Event 9: ConnectRetryTimer_Expires


     Event 10: HoldTimer_Expires


     Event 11: KeepaliveTimer_Expires


     Event 12: DelayOpenTimer_Expires








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     Event 13: IdleHoldTimer_Expires


[TCP Connection based Events]


     Event 14: TcpConnection_Valid


     Event 16: Tcp_CR_Acked


     Event 17: TcpConnectionConfirmed


     Event 18: TcpConnectionFails


[BGP Messages based Events]


     Event 19: BGPOpen


     Event 20: BGPOpen with DelayOpenTimer running


     Event 21: BGPHeaderErr


     Event 22: BGPOpenMsgErr


     Event 23: OpenCollisionDump


     Event 24: NotifMsgVerErr


     Event 25: NotifMsg


     Event 26: KeepAliveMsg


     Event 27: UpdateMsg


     Event 28: UpdateMsgErr


3.1.  Vulnerabilities in BGP messages


There are four different BGP message types - OPEN, KEEPALIVE,
NOTIFICATION, and UPDATE.  This section contains a discussion of the
vulnerabilities arising from each message and the ability of outsiders
or BGP peers to exploit the vulnerabilities.  To summarize, outsiders
can use bogus OPEN, KEEPALIVE, NOTIFICATION, or UPDATE messages to
disrupt the BGP peer-peer connections and can use bogus UPDATE messages
to disrupt routing without breaking the peer-peer connection.  Outsiders
can also disrupt BGP peer-peer connections by inserting bogus TCP
packets that disrupt the TCP connection processing.  In general, the






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ability of outsiders to use bogus BGP and TCP messages is limited, but
not eliminated, by the TCP sequence number processing.  The use of
[TCPMD5] can counter these outsider attacks.  BGP peers themselves are
permitted to break peer-peer connections at any time using NOTIFICATION
messages, so there is no additional risk of broken connections through
their use of OPEN, KEEPALIVE, or UPDATE messages.  However, BGP peers
can disrupt routing (in impermissible ways) by issuing UPDATE messages
that contain bogus routing information.  In particular, bogus
ATOMIC_AGGREGATE, NEXT_HOP and AS_PATH attributes and bogus NLRI in
UPDATE messages can disrupt routing.  The use of [TCPMD5] will not
counter these attacks from BGP peers.


Each message introduces certain different vulnerabilities and risks and
is discussed in the following sections.


3.1.1.  Message Header


Event 21:  Each BGP message starts with a standard header.  In all
cases, syntactic errors in the message header will cause the BGP speaker
to close the connection, release all associated BGP resources, delete
all routes learned through that connection,  run its decision process to
decide on new routes and cause the state to return to Idle.  Also,
optionally, an implementation specific peer oscillation damping may be
performed.  The peer oscillation damping process can affect how soon the
connection can be restarted.  An outsider who could spoof messages with
message header errors could cause disruptions in routing over a wide
area.


3.1.2.  OPEN


Event 19:  Receipt of an OPEN message in state Connect or Active will
cause the BGP speaker to bring down the connection, release all
associated BGP resources, delete all associated routes, run its decision
process and cause the state to return to Idle. The deletion of routes
can cause a cascading effect of routing changes propagating through
other peers.  Also, optionally, an implementation specific peer
oscillation damping may be performed.  The peer oscillation damping
process can affect how soon the connection can be restarted.


In state OpenConfirm or Established, the arrival of an OPEN may indicate
a connection collision has occurred.  If this connection is to be
dropped, then Event 23 will be issued.  (Event 23, discussed below,
results in the same set of disruptive actions as mentioned above for
states Connect or Active.)







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In state OpenSent, the arrival of an OPEN message will cause the BGP
speaker to transition to the OpenConfirm state.  If an outsider was able
to spoof an OPEN message (requiring very careful timing), then the later
arrival of the legitimate peer's OPEN message might lead the BGP speaker
to declare a connection collision.  The collision detection procedure
may cause the legitimate connection to be dropped.


