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Versions: 00 01 02 03 draft-ietf-6man-predictable-fragment-id

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Updates: 2460 (if approved)                              January 9, 2013
Intended status: Standards Track
Expires: July 13, 2013


  Security Implications of Predictable Fragment Identification Values
               draft-gont-6man-predictable-fragment-id-03

Abstract

   IPv6 specifies the Fragment Header, which is employed for the
   fragmentation and reassembly mechanisms.  The Fragment Header
   contains an "Identification" field which, together with the IPv6
   Source Address and the IPv6 Destination Address of the packet,
   identifies fragments that correspond to the same original datagram,
   such that they can be reassembled together at the receiving host.
   The only requirement for setting the "Identification" value is that
   it must be different than that of any other fragmented packet sent
   recently with the same Source Address and Destination Address.  Some
   implementations simply use a global counter for setting the Fragment
   Identification field, thus leading to predictable values.  This
   document analyzes the security implications of predictable
   Identification values, and updates RFC 2460 specifying additional
   requirements for setting the Fragment Identification, such that the
   aforementioned security implications are mitigated.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may not be modified,
   and derivative works of it may not be created, and it may not be
   published except as an Internet-Draft.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 13, 2013.

Copyright Notice



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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Security Implications of Predictable Fragment
       Identification values  . . . . . . . . . . . . . . . . . . . .  4
   3.  Updating RFC 2460  . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Constraints for the selection of Fragment Identification
       Values . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Algorithms for Selecting Fragment Identification Values  . . . 10
     5.1.  Per-destination counter (initialized to a random value)  . 10
     5.2.  Randomized Identification values . . . . . . . . . . . . . 11
     5.3.  Hash-based Fragment Identification selection algorithm . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Information leakage produced by vulnerable
                implementations . . . . . . . . . . . . . . . . . . . 19
   Appendix B.  Survey of Fragment Identification selection
                algorithms employed by popular IPv6
                implementations . . . . . . . . . . . . . . . . . . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22













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1.  Introduction

   IPv6 specifies the Fragment Header, which is employed for the
   fragmentation and reassembly mechanisms.  The Fragment Header
   contains an "Identification" field which, together with the IPv6
   Source Address and the IPv6 Destination Address of the packet,
   identifies fragments that correspond to the same original datagram,
   such that they can be reassembled together at the receiving host.
   The only requirement for setting the "Identification" value is that
   it must be different than that of any other fragmented packet sent
   recently with the same Source Address and Destination Address.

   The most trivial algorithm to avoid reusing Fragment Identification
   values too quickly is to maintain a global counter that is
   incremented for each fragmented packet that is sent.  However, this
   trivial algorithm leads to predictable Identification values, which
   can be leveraged for performing a variety of attacks.

   Section 2 of this document analyzes the security implications of
   predictable Identification values.  Section 3 updates RFC 2460 by
   adding the requirement that Identification values not be predictable
   by an off-path attacker.  Section 4 discusses constraints in the
   possible algorithms for selecting Fragment Identification values.
   Section 5 specifies a number of algorithms that could be used for
   generating Identification values.  Finally, Appendix B contains a
   survey of the Fragment Identification algorithms employed by popular
   IPv6 implementations.

   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 RFC 2119 [RFC2119].




















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2.  Security Implications of Predictable Fragment Identification values

   Predictable Identification values result in an information leakage
   that can be exploited in a number of ways.  Among others, they may
   potentially be exploited to:

   o  determine the packet rate at which a given system is transmitting
      information,

   o  perform stealth port scans to a third-party,

   o  uncover the rules of a number of firewalls,

   o  count the number of systems behind a middle-box, or,

   o  perform a Denial of Service (DoS) attack

   [CPNI-IPv6] contains a detailed analysis of possible vulnerabilities
   introduced by predictable Fragment Identification values.  In
   summary, their security implications are very similar to those of
   predictable Identification values in IPv4.

