Network Working Group R. Stewart
Internet-Draft Cisco Systems, Inc.
Expires: December 15, 2007 M. Tuexen
Muenster Univ. of Applied Sciences
G. Camarillo
Ericsson
June 13, 2007
Security Attacks Found Against SCTP and Current Countermeasures
draft-ietf-tsvwg-sctpthreat-05.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes certain security threats to SCTP. It also
describes ways to mitigate these threats, in particular by using
techniques from the SCTP Specification Errata and Issues memo (RFC
4460). These techniques are included in RFC 4960, which obsoletes
RFC 2960. It is hoped that this information will provide some useful
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background information for many of the newest requirements spelled
out in the SCTP Specification Errata and Issues and included in RFC
4960.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Address Camping or stealing . . . . . . . . . . . . . . . . . 3
3. Association hijacking 1 . . . . . . . . . . . . . . . . . . . 5
4. Association hijacking 2 . . . . . . . . . . . . . . . . . . . 7
5. Bombing attack (amplification) 1 . . . . . . . . . . . . . . . 8
6. Bombing attack (amplification) 2 . . . . . . . . . . . . . . . 10
7. Association redirection . . . . . . . . . . . . . . . . . . . 11
8. Bombing attack (amplification) 3 . . . . . . . . . . . . . . . 11
9. Bombing attack (amplification) 4 . . . . . . . . . . . . . . . 12
10. Bombing attack (amplification) 5 . . . . . . . . . . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13
12. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
13.1. Normative References . . . . . . . . . . . . . . . . . . 13
13.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
Stream Control Transmission Protocol originally defined in [RFC2960]
is a multi-homed transport protocol. As such, unique security
threats exists that are addressed in various ways within the protocol
itself. This document describes certain security threats to SCTP.
It also describes ways to mitigate these threats, in particular by
using techniques from the SCTP Specification Errata and Issues memo
([RFC4460]). These techniques are included in
[I-D.ietf-tsvwg-2960bis], which obsoletes [RFC2960]. It is hoped
that this information will provide some useful background information
for many of the newest requirements spelled out in the [RFC4460] and
included in [I-D.ietf-tsvwg-2960bis].
This work and some of the changes that went into the [RFC4460] and
[I-D.ietf-tsvwg-2960bis] are much indebted to the paper on potential
SCTP security risks Effects [effects] by Aura, Nikander and
Camarillo. Without their work some of these changes would remain
undocumented and potential threats.
The rest of this document will concentrate on the various attacks
that were illustrated in Effects [effects] and detail what
preventative measures are now in place within the current SCTP
standards (if any).
2. Address Camping or stealing
This attack is a form of denial of service attack crafted around
SCTP's multi-homing. In effect an illegitimate endpoint connects to
a server and "camps upon" or holds up a valid peers address. This is
done to prevent the legitimate peer from communicating with the
server.
2.1. Attack details
+----------+ +----------+ +----------+
| Evil | | Server | | Client |
| IP-A=+------------+ +-----------+=IP-C & D |
| Attacker | | | | Victim |
+----------+ +----------+ +----------+
Figure 1: Camping
Consider the scenario illustrated in Figure 1. The attacker
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legitimately holds IP-A and wishes to prevent the 'Client-Victim'
from communication with the 'Server'. Note also that the client is
multi-homed. The attacker first guesses the port number our client
will use in its association attempt. It then uses this port and sets
up an association with the server listing not only IP-A but also IP-C
as well in its initial INIT chunk. The server will respond and setup
the association noting that the attacker is multi-homed holding both
IP-A and IP-C.
Next the victim sends in an INIT message listing its two valid
addresses IP-C and IP-D. In response it will receive an ABORT
message with possibly an error code indicating that a new address was
added in its attempt to setup an existing association (a restart with
new addresses). At this point 'Client-Victim' is now prevented from
setting up an association with the server until the server realizes
that the attacker does not hold the address IP-C at some future point
by using a HEARTBEAT based mechanism. See the mitigation option
subsection of this section.