Consequently, the ability of an outsider to spoof this message can lead
to a severe disruption of routing over a wide area.


Event 20:  If an OPEN message arrives when the DelayOpen timer is
running when the connection is in state OpenSent, OpenConfirm or
Established, the BGP speaker will bring down the connection, release all
associated BGP resources, delete all associated routes, run its decision
process and cause the state to return to Idle.  The deletion of routes
can cause a cascading effect of routing changes propagating through
other peers.  Also, optionally, an implementation specific peer
oscillation damping may be performed.  The peer oscillation damping
process can affect how soon the connection can be restarted.  However,
as the OpenDelay timer should never be running in these states, this
could only be caused by an error in the implementation (a NOTIFICATION
is sent with the error code "Finite State Machine Error").  It would be
difficult, if not impossible, for an outsider to induce this FSM error.


In states Connect and Active, this event will cause a transition to the
OpenConfirm state. As in Event 19, if an outsider were able to spoof an
OPEN which arrived while the DelayOpen timer was running, then a later
arriving OPEN from the legitimate peer might be considered a connection
collision and the legitimate connection could be dropped.


Consequently, the ability for an outsider to spoof this message can lead
to a severe disruption of routing over a wide area.


Event 22:  Errors in the OPEN message (e.g., unacceptable Hold state,
malformed Optional Parameter, unsupported version, etc.) will cause the
BGP speaker to bring down the connection, release all associated BGP
resources, delete all associated routes, run its decision process and
cause the state to return to Idle.  The deletion of routes can cause a
cascading effect of routing changes propagating through other peers.
Also, optionally, an implementation specific peer oscillation damping
may be performed.  The peer oscillation damping process can affect how
soon the connection can be restarted.  Consequently, the ability of an
outsider to spoof this message can lead to a severe disruption of
routing over a wide area.







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3.1.3.  KEEPALIVE


Event 26:  Receipt of a KEEPALIVE message when the peering connection is
in the Connect, Active, and OpenSent states would cause the BGP speaker
to transition to the Idle state and fail to establish a connection.
Also, optionally, an implementation specific peer oscillation damping
may be performed.  The peer oscillation damping process can affect how
soon the connection can be restarted.  The ability of an outsider to
spoof this message can lead to a disruption of routing.  To exploit this
vulnerability deliberately, the KEEPALIVE must be carefully timed in the
sequence of messages exchanged between the peers; otherwise, it causes
no damage.


3.1.4.  NOTIFICATION


Event 25:  Receipt of a NOTIFICATION message in any state will cause the
BGP speaker to bring down the connection, release all associated BGP
resources, delete all associated routes, run its decision process and
cause the state to return to Idle.  The deletion of routes can cause a
cascading effect of routing changes propagating through other peers.
Also, optionally, in any state but Established, an implementation
specific peer oscillation damping may be performed.  The peer
oscillation damping process can affect how soon the connection can be
restarted.  Consequently, the ability of an outsider to spoof this
message can lead to a severe disruption of routing over a wide area.


Event 24:  A NOTIFICATION message carrying an error code of "Version
Error" behaves the same as in Event 25, with the exception that the
optional peer oscillation damping is not performed in states OpenSent or
OpenConfirm, or in state Connect or Active if the DelayOpen timer is
running.  The damage caused is therefore one small bit less, because
restarting the connection is not affected.


3.1.5.  UPDATE


Event 8:  A BGP speaker may optionally choose to automatically
disconnect a BGP connection if the total number of prefixes exceeds a
configured maximum. If such a case, an UPDATE may carry a number of
prefixes that would result in that maximum being exceeded.  The BGP
speaker would disconnect the connection, release all associated BGP
resources, delete all associated routes, run its decision process and
cause the state to return to Idle.  The deletion of routes can cause a
cascading effect of routing changes propagating through other peers.
Also, optionally, an implementation specific peer oscillation damping
may be performed.  The peer oscillation damping process can affect how






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soon the connection can be restarted.  Consequently, the ability of an
outsider to spoof this message can lead to a severe disruption of
routing over a wide area.


