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Network Working Group S. Murphy
Request for Comments: 4272 Sparta, Inc.
Category: Informational January 2006
BGP Security Vulnerabilities Analysis
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
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 BGP that protect against attacks that modify,
delete, forge, or replay data, any of which has the potential to
disrupt overall network routing behavior.
This document discusses some of the security issues with BGP routing
data dissemination. This document does not discuss security issues
with forwarding of packets.
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Table of Contents
1. Introduction ....................................................3
1.1. Specification of Requirements ..............................5
2. Attacks .........................................................6
3. Vulnerabilities and Risks .......................................7
3.1. Vulnerabilities in BGP Messages ............................8
3.1.1. Message Header ......................................9
3.1.2. OPEN ................................................9
3.1.3. KEEPALIVE ..........................................11
3.1.4. NOTIFICATION .......................................11
3.1.5. UPDATE .............................................11
3.1.5.1. Unfeasible Routes Length, Total
Path Attribute Length .....................12
3.1.5.2. Withdrawn Routes ..........................13
3.1.5.3. Path Attributes ...........................13
3.1.5.4. NLRI ......................................16
3.2. Vulnerabilities through Other Protocols ...................16
3.2.1. TCP Messages .......................................16
3.2.1.1. TCP SYN ...................................16
3.2.1.2. TCP SYN ACK ...............................17
3.2.1.3. TCP ACK ...................................17
3.2.1.4. TCP RST/FIN/FIN-ACK .......................17
3.2.1.5. DoS and DDos ..............................18
3.2.2. Other Supporting Protocols .........................18
3.2.2.1. Manual Stop ...............................18
3.2.2.2. Open Collision Dump .......................18
3.2.2.3. Timer Events ..............................18
4. Security Considerations ........................................19
4.1. Residual Risk .............................................19
4.2. Operational Protections ...................................19
5. References .....................................................21
5.1. Normative References ......................................21
5.2. Informative References ....................................21
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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 design did not include protections against
deliberate or accidental errors that could cause disruptions of
routing behavior.
This document discusses the vulnerabilities of BGP, based on the BGP
specification [RFC4271]. 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 peer-peer communication is not an
integral part of BGP. As a TCP/IP protocol, BGP is subject to all
TCP/IP attacks, e.g., 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-peer connection. Any break in the peer-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 a default.
The current BGP specification also allows for implementations that
would accept connections from "unconfigured peers" ([RFC4271] Section
8). However, the specification is not clear as to what an
unconfigured peer might be, or how the protections of [TCPMD5] would
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apply in such a case. Therefore, it is not possible to include an
analysis of the security issues of this feature. When a
specification that describes this feature more fully is released, a
security analysis should be part of that 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, yet 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
by a path that does not follow the expected policy, or by a path that
will not forward the traffic. Consequently, traffic to that network
could be delayed by a path that is longer than necessary. 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 to view or 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, in fact, it is
not.
cut: Some portion of the network believes that it has no route to
some network to which it 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 becomes unstable in such a way 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, thus 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.
1.1. 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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2. Attacks
BGP, in and of itself, is subject to the following attacks. (The
list is taken from the IAB RFC that provides guidelines for the
"Security Considerations" section of RFCs [SecCons].)
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 not perform peer entity
authentication, a man-in-the-middle attack is child's 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 (i.e., longer prefixes) can cause BGP traffic
and router table size to increase, even explode.
The mandatory-to-support mechanism of [TCPMD5] will counter message
insertion, deletion, and modification, man-in-the-middle and 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
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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 [TCPMD5] cannot protect against bogus routing
information that originates from an insider.
3. Vulnerabilities and Risks
The risks in BGP arise from three fundamental vulnerabilities:
(1) 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.
(2) no mechanism has been specified within BGP to validate the
authority of an AS to announce NLRI information.
(3) 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 the support of [TCPMD5] is
employed, message integrity and peer entity authentication are
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. The events are defined
and discussed in section 8 of [RFC4271]. 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. They 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 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
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outsider attacks. BGP peers themselves are permitted to break peer-
peer connections, at any time, using NOTIFICATION messages. Thus,
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 vulnerabilities and risks, which are
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 states 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 in which routing
changes propagate 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.)
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
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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 in which routing
changes propagate 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, because the OpenDelay timer
should never be running in these states, this effect 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 Finite State
Machine 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 of 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 in which routing changes
propagate 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 in which routing changes
propagate 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 states Connect or Active if the DelayOpen timer
is running. Therefore, the damage caused is 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. In 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 in which routing changes propagate through
other peers. Also, optionally, an implementation-specific peer
oscillation damping may be performed. The peer oscillation damping
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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 28: If the UPDATE message is malformed, 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. (Here, "malformed" refers to improper
Withdrawn Routes Length, Total Attribute Length, or Attribute Length,
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.) The deletion of routes can cause
a cascading effect in which routing changes propagate 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
additional opportunity to cause damage.