      [Sanfilippo1998a] originally pointed out how the IPv4
      Identification field could be examined to determine the packet
      rate at which a given system is transmitting information.  Later,
      [Sanfilippo1998b] described how a system with such an
      implementation could be used to perform a stealth port scan to a
      third (victim) host.  [Sanfilippo1999] explained how to exploit
      this implementation strategy to uncover the rules of a number of
      firewalls.  [Bellovin2002] explains how the IPv4 Identification
      field can be exploited to count the number of systems behind a
      NAT.  [Fyodor2004] is an entire paper on most (if not all) the
      ways to exploit the information provided by the Identification
      field of the IPv4 header (and these results apply in a similar way
      to IPv6).  [RFC6274] covers the security implications of IPv4 in
      detail.

   One key difference between the IPv4 case and the IPv6 case is that in
   IPv4 the Identification field is part of the fixed IPv4 header (and
   thus usually set for all packets), while in IPv6 the Identification
   field is set only in those packets that employ a Fragment Header.  As
   a result, successful exploitation of the Identification field against
   communication instances with arbitrary destinations depends on two
   different factors:

   o  IPv6 implementations using predictable Identification values, and,





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   o  the ability of the attacker to cause the victim host to fragment
      packets destined to other nodes

   As noted in the previous section, some implementations are known to
   use predictable identification values.

      For example, Linux 2.6.38-8 sets the Identification field
      according to a global counter that is incremented by one for each
      datagram that is sent with a fragment header (either a single
      fragment or as multiple fragments).

   Finally, we note that an attacker could cause a victim host to
   fragment its outgoing packets by sending it a forged ICMPv6 'Packet
   Too Big' error message advertising a Next-Hop MTU smaller than 1280
   bytes.

      RFC 1981 [RFC1981] states that when an ICMPv6 Packet Too Big error
      message with an MTU smaller than 1280 bytes is received, the
      receiving host is not required to reduce the Path-MTU for the
      corresponding destination address, but must simply include a
      Fragment Header in all subsequent packets sent to that
      destination.  In order to make sure that the forged ICMPv6 Packet
      Too Big error message triggers fragmentation at the victim host,
      the attacker could set the MTU field of the error message to a
      value smaller than 1280 bytes.  Since the minimum IPv6 MTU is 1280
      bytes, such value would always be smaller than the Path-MTU in use
      for that destination.

   There are a few issues that should be considered, though:

   o  In all the implementations the author is aware of, an attacker can
      only cause the victim to enable fragmentation on a per-destination
      basis.  That is, the victim will use fragmentation only for those
      packets sent to the Source Address of IPv6 packet embedded in the
      payload of the ICMPv6 Packet Too Big error message.

         Section 5.2 of [RFC1981] notes that an implementation could
         maintain a single system-wide PMTU value to be used for all
         packets originating from that nodes.  Clearly, such an
         implementations would exacerbate the problem of any attacks
         based on PMTUD [RFC5927] or IPv6 fragmentation.

   o  If the victim node implements some of the counter-measures for
      ICMP attacks described in RFC 5927 [RFC5927], it might be
      difficult for an attacker to cause the victim node to use
      fragmentation for its outgoing packets.





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         Some implementations do not incorporate countermeasures for
         attacks based on ICMPv6 error messages.  For example, Linux
         2.6.38-8 does not even require received ICMPv6 error messages
         to correspond to ongoing communication instances.

   Implementations that employ predictable Identification values and
   also fail to include countermeasures against attacks based on ICMPv6
   error messages will be vulnerable to attacks similar to those based
   on the IPv4 Identification field for IPv4 networks, such as the
   stealth port-scanning technique described in [Sanfilippo1998b].

   One possible way in which predictable Identification values could be
   leveraged for performing a Denial of Service (DoS) attack is as
   follows: once the Identification value currently in use at the victim
   host has been learned, the attacker would send a forged ICMPv6 Packet
   Too Big error message to the victim host, with the IPv6 Destination
   Address of the embedded IPv6 packet set to the IPv6 address of a
   third-party host with which the victim is communicating.  This ICMPv6
   Packet Too Big error message would cause any packets sent from the
   victim to the third-party host to include a Fragment Header.  The
   attacker would then send forged IPv6 fragments to the third-party
   host, with their IPv6 Source Address set to that of the victim host,
   and with the Identification field of the forged fragments set to
   values that would result in collisions at the third-party host.  If
   the third-party host discards fragments that result in collisions of
   Identification values, the attacker could simply trash the
   Identification space by sending multiple forged fragments with
   different Identification values, such that any subsequent packets
   from the victim host are discarded at the third-party host as a
   result of the malicious fragments sent by the attacker.