Event 28:  If the UPDATE message is malformed (Withdrawn Routes Length,
Total Attribute Length, or Attribute Length that are improper, missing
mandatory well-known attributes, Attribute Flags that conflict with the
Attribute Type Codes, syntactic errors in the ORIGIN, NEXT_HOP or
AS_PATH, etc.), then the BGP speaker will bring down the connection,
release all associated BGP resources, delete all associated routes, run
its decision process and cause the state to return to Idle.  The
deletion of routes can cause a cascading effect of routing changes
propagating through other peers.  Also, optionally, an implementation
specific peer oscillation damping may be performed.  The peer
oscillation damping process can affect how soon the connection can be
restarted.  Consequently, the ability of an outsider to spoof this
message could cause widespread disruption of routing.  As a BGP speaker
has the authority to close a connection whenever it wants, this message
gives BGP speakers no more opportunity to cause damage than they already
had.


Event 27:  An Update message that arrives in any state but Established
will cause the BGP speaker to bring down the connection, release all
associated BGP resources, delete all associated routes, run its decision
process and cause the state to return to Idle.  The deletion of routes
can cause a cascading effect of routing changes propagating through
other peers.  Also, optionally, an implementation specific peer
oscillation damping may be performed.  The peer oscillation damping
process can affect how soon the connection can be restarted.
Consequently, the ability of an outsider to spoof this message can lead
to a severe disruption of routing over a wide area.


In the Established state, the Update message carries the routing
information.  The ability to spoof any part of this message can lead to
a disruption of routing, whether the source of the message is an
outsider or a legitimate BGP speaker.


3.1.5.1.  Unfeasible Routes Length, Total Path Attribute Length


There is a vulnerability arising from the ability to modify these
fields.  If a length is modified, the message is not likely to parse
properly, resulting in an error, the transmission of a NOTIFICATION
message and the close of the connection (see Event 28, above).  As a
true BGP speaker is always able to close a connection at any time, this
vulnerability represents an additional risk only when the source is not






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the configured BGP peer, i.e., it presents no additional risk from BGP
speakers.


3.1.5.2.  Withdrawn Routes


An outsider could cause the elimination of existing legitimate routes by
forging or modifying this field.  An outsider could also cause the
elimination of reestablished routes by replaying this withdrawal
information from earlier packets.


A BGP speaker could "falsely" withdraw feasible routes using this field.
However, as the BGP speaker is authoritative for the routes it will
announce, it is allowed to withdraw any previously announced routes that
it wants.  As the receiving BGP speaker will only withdraw routes
associated with the sending BGP speaker, there is no opportunity for a
BGP speaker to withdraw another BGP speaker's routes.  Therefore, there
is no additional risk from BGP peers via this field.


3.1.5.3.  Path Attributes


The path attributes present many different vulnerabilities and risks.


Attribute Flags, Attribute Type Codes, Attribute Length


A BGP peer or an outsider could modify the attribute length or attribute
type (flags and type codes) so they did not reflect the attribute values
that followed.  If the flags were modified, the flags and type code
could become incompatible (i.e., a mandatory attribute marked as
partial), or a optional attribute could be interpreted as a mandatory
attribute or vice versa.  If the type code were modified,  the attribute
value could be interpreted as if it were the data type and value of a
different attribute.


The most likely result from modifying the attribute length, flags, or
type code would be a parse error of the UPDATE message.  A parse error
would cause the transmission of a NOTIFICATION message and the close of
the connection (see Event 28, above).  As a true BGP speaker is always
able to close a connection at any time, this vulnerability represents an
additional risk only when the source is an outsider, i.e., it presents
no additional risk from a BGP peer.