2.2. Analysis
This particular attack was discussed in detail on the SCTP
implementors list in March of 2003. Out of that discussion changes
were made in the BSD implementation that are now present in the
[I-D.ietf-tsvwg-2960bis]. In close examination, this attack depends
on a number of specific things to occur.
1) The attacker must setup the association before the victim and must
correctly guess the port number that the victim will use. If the
victim uses any other port number the attack will fail.
2) SCTP's existing HEARTBEAT mechanism as defined already in
[RFC2960] will eventually catch this situation and abort the evil
attackers association. This may take several seconds based on
default HEARTBEAT timers but the attacker himself will lose any
association.
3) If the victim is either not multi-homed, or the address set that
it uses is completely camped upon by the attacker (in our example
if the attacker had included IP-D in its INIT as well), then the
client's INIT message would initiate an association between the
client and the server while destroying the association between the
attacker and the server. From the servers' perspective this is a
restart of the association.
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2.3. Mitigation option
[I-D.ietf-tsvwg-2960bis] adds a new set of requirements to better
counter this attack. In particular the HEARTBEAT mechanism was
modified so that addresses unknown to an endpoint (i.e. presented in
an INIT with no pre-knowledge given by the application) enter a new
state called "UNCONFIRMED". During the time that any address is
UNCONFIRMED and yet considered available, heartbeating will be done
on those UNCONFIRMED addresses at an accelerated rate. This will
lessen the time that an attacker can "camp" on an address. In
particular the rate of heartbeats to UNCONFIRMED addresses is done
every RTO. Along with this expanded rate of heartbeating, a new 64
bit random nonce is required to be inside HEARTBEATs to UNCONFIRMED
addresses. In the HEARTBEAT-ACK the random nonce must match the
value sent in the HEARTBEAT before an address can leave the
UNCONFIRMED state. This will prevent an attacker from generating
false HEARTBEAT-ACKs with the victims source address(es). In
addition, clients which do not need to use a specific port number
should choose their port numbers on a random base. This makes it
hard for an attacker to guess that number.
3. Association hijacking 1
Association hijacking is the ability of some other user to assume the
session created by another endpoint. In cases of a true man-in-the-
middle only a strong end-to-end security model can prevent this.
However with the addition of the [I-D.ietf-tsvwg-addip-sctp]
extension to SCTP an endpoint that is NOT a man-in-the-middle may be
able to assume another endpoints association.
3.1. Attack details
The attack is made possible by any mechanism that lets an endpoint
acquire some other IP address that was recently in use by an SCTP
endpoint. For example in a mobile network DHCP may be in use with
short IP address lifetimes to reassign IP addresses to migrant hosts.
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IP-A DHCP-Server's Peer-Server
|
|
1 |-DHCP-Rel(IP-A)---->|
2 |------ASCONF(ADD-IP(IP-B), DEL-IP(IP-A)---->XXlost
time
|
|-DHCP-new-net------>|
3 |<---Assign (IP-A)
|
4 |<------------Tag:X-DATA()------------------
|
|-------------INIT()------------------------>
5 |<------------INIT-ACK()---------------------
|
6 |----ASCONF(ADD-IP(IP-Z),DEL-IP(IP-A))------>
Figure 2: Association Hijack via DHCP
At point 1, our valid client releases the IP address IP-A. It
presumably acquires a new address (IP-B) and sends an ASCONF to ADD
the new address and delete to old address at point 2, but this packet
is lost. Thus our peer (Peer-Server) has no idea that the former
peer is no longer at IP-A. Now at point 3 a new "evil" peer DHCP's
an address and happens to get the re-assigned address IP-A. Our
Peer-Server sends a chunk of DATA at point 4. This reveals to the
new owner of IP-A that the former owner of IP-A had an association
with Peer-Server. So at point 5 the new owner of IP-A sends an INIT.
The INIT-ACK is sent back and inside it is a COOKIE. The cookie
would of course hold tie-tags which would list both sets of tags
which could then be used at point 6 to add in any other IP addresses
that the owner of IP-A holds and thus acquire the association.
It should be noted that this attack is possible in general whenever
the attacker is able to send packets with source address IP-A and
receive packets with destination address IP-A.
