





Network Working Group                                         R. Stewart
Request for Comments: 5062                           Cisco Systems, Inc.
Category: Informational                                        M. Tuexen
                                      Muenster Univ. of Applied Sciences
                                                            G. Camarillo
                                                                Ericsson
                                                          September 2007


                    Security Attacks Found Against
            the Stream Control Transmission Protocol (SCTP)
                      and Current Countermeasures

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.

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
   background information for many of the newest requirements spelled
   out in the SCTP Specification Errata and Issues and included in RFC
   4960.






















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Address Camping or Stealing  . . . . . . . . . . . . . . . . .  2
   3.  Association Hijacking 1  . . . . . . . . . . . . . . . . . . .  3
   4.  Association Hijacking 2  . . . . . . . . . . . . . . . . . . .  6
   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. Bombing Attack (amplification) 5 . . . . . . . . . . . . . . . 11
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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 [RFC4960],
   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 [RFC4960].

   This work and some of the changes that went into [RFC4460] and
   [RFC4960] are much indebted to the paper on potential SCTP security
   risks [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] and detail the preventative
   measures now in place, if any, within the current SCTP standards.

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 peer's address.  This
   is done to prevent the legitimate peer from communicating with the
   server.






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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
   legitimately holds IP-A and wishes to prevent the 'Client-Victim'
   from communicating 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
   in its initial INIT chunk.  The server will respond and set up the
   association, noting that the attacker is multi-homed and holds 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 set up 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
   [RFC4960].  In close examination, this attack depends on a number of
   specific things to occur.

   1) The attacker must set up 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.










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   2) SCTP's existing HEARTBEAT mechanism as defined already in
      [RFC2960] will eventually catch this situation and abort the evil
      attacker's 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.

2.3.  Mitigation Option

   [RFC4960] 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 victim's source address(es).  In addition, clients that do not
   need to use a specific port number should choose their port numbers
   on a random basis.  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 SCTP extension specified in
   [RFC5061], an endpoint that is NOT a man-in-the-middle may be able to
   assume another endpoint's association.








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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, DHCP may be used in a mobile network 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

   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 obtains
   an address via DHCP and it 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 that 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.







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3.2.  Analysis

   This attack depends on a number of events:

   1) Both endpoints must support the SCTP extension specified in
      [RFC5061].

   2) One of the endpoints must be using the SCTP extension for mobility
      specified in [RFC5061].

   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 receive 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 alternate address, aside from the
      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

   [RFC4960] 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 SCTP extension specified in [RFC5061]
   requires the use of the authentication mechanism defined in
   [RFC4895].  This requires the attacker to be able to capture the
   traffic during the association setup.  If in addition an endpoint-
   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 a 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.





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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 to destination address A and send packets with source address
   A, the attacker can perform a full four-way handshake using the IP-
   addresses and port numbers from the received packet.  E2 will
   consider this 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 the association just restarted.  From this
   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 endpoint's 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 [RFC4960].  Note that other IP protocols may also be
   affected 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.



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   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.

   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 "out of the blue" (OOTB) abort.

   2) The attacker must time its sending of acknowledgments correctly in
      order to get its address into the failed state and the victim's
      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 is no longer seeing.

5.3.  Mitigation Option

   [RFC4960] makes two changes to prevent this attack.  First, it
   details 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 victim's address.

   The second change made in [RFC4960] 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.







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   An SCTP implementation should abort the association if it receives a
   SACK acknowledging a TSN that 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 requests a long cookie lifetime.  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 endpoint HB.Max.Burst.  Also, the shorter the
   cookie lifetime, the earlier the attacker has to go through the
   initial stage of sending an INIT instead of 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 [RFC4960] and an endpoint 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 set up 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 possibly sets
   up the association using the modified port numbers.

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.  If the
   source and destination ports do not match those within the COOKIE
   chunk when the COOKIE is returned, 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 sent 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 that require an ERROR chunk to be sent, known chunks that
   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 chunks
   other than HEARTBEATs for path verification.  This ensures that the
   above attack cannot 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 [RFC4960].

9.  Bombing Attack (Amplification) 4

   This attack allows an attacker to use an SCTP server to send a larger
   packet 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 a 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, is hard to avoid.  A possible method to
   avoid this attack would be the usage 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
   limit the number of response packets correctly.

10.3.  Mitigation Option

   First of all, path verification must happen before sending chunks
   other than HEARTBEATs for path verification.  This makes sure that
   the above attack cannot 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; as such, there are no additional
   security considerations.

12.  References

12.1.  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.

   [RFC4820]  Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
              Parameter for the Stream Control Transmission Protocol
              (SCTP)", RFC 4820, March 2007.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.



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   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, June 2007.

12.2.  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


   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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