ORIGIN


This field indicates whether the information was learned from IGP or EGP
information.  This field is used in making routing decisions, so there






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is some small vulnerability in being able to affect the receiving BGP
speaker's routing decision by modifying this field.


AS_PATH


A BGP peer or outsider could announce an AS_PATH that was not accurate
for the associated NLRI.


As it is possible for a BGP peer not to verify that a received AS_PATH
begins with the AS number of its peer, a malicious BGP peer could
announce a path that begins with the AS of any BGP speaker with little
impact on itself.  This could affect the receiving BGP speaker's
decision procedure and choice of installed route.  The malicious peer
could considerably shorten the AS_PATH, which will increase that route's
chances of being chosen, possibly giving the malicious peer access to
traffic it would otherwise not receive.  The shortened AS_PATH also
could result in routing loops, as it does not contain the information
needed to prevent loops.


It is possible for a BGP speaker to be configured to accept routes with
its own AS number in the AS path.  Such operational considerations are
defined to be "outside the scope" of the BGP specification, but the fact
that AS_PATHs can have loops means that implementations cannot
automatically reject routes with loops.  Each BGP speaker verifies only
that its own AS number does not appear in the AS_PATH.


Coupled with the ability to use any value for the NEXT_HOP, this gives a
malicious BGP speaker considerable control over the path traffic will
take.


Originating Routes


A special case of announcing a false AS_PATH occurs when the AS_PATH
advertises a direct connection to a specific network address.  A BGP
peer or outsider could disrupt routing to the network(s) listed in the
NLRI field by falsely advertising a direct connection to the network.
The NLRI would become unreachable to the portion of the network that
accepted this false route, unless the ultimate AS on the AS_PATH
undertook to tunnel the packets it was forwarded for this NLRI on toward
their true destination AS by a valid path.  But even when the packets
are tunneled to the correct destination AS, the route followed may not
be optimal or may not follow the intended policy.  Additionally, routing
for other networks in the Internet could be affected if the false
advertisement fragmented an aggregated address block, forcing the
routers to handle (issue UPDATES, store, manage) the multiple fragments






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rather than the single aggregate.  False originations for multiple
addresses can result in routers and transit networks along the announced
route to become flooded with mis-directed traffic.


NEXT_HOP


The NEXT_HOP attribute defines the IP address of the border router that
should be used as the next hop when forwarding the NLRI listed in the
UPDATE message.  If the recipient is an external peer, then the
recipient and the NEXT_HOP address must share a subnet.  It is clear
that an outsider modifying this field could disrupt the forwarding of
traffic between the two AS's.


In the case that the recipient of the message is an external peer of an
AS and the route was learned from another peer AS (this is one of two
forms of "third party" NEXT_HOP), then the BGP speaker advertising the
route has the opportunity to direct the recipient to forward traffic to
a BGP speaker at the NEXT_HOP address.  This affords the opportunity to
direct traffic at a router that may not be able to continue forwarding
the traffic.  A malicious BGP speaker can also use this technique to
force another AS to carry traffic it would otherwise not have to carry.
In some cases, this could be to the malicious BGP speaker's benefit, as
it could cause traffic to be carried long-haul by the victim AS to some
other peering point it shared with the victim.


MULTI_EXIT_DISC


The MULTI_EXIT_DISC attribute is used in UPDATE messages transmitted
between inter-AS BGP peers.  While the MULTI_EXIT_DISC received from an
inter-AS peer may be propagated within an AS, it may not be propagated
to other AS's.  Consequently, this field is only used in making routing
decisions internal to one AS.  Modifying this field, whether by an
outsider or a BGP peer, could influence routing within an AS to be
sub-optimal, but the effect should be limited in scope.