3.2. Analysis
This attack depends on a number of events:
1) Both endpoints must support the [I-D.ietf-tsvwg-addip-sctp]
extension.
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2) One of the endpoints must be using the [I-D.ietf-tsvwg-addip-sctp]
extension for mobility.
3) The IP address must be acquired in such a way as to make the
endpoint the owner of that IP address as far as the network is
concerned.
4) The true peer must not get the ASCONF packet that deletes IP-A and
adds its new address to the peer before the new "evil" peer gets
control of the association.
5) The new "evil" peer must have an alternative address besides IP-A
that it can add to the association so it can delete IP-A
preventing the real peer from re-acquiring the association when it
finally retransmits the ASCONF (from step 2).
3.3. Mitigation option
[I-D.ietf-tsvwg-2960bis] adds a new counter measure to this threat.
It is now required that Tie-Tags in the State-Cookie parameter not be
the actual tags. Instead a new pair of two 32 bit nonces must be
used to represent the real tags within the association. This
prevents the attacker from acquiring the real tags and thus prevents
this attack. Furthermore the use of the [I-D.ietf-tsvwg-addip-sctp]
extensions requires the use of the authentication mechanism defined
in [I-D.ietf-tsvwg-sctp-auth]. This requires the attacker to be able
to capture the traffic during the association setup. If in addition
an end-point pair shared key is used, capturing or intercepting these
setup messages does not enable the attacker to hijack the
association.
4. Association hijacking 2
Association hijacking is the ability of some other user to assume the
session created by another endpoint. In cases where an attacker can
send packets using the victims IP-address as a source address and can
receive packets with the victims' address as destination address the
attacker can easily restart the association. If the peer does not
pay attention to the restart notification the attacker has taken over
the association.
4.1. Attack details
Assume that an endpoint E1 having an IP-address A has an SCTP
association with endpoint E2. After the attacker is able to receive
packets with destination address A and send packet with source
address A the attacker can perform a full four-way handshake using
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the the IP-addresses and port numbers from the received packet. E2
will consider this as a restart of the association. If and only if
the SCTP user of E2 does not process the restart notification the
user will not recognize that that association just restarted. From
his perspective the association has been hijacked.
4.2. Analysis
This attack depends on a number of circumstances:
1) The IP address must be acquired in such a way as to make the evil
endpoint the owner of that IP address as far as the network or
local LAN is concerned.
2) The attacker must receive a packet belonging to the association or
connection.
3) The other endpoints user does not pay attention to restart
notifications.
4.3. Mitigation option
It is important to note that this attack is not based on a weakness
of the protocol but on the ignorance of the upper layer. This attack
is not possible if the upper layer processes the restart
notifications provided by SCTP as described in section 10 of
[RFC2960] or [I-D.ietf-tsvwg-2960bis]. Note that other IP protocols
may also be effected by this attack.
5. Bombing attack (amplification) 1
The bombing attack is a method to get a server to amplify packets to
an innocent victim.
5.1. Attack details
This attack is performed by setting up an association with a peer and
listing the victims IP address in the INIT's list of addresses.
After the association is setup, the attacker makes a request for a
large data transfer. After making the request the attacker does not
acknowledge data sent to it. This then causes the server to re-
transmit the data to the alternate address i.e. that of the victim.
After waiting an appropriate time period the attacker acknowledges
the data for the victim. At some point the attackers address is
considered unreachable since only data sent to the victims address is
acknowledged. At this point the attacker can send strategic
acknowledgments so that the server continues to send data to the
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victim.
Alternatively, instead of stopping the sending of SACKs to enforce a
path failover, the attacker can use the ADD-IP extension to add the
address of the victim and make that address the primary path.
5.2. Analysis
This attack depends on a number of circumstances:
1) The victim must NOT support SCTP, otherwise it would respond with
an OOTB abort.
2) The attacker must time its sending of acknowledgments correctly in
order to get its address into the failed state and the victims
address as the only valid alternative.
3) The attacker must guess TSN values that are accepted by the
receiver once the bombing begins since it must acknowledge packets
it no longer is seeing.