Event 27: An Update message that arrives in any state except
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 in which routing
changes propagate 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 able to close a connection at any time, this
vulnerability represents an additional risk only when the source is
not the configured BGP peer, i.e., it presents no additional risk
from BGP speakers.
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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.
o 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) not to 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 an 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 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.
o ORIGIN
This field indicates whether the information was learned from IGP
or EGP information. This field is used in making routing
decisions, so there is some small vulnerability of being able to
affect the receiving BGP speaker's routing decision by modifying
this field.
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o AS_PATH
A BGP peer or outsider could announce an AS_PATH that was not
accurate for the associated NLRI.
Because a BGP peer might not 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 because AS_PATHs can legitimately have loops,
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
provides a malicious BGP speaker considerable control over the
path traffic will take.
o 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 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 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
misdirected traffic.
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o 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 who modified this field
could disrupt the forwarding of traffic between the two ASes.
If 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.
o 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 ASes. 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.
o 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. Because BGP peers are free to
choose the LOCAL_PREF, modification of this field is a
vulnerability with respect to outsiders only.
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o 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 being formed as usual from the ASes 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.
o AGGREGATOR
This field may be included by a BGP speaker who has computed the
routes represented in the UPDATE message by aggregating 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: Like other protocols, BGP is subject to the effects on
the TCP implementation of SYN flooding attacks, and must rely on the
implementation's protections against these attacks.
Event 14: If an outsider were able to send a SYN to the BGP speaker
at the appropriate time during connection establishment, then the
legitimate peer's SYN would appear to be a second connection. If the
outsider were able to continue with a sequence of packets resulting
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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., if the outsider chooses 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 an 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. Thus, this message does not
present a 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 it would trigger 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 Connect state or the OpenSent state at the time,
the connection will return to an Active state.
Spoofing a RST in this situation requires an outsider to guess a
sequence number that need only be within the receive window
[Watson04]. This is generally an easier task than guessing the exact
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sequence number required to spoof a FIN. The use of [TCPMD5] can
counter this attack.
3.2.1.5. DoS and DDos
Because the packets directed to TCP port 179 are passed to the BGP
process, which 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 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 is not carefully
protected, the BGP connection could be destroyed by an outsider.
Consequently, BGP security is secondarily dependent on the security
of the management and configuration protocols that are used to 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
the 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
management and configuration protocols that are used to 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, thus releasing resources, deleting associated
routes, etc. Consequently, BGP security is secondarily dependent on
the security of the operation, management, and configuration
protocols that are used to modify the timers.
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4. Security Considerations
This entire memo is about security, describing an analysis of the
vulnerabilities that exist in BGP.
Use of the mandatory-to-support mechanisms of [TCPMD5] counters the
message insertion, deletion, and modification attacks, as well as
man-in-the-middle attacks by outsiders. If routing data
confidentiality is desired (there is some controversy as to whether
it 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.
4.2. Operational Protections
BGP is primarily used 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
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distribute global routing information in the Internet. Therefore,
BGP is used by all major Internet Service Providers (ISP), as well as
many smaller providers and other organizations.
BGP's role 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. Under such
conditions, one means to protect the network elements from DoS
attacks 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. This removes all traffic destined for
network elements' internal addresses (typically contained within a
single prefix) and the BGP port number (179). If the BGP port number
is found, 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.
Appropriate router design can limit the risk of compromise when a BGP
peer fails to provide adequate filtering. The risk can be limited to
the peering session on which filtering is not performed by the peer,
or to the interface or line card on which the peering is supported.
There is substantial motivation, and little effort is required, 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
attack, the attacker would have to undertake more difficult tasks,
such as compromising the ISP network elements or undetected tapping
into physical media.
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5. References
5.1. Normative References
[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", RFC 2385, August 1998.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, Eds., "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
5.2. Informative References
[IPsec] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, 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 B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, 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., and S.
Murphy, "Routing Policy System Security", RFC 2725,
December 1999.
[Watson04] Watson, P., "Slipping In The Window: TCP Reset Attacks",
CanSecWest 2004, April 2004.
Author's Address
Sandra Murphy
Sparta, Inc.
7075 Samuel Morse Drive
Columbia, MD 21046
EMail: Sandy@tislabs.com
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