LOCAL_PREF


The LOCAL_PREF attribute must be included in all messages with internal
peers and excluded from messages with external peers.  Consequently,
modification of the LOCAL_PREF could effect the routing process within
the AS only.  Note that there is no requirement in the BGP RFC that the
LOCAL_PREF be consistent among the internal BGP speakers of an AS.  As
BGP peers are free to choose the LOCAL_PREF as they wish, modification
of this field is a vulnerability with respect to outsiders only.







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ATOMIC_AGGREGATE


The ATOMIC_AGGREGATE field indicates that an AS somewhere along the way
has aggregated several routes and advertised the aggregate NLRI without
the AS_SET formed as usual from the AS's in the aggregated routes'
AS_PATHs.  BGP speakers receiving a route with ATOMIC_AGGREGATE are
restricted from making the NLRI any more specific.  Removing the
ATOMIC_AGGREGATE attribute would remove the restriction, possibly
causing traffic intended for the more specific NLRI to be routed
incorrectly.   Adding the ATOMIC_AGGREGATE attribute when no aggregation
was done would have little effect, beyond restricting the un-aggregated
NLRI from being made more specific.  This vulnerability exists whether
the source is a BGP peer or an outsider.


AGGREGATOR


This field may be included by a BGP speaker who has computed the routes
represented in the UPDATE message from aggregation of other routes.  The
field contains the AS number and IP address of the last aggregator of
the route.  It is not used in making any routing decisions, so it does
not represent a vulnerability.


3.1.5.4.  NLRI


By modifying or forging this field, either an outsider or BGP peer
source could cause disruption of routing to the announced network,
overwhelm a router along the announced route, cause data loss when the
announced route will not forward traffic to the announced network, route
traffic by a sub-optimal route, etc.


3.2.  Vulnerabilities through Other Protocols


3.2.1.  TCP messages


BGP runs over TCP, listening on port 179.  Therefore, BGP is subject to
attack through attacks on TCP.


3.2.1.1.  TCP SYN


SYN flooding:  BGP is as subject to the effects on the TCP
implementation of SYN flooding attacks as other protocols, and must rely
on the implementation's protections against this attack.


Event 14:  If an outsider were able to send a SYN to the BGP speaker at
the appropriate time during connection establishment, then the






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legitimate peer's SYN would appear to be a second connection.  If the
outsider were able to continue with a sequence of packets resulting in a
BGP connection (guessing the BGP speaker's choice for sequence number on
the SYN ACK, for example), then, the outsider's connection and the
legitimate peer's connection would appear to be a connection collision.
Depending on the outcome of the collision detection (i.e., the outsider
chose a BGP identifier so as to win the race), the legitimate peer's
true connection could be destroyed.  The use of [TCPMD5] can counter
this attack.


3.2.1.2.  TCP SYN ACK


Event 16:  If an outsider were able to respond to a BGP speaker's SYN
before the legitimate peer, then the legitimate peer's SYN-ACK would
receive a empty ACK reply, causing the legitimate peer to issue a RST
that would break the connection.  The BGP speaker would bring down the
connection, release all associated BGP resources, delete all associated
routes and run its decision process.  This attack requires that the
outsider be able to predict the sequence number used in the SYN.  The
use of [TCPMD5] can counter this attack.


3.2.1.3.  TCP ACK


Event 17:  If an outsider were able to spoof an ACK at the appropriate
time during connection establishment, then the BGP speaker would
consider the connection complete, send an OPEN (Event 17) and transition
to the OpenSent state.  The arrival of the legitimate peer's ACK would
not be delivered to the BGP process, as it would look like a duplicate
packet.  This message, then, presents no particular vulnerability to BGP
during connection establishment.  Spoofing an ACK after connection
establishment requires knowledge of the sequence numbers in use, and is
in general a very difficult task.  The use of [TCPMD5] can counter this
attack.