5.3. Mitigation option
[I-D.ietf-tsvwg-2960bis] makes two changes to prevent this attack.
First it details out proper handling of ICMP messages. With SCTP the
ICMP messages provide valuable clues to the SCTP stack that can be
verified with the tags for authenticity. Proper handling of an ICMP
protocol unreachable (or equivalent) would cause the association
setup by the attacker to be immediately failed upon the first
retransmission to the victims address.
The second change made in [I-D.ietf-tsvwg-2960bis] is the requirement
that no address that is not CONFIRMED is allowed to have DATA chunks
sent to it. This prevents the switch-over to the alternate address
from occurring even when ICMP messages are lost in the network and
prevents any DATA chunks from being sent to any other destination
other then the attacker itself. This also prevents the alternative
way of using ADD-IP to add the new address and make it the primary
address.
An SCTP implementation should abort the association if it receives a
SACK acknowledging a TSN which has not been sent. This makes TSN
guessing for the attacker quite hard because if the attacker
acknowledges one TSN too fast the association will be aborted.
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6. Bombing attack (amplification) 2
This attack allows an attacker to use an arbitrary SCTP endpoint to
send multiple packets to a victim in response to one packet.
6.1. Attack details
The attacker sends an INIT listing multiple IP addresses of the
victim in the INIT's list of addresses to an arbitrary endpoint.
Optionally it request a long cookie life time. Upon reception of the
INIT-ACK it stores the cookie and sends it back to the other
endpoint. When the other endpoint receives the COOKIE it will send
back a COOKIE-ACK to the attacker and up to HB.Max.Burst HEARTBEATS
to the victim's address(es) (to confirm these addresses). The victim
responds with ABORTs or ICMP messages resulting in the removal of the
TCB at the other endpoint. The attacker can now resend the stored
cookie as long as it is valid and this will again result in up to
HB.Max.Burst HEARTBEATs sent to the victim('s).
6.2. Analysis
The multiplication factor is limited by the number of addresses of
the victim and of the end point HB.Max.Burst. Also the shorter the
cookie life time is, the earlier the attacker has to go through the
initial stage of sending an INIT instead of the just sending the
COOKIE. It should also be noted that the attack is more effective if
large HEARTBEATs are used for path confirmation.
6.3. Mitigation option
To limit the effectiveness of this attack the new parameter
HB.Max.Burst was introduced in [I-D.ietf-tsvwg-2960bis] and an end
point should:
1) not allow very large cookie lifetimes, even if they are requested.
2) not use larger HB.Max.Burst parameter values than recommended.
Note that an endpoint may decide to send only one Heartbeat per
RTT instead of the maximum (i.e. HB.Max.Burst). An endpoint that
chooses this approach will however slow down detection of
endpoints camping on valid addresses.
3) not use large HEARTBEATs for path confirmation.
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7. Association redirection
This attack allows an attacker to wrongly setup an association to a
different endpoint.
7.1. Attack details
The attacker sends an INIT sourced from port 'X' and directed towards
port 'Y'. When the INIT-ACK is returned the attacker sends the
COOKIE-ECHO chunk and either places a different destination or source
port in the SCTP common header, i.e., X+1 or Y+1. This then sets up
the association with possibly other endpoints.
7.2. Analysis
This attack depends on the failure of an SCTP implementation to store
and verify the ports within the COOKIE structure.
7.3. Mitigation option
This attack is easily defeated by an implementation including the
ports of both the source and destination within the COOKIE. When the
COOKIE is returned if the source and destination ports do not match
those within the COOKIE chunk, the SCTP implementation silently
discards the invalid COOKIE.
8. Bombing attack (amplification) 3
This attack allows an attacker to use an SCTP endpoint to send a
large number of packets in response to one packet.
8.1. Attack details
The attacker sends a packet to an SCTP endpoint which requires the
sending of multiple chunks. If the SCTP endpoint does not support
bundling on the sending side it might send each chunk per packet.
These packets can either be sent to a victim by using the victim's
address as the sources address or it can be considered an attack
against the network. Since the chunks which need to be send in
response to the received packet may not fit into one packet an
endpoint supporting bundling on the sending side might send multiple
packets.
