Network Working Group R. Stewart
Internet-Draft Cisco Systems, Inc.
Expires: July 22, 2006 M. Tuexen
Muenster Univ. of Applied Sciences
G. Camarillo
Ericsson
January 18, 2006
Stream Control Transmission Protocol (SCTP) Security Threats
draft-ietf-tsvwg-sctpthreat-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
Stream Control Transmission Protocol RFC2960 [2] is a multi-homed
transport protocol. As such, unique security threats exists that are
addressed in various ways within the protocol itself. This document
attempts to detail the known security threats and there
countermeasures as detailed in the current version of the SCTP
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Implementors guide (SCTP-IG). It is hoped that this information will
provide some useful background information for many of the newest
requirements spelled out in the SCTP-IG.
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 . . . . . . . . . . . . . . . 7
6. Bombing attack (amplification) 2 . . . . . . . . . . . . . . . 9
7. Association redirection . . . . . . . . . . . . . . . . . . . 10
8. Bombing attack (amplification) 3 . . . . . . . . . . . . . . . 10
9. Bombing attack (amplification) 4 . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
Stream Control Transmission Protocol RFC2960 [2] is a multi-homed
transport protocol. As such, unique security threats exists that are
addressed in various ways within the protocol itself. This document
attempts to detail the known security threats and there
countermeasures as detailed in the current version of the SCTP
Implementors guide (SCTP-IG). It is hoped that this information will
provide some useful background information for many of the newest
requirements spelled out in the SCTP-IG.
This work and some of the changes that went into the tenth version of
the SCTP-IG are much indebted to the paper on potential SCTP security
risks Effects [1] by Aura, Nikander and Camarillo. Without there
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 [1] 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-X & Y=+-----------+=IP-C & D |
| Attacker | | | | Victim |
+----------+ +----------+ +----------+
Figure 1: Camping
Consider the scenario illustrated in Figure 1. The attacker
legitimately holds IP-A and wishes to prevent the 'Client-Victim'
from communication with the 'Server'. Note also that both the client
and server are multi-homed. The attacker first guesses the port
number our client uses in its association attempt. It then uses this
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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.
2.2. Errata
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
SCTP-IG. In closely 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 in RFC2960 [2] 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 restart the attackers association
destroying it.
2.3. Counter measure
Version 10 of the SCTP-IG 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
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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-ACK's 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 ADD-IP extension to SCTP a 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.
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
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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.
3.2. Errata
This attack depends on a number of events:
1) Both endpoints must support the ADD-IP extension.
2) One of the endpoints must be using the ADD-IP 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. Counter measure
The latest SCTP-IG 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 set of two 32 bit nonce 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 ADD-IP extensions requires the use of the
authentication mechanism defined in SCTP-AUTH [3]. 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
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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
take over an IP-address he 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 has taken over the
IP-address A he waits for a packet from E2. After reception of the
packet the attacker can perform a full four way handshake using 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. Errata
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. Counter measure
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. Note that other IP protocols may
also be effected by this attack.
5. Bombing attack (amplification) 1
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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
victim.
5.2. Errata
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. Counter measure
The current SCTP-IG 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 the newest SCTP-IG 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
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other then the attacker itself.
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.
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 Max.Burst HEARTBEATS to
the victim('s) (to confirm 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 Max.Burst
HEARTBEATs sent to the victim('s).
6.2. Errata
The multiplication factor is limited by the number of addresses of
the victim and of the end point 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. Counter measure
To limit the effectiveness of this attack and end point should
1) not allow very large cookie lifetimes, even if they are requested.
2) not use larger Max.Burst parameter values than recommended or
alternatively only send one CONFIRMATION Heartbeat per RTT. Note
that an endpoint may decide to send only one Heartbeat per RTT
instead of the maximum (i.e. Max.Burst). An endpoint that
chooses this approach will however slow down detection of
endpoints camping on valid addresses.
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3) not use large HEARTBEATs for path confirmation.
7. Association redirection
This attack allows an attacker to mis-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 set's up
the association with possibly other endpoints.
7.2. Errata
This attack depends on the failure of an SCTP implementation to store
and verify the ports within the COOKIE structure.
7.3. Counter measure
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 unkown
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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.
8.2. Errata
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. Counter measure
First of all, path verification must happen before sending other
chunks then 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 IG.
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. Errata
This attack is a byte and not a packet amplification attack and
without protocol changes hard to avoid.
9.3. Counter measure
A server should be implemented in a way that the generated INIT-ACK
chunks are as small as possible.
10. References
[1] Aura, T., Nikander, P., and G. Camarillo, "Effects of Mobility
and Multihoming on Transport-Layer Security", December 2003.
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[2] 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.
[3] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for Stream Control Transmission Protocol
(SCTP)", draft-tuexen-sctp-auth-chunk-03 (work in progress),
February 2005.
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Authors' Addresses
Randall R. Stewart
Cisco Systems, Inc.
4785 Forest Drive
Suite 200
Columbia, SC 29206
USA
Email: rrs@cisco.com
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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