3.2.1.4.  TCP RST/FIN/FIN-ACK


Event 18:  If an outsider were able to spoof a RST, the BGP speaker
would bring down the connection, release all associated BGP resources,
delete all associated routes and run its decision process.  If an
outsider were able to spoof a FIN, then data could still be transmitted,
but any attempt to receive would receive a notification that the
connection is closing.  In most cases, this results in the connection
being placed in an Idle state, but if the connection is in the OpenSent
state at the time, the connection returns to an Active state.  Spoofing
a RST in this situation requires an outsider to guess a sequence number






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that need only be within the receive window [Watson04], generally an
easier task than guessing the exact sequence number so as to spoof a
FIN.  The use of [TCPMD5] can counter this attack.


3.2.1.5.  DoS and DDos


Because the packet directed to TCP port 179 are passed to the BGP
process, that potentially resides on a slower processor in the router,
flooding a router with TCP port 179 packets is an avenue for DoS attacks
against the router.  No BGP protocol mechanism can defeat such attacks;
other mechanisms must be employed.


3.2.2.  Other supporting protocols


3.2.2.1.  Manual stop


Event 2:  A manual stop event causes the BGP speaker to bring down the
connection, release all associated BGP resources, delete all associated
routes and run its decision process.  If the mechanism by which a BGP
speaker was informed of a manual stop were not carefully protected, the
BGP connection could be destroyed by an outsider. Consequently, BGP
security is secondarily dependent on the security of the protocols by
which the platform is managed and configured that might signal this
event.


3.2.2.2.  Open Collision Dump


Event 23:  The OpenCollisionDump event may be generated administratively
when a connection collision event is detected and this connection has
been selected to be disconnected.  When this event occurs in any state,
the BGP connection is dropped, the BGP resources are released, the
associated routes are deleted, etc.  Consequently, BGP security is
secondarily dependent on the security of the protocols by which the
platform is managed and configured that might signal this event.


3.2.2.3.  Timer events


Events 9-13:  BGP employs five timers (ConnectRetry, Hold, Keepalive,
MinASOrigination-Interval, and MinRouteAdvertisementInterval) and two
optional timers (DelayOpen and IdleHold).  These timers are critical to
BGP operation.  For example, if the Hold timer value were changed, the
remote peer might consider the connection unresponsive and bring the
connection down, releasing resources, deleting associated routes, etc.
Consequently, BGP security is secondarily dependent on the security of
the protocols by which the platform is operated, managed and configured






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that might modify these timers.


4.  Security Considerations


This entire memo is about security, describing an analysis of the
vulnerabilities that exist in the BGP protocol.


Use of the mandatory-to-support mechanisms of [TCPMD5] counters the
message insertion, deletion, and modification attacks and
man-in-the-middle attacks from outsiders.  If routing data
confidentiality were desired (there being some controversy as to whether
that is a desirable security service), the use of IPsec ESP could
provide that service.


4.1.  Residual Risk


As cryptographic-based mechanisms, both [TCPMD5] and IPsec [IPsec]
assume that the cryptographic algorithms are secure, that 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.


These mechanisms do not prevent attacks that arise from a router's
legitimate BGP peers.  There are several possible solutions to prevent a
BGP speaker from inserting bogus information in its advertisements to
its peers, i.e., from mounting an attack on a network's origination or
AS-PATH.


(1)  Origination Protection:  sign the originating AS.


(2)  Origination and Adjacency Protection:  sign the originating AS and
     predecessor information ([Smith96])


(3)  Origination and Route Protection:  sign the originating AS, and
     nest signatures of AS_PATHs to the number of consecutive bad
     routers you want to prevent from causing damage. ([SBGP00])


(4)  Filtering:  rely on a registry to verify the AS_PATH and NLRI
     originating AS ([RPSL]).


Filtering is in use near some customer attachment points, but is not
effective near the Internet center.  The other mechanisms are still
controversial and are not yet in common use.








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4.2.  Operational Protections


The primary usage of BGP is as a means to provide reachability
information to Autonomous Systems (AS) and to distribute external
reachability internally within an AS.  BGP is the routing protocol used
to distribute global routing information in the Internet.  BGP is
therefore used by all major Internet Service Providers (ISP) and many
smaller providers and other organizations.