Examples of these packets are packets containing a lot of unknown
chunks which require an ERROR chunk to be sent, known chunks which
initiate the sending of ERROR chunks, packets containing a lot of
HEARTBEAT chunks and so on.
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8.2. Analysis
This attack depends on the fact that the SCTP endpoint does not
support bundling on the sending side or provides a bad implementation
of bundling on the sending side.
8.3. Mitigation option
First of all, path verification must happen before sending other
chunks than HEARTBEATs for path verification. This makes sure that
the above attack can not be used against other hosts. To avoid the
attack, an SCTP endpoint should implement bundling on the sending
side and should not send multiple packets in response. If the SCTP
endpoint does not support bundling on the sending side it should not
send in general more than one packet in response to a received one.
The details of the required handling are described in the
[I-D.ietf-tsvwg-2960bis].
9. Bombing attack (amplification) 4
This attack allows an attacker to use an SCTP server to send a larger
packets to a victim than it sent to the SCTP server.
9.1. Attack details
The attacker sends packets using the victim's address as the source
address containing an INIT chunk to an SCTP Server. The server then
sends an packet containing an INIT-ACK chunk to the victim, which is
most likely larger than the packet containing the INIT.
9.2. Analysis
This attack is a byte and not a packet amplification attack and
without protocol changes hard to avoid. A possible method would be
the usage of the PAD parameter defined in [RFC4820].
9.3. Mitigation option
A server should be implemented in a way that the generated INIT-ACK
chunks are as small as possible.
10. Bombing attack (amplification) 5
This attack allows an attacker to use an SCTP endpoint to send a
large number of packets in response to one packet.
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10.1. Attack details
The attacker sends a packet to an SCTP endpoint which requires the
sending of multiple chunks. If the MTU towards the attacker is
smaller than the MTU towards the victim, the victim might need to
send more than one packet to send all the chunks. The difference
between the MTUs might be extremely large if the attacker sends
malicious ICMP packets to make use of the path MTU discovery.
10.2. Analysis
This attack depends on the fact that an SCTP implementation might not
not limit the number of response packets correctly.
10.3. Mitigation option
First of all, path verification must happen before sending other
chunks than HEARTBEATs for path verification. This makes sure that
the above attack can not be used against other hosts. To avoid the
attack, an SCTP endpoint should not send multiple packets in response
to a single packet. The chunks not fitting in this packet should be
dropped.
11. Security Considerations
This document is about security and there is nothing to be added to
it in this section.
12. IANA considerations
There are no actions required from IANA.
13. References
13.1. Normative References
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC4460] Stewart, R., Arias-Rodriguez, I., Poon, K., Caro, A., and
M. Tuexen, "Stream Control Transmission Protocol (SCTP)
Specification Errata and Issues", RFC 4460, April 2006.
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[I-D.ietf-tsvwg-2960bis]
Stewart, R., "Stream Control Transmission Protocol",
draft-ietf-tsvwg-2960bis-05 (work in progress), June 2007.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, March 2007.
[I-D.ietf-tsvwg-sctp-auth]
Tuexen, M., "Authenticated Chunks for Stream Control
Transmission Protocol (SCTP)",
draft-ietf-tsvwg-sctp-auth-08 (work in progress),
February 2007.
[I-D.ietf-tsvwg-addip-sctp]
Stewart, R., "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration",
draft-ietf-tsvwg-addip-sctp-21 (work in progress),
June 2007.
13.2. Informative References
[effects] Aura, T., Nikander, P., and G. Camarillo, "Effects of
Mobility and Multihoming on Transport-Layer Security",
Security and Privacy 2004, IEEE Symposium , URL http://
research.microsoft.com/users/tuomaura/Publications/
aura-nikander-camarillo-ssp04.pdf, May 2004.
Authors' Addresses
Randall R. Stewart
Cisco Systems, Inc.
4785 Forest Drive
Suite 200
Columbia, SC 29206
USA
Email: rrs@cisco.com
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Michael Tuexen
Muenster Univ. of Applied Sciences
Stegerwaldstr. 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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