      For example, Linux 2.6.38-10 is vulnerable to the aforementioned
      issue.

      [I-D.ietf-6man-ipv6-atomic-fragments] describes an improved
      processing of these packets that would eliminate this specific
      attack vector, at least in the case of TCP connections that employ
      the Path-MTU Discovery mechanism.

   The aforementioned attack scenario is simply included to illustrate
   the problem of employing predictable fragment Identification values,
   rather than to indicate a specific attack vector that needs to be
   mitigated.

   We note that regardless of the attacker's ability to cause a victim
   host to employ fragmentation when communicating with third-parties,
   use of predictable Identification values makes communication flows
   that employ fragmentation vulnerable to any fragmentation-based



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   attacks.


















































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3.  Updating RFC 2460

   Hereby we update RFC 2460 [RFC2460] as follows:

   The Identification value of the Fragment Header MUST NOT be
   predictable by an off-path attacker.













































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4.  Constraints for the selection of Fragment Identification Values

   the "Identification" field of the Fragmentation Header is 32-bits
   long.  However, when translators [RFC6145] are employed, the
   "effective" length of the IPv6 Fragment Identification field is 16
   bits.

      [RFC6145] notes that, when translating in the IPv6-to-IPv4
      direction, "if there is a Fragment Header in the IPv6 packet, the
      last 16 bits of its value MUST be used for the IPv4 identification
      value".  This means that the high-order 16 bits are effectively
      ignored.

   As a result, at least during the IPv6/IPv4 transition/co-existence
   phase, it is probably safer to assume that only the last 16 bits of
   the IPv6 Fragment Identification may be used in some cases.

   Regarding the selection of Fragment Identification values, the only
   requirement specified in [RFC2460] is that the Fragment
   Identification must be different than that of any other fragmented
   packet sent recently with the same Source Address and Destination
   Address.

      Failure to comply with that requirement might lead to the
      interoperability problems discussed in [RFC4963].

   From a security standpoint, unpredictable Identification values are
   desirable.  However, this is somewhat at odds with the "re-use"
   requirements specified in [RFC2460].

   Finally, since Fragment Identification values need to be selected for
   each outgoing datagram that requires fragmentation, the performance
   aspect should be considered when choosing an algorithm for the
   selection of Fragment Identification values.

















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5.  Algorithms for Selecting Fragment Identification Values

   This section specifies a number of algorithms that MAY be used for
   selecting Fragment Identification values.

5.1.  Per-destination counter (initialized to a random value)

   1.  Whenever a packet must be sent with a Fragment Header, the
       sending host should perform a look-up in the Destinations Cache
       an entry corresponding to the intended Destination Address.

   2.  If such an entry exists, it contains the last Fragment
       Identification value used for that Destination.  Therefore, such
       value should be incremented by 1, and used for setting the
       Fragment Identification value of the outgoing packet.
       Additionally, the updated value should be recorded in the
       corresponding entry of the Destination Cache.

   3.  If such an entry does not exist, it should be created, and the
       "Identification" value for that destination should be initialized
       with a random value (e.g., with a pseudorandom number generator),
       and used for setting the Identification field of the Fragment
       Header of the outgoing packet.

   The advantages of this algorithm are:

   o  It is simple to implement, with the only complexity residing in
      the Pseudo-Random Number Generator (PRNG) used to initialize the
      "Identification" value contained in each entry of the Destinations
      Cache.

   o  The "Identification" re-use frequency will typically be lower than
      that achieved by a global counter (when sending traffic to
      multiple destinations), since this algorithm uses per-destination
      counters (rather than a single system-wide counter).

   o  It has good performance properties (once the corresponding entry
      in the Destinations Cache has been created, each subsequent
      "Identification" value simply involves the increment of a
      counter).

   The possible drawbacks of this algorithm are:

   o  If as a result of resource management an entry of the Destinations
      Cache must be removed, the last Fragment Identification value used
      for that Destination is obviously lost.  Thus, if subsequent
      traffic to that destination causes the aforementioned entry to be
      re-created, the Fragment Identification value will be randomized,



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      thus possibly leading to Fragment Identification "collisions".

   o  Since the Fragment Identification values are predictable by the
      destination host, a vulnerable host might possible leak to third-
      parties the Fragment Identification values used by other hosts to
      send traffic to it (i.e., Host B could leak to Host C the Fragment
      Identification values that Host A is using to send packets to Host
      B).

         Appendix A describes a scenario in which that information
         leakage could take place.

5.2.  Randomized Identification values

   Clearly, use of a Pseudo-Random Number Generator for selecting the
   Fragment Identification could be desirable from a security
   standpoint.  With such a scheme, the Fragment Identification of each
   fragmented datagram would be selected as:

                         Identification = random()

   where "random()" is the PRNG.

   The specific properties of such scheme would clearly depend on the
   specific PRNG algorithm used.  For example, some PRNGs may result in
   higher Fragment Identification reuse frequencies than others, in the
   same way as some PRNGs may be more expensive (in terms of processing
   requirements and/or implementation complexity) than others.

   Discussion of the properties of possible PRNGs is considered out of
   the scope of this document.  However, we do note that some PRNGs
   employed in the past by some implementations have been found to be
   predictable [Klein2007].  Please see [RFC4086] for randomness
   requirements for security.

5.3.  Hash-based Fragment Identification selection algorithm

   Another alternative is to implement a hash-based algorithm similar to
   that specified in for the selection of transport port numbers.  With
   such a scheme, the Fragment Identification value of each fragment
   datagram would be selected with the expression:

           Identification = F(Src IP, Dst IP, secret1)  +
                            counter[G(src IP, Dst Pref, secret2)]

   where:





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   Identification:
      Identification value to be used for the fragmented datagram

   F():
      Hash function

   Src IP:
      IPv6 Source Address of the datagram to be fragmented

   Dst IP:
      IPv6 Destination Address of the datagram to be fragmented

   secret1:
      Secret data unknown to the attacker

   counter[]:
      System-wide array of 32-bit counters (e.g. with 8K elements or
      more)

   G():
      Hash function.  May or may not be the same hash function as that
      used for F()

   Dst Pref:
      IPv6 "Destination Prefix" of datagram to be fragmented (can be
      assumed to be the first eight bytes of the Destination Address of
      such packet).  Note: the "Destination Prefix" (rather than
      Destination Address) is used, such that the ability of an attacker
      of searching the "increments" space by using multiple addresses of
      the same subnet is reduced.

   secret1:
      Secret data unknown to the attacker

   Note: counter[G(src IP, Dst Pref, secret2)] should be incremented by
   one each time an Identification value is selected.

   The advantages of this algorithm are:

   o  The "Identification" re-use frequency will typically be lower than
      that achieved by a global counter (when sending traffic to
      multiple destinations), since this algorithm uses multiple system-
      wide counters (rather than a single system-wide counter).  The
      extent to which the re-use frequency will be lower will depend on
      the number of elements in counter[], and the number of other
      active flows that result in the same value of G() (and hence cause
      the same counter to be incremented for each fragmented datagram
      that is sent).



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   o  It is possible to implement the algorithm such that good
      performance is achieved.  For example, the result of F() could be
      stored in the Destinations Cache (such that it need not be
      recomputed for each packet that must be sent) along with computed
      *index* for counter[].

         It should be noted that if this implementation approach is
         followed, and an entry of the Destinations Cache must be
         removed as a result of resource management, the last Fragment
         Identification value used for that Destination will *not* lost.
         This is an improvement over the algorithm specified in
         Section 5.1