The role which BGP plays in the Internet puts BGP implementations in
unique conditions and places unique security requirements on BGP.  BGP
is operated over interprovider interfaces in which traffic levels push
the state of the art in specialized packet forwarding hardware and
exceed the performance capabilities of hardware implementation of
decryption by many orders of magnitude.  The capability of an attacker
using a single workstation with high speed interface to generate false
traffic for denial of service (DoS) far exceeds the capability of
software based decryption or appropriately priced cryptographic hardware
to detect the false traffic.  One means to protect the network elements
from DoS attacks under such conditions is to use packet based filtering
techniques based on relatively simple inspections of packets.  As a
result, for an ISP carrying large volumes of traffic, the ability to
packet filter on the basis of port numbers is an important protection
against DoS attacks, and a necessary adjunct to cryptographic strength
in encapsulation.


Current practice in ISP operation is to use certain common filtering
techniques to reduce the exposure to attacks from outside the ISP.  To
protect Internal BGP (IBGP) sessions, filters are applied at all borders
to an ISP network which remove all traffic destined for addresses of
network elements internal addresses (typically contained within a single
prefix) and the BGP port number (179).  Packets from within an ISP are
not forwarded from an internal interface to the BGP speaker's address on
which External BGP (EBGP) sessions are supported, or to a peer's EBGP
address if the BGP port number is found.  With appropriate consideration
in router design, in the event of failure of a BGP peer to provide the
equivalent filtering, the risk of compromise can be limited to the
peering session on which filtering is not performed by the peer or the
interface or line card on which the peering is supported.  There is
substantial motivation and little effort for ISPs to maintain such
filters.


These operational practices can considerably raise the difficulty for an
outsider to launch a DoS attack against an ISP.  Prevented from
injecting sufficient traffic from outside a network to effect a DoS






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attack, the attacker would have to undertake much more difficult tasks,
such as compromise of the ISP network elements or undetected tapping
into physical media.


5.  IANA Considerations   This document has no actions for IANA.


6.  References


6.1.  Normative


[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", RFC 2119, BCP 14, March 1997.


[TCPMD5]   Heffernan, A., "Protection of BGP Sessions via the TCP MD5
           Signature Option", RFC2385, August 1998.


[BGP]      Rekhter, Y.  and Li, T., "A Border Gateway Protocol 4 (BGP-
           4)", work in progress, September 2004.  available as
           <<draft-ietf-idr-bgp4-25.txt>> at Internet-Draft shadow
           sites.


6.2.  Informative


[IPsec]    Kent, S. and  Atkinson, R., "Security Architecture for the
           Internet Protocol", RFC2401, November 1998.


[SBGP00]   Kent, S., Lynn, C. and Seo, K., "Secure Border Gateway
           Protocol (Secure-BGP)", IEEE Journal on Selected Areas in
           Communications, Vol. 18, No. 4, April 2000, pp. 582-592.


[SecCons]  Rescorla, E. and Korver, B., "Guidelines for Writing RFC Text
           on Security Considerations", RFC3552, BCP72, July 2003.


[Smith96]  Smith, B. and Garcia-Luna-Aceves, J.J., "Securing the Border
           Gateway Routing Protocol", Proc. Global Internet'96, London,
           UK, 20-21 November 1996.


[RPSL]     Villamizar, C.,  Alaettinoglu, C., Meyer, D., Murphy, S. and
           Orange, C., "Routing Policy System Security", RFC 2725,
           December 1999.


[Watson04] Watson, P., "Slipping In The Window: TCP Reset Attacks",
           CanSecWest 2004, April 2004.








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7.  Author's Address


Sandra Murphy
Sparta, Inc.
7075 Samuel Morse Drive
Columbia, MD 21046
EMail: Sandy@tislabs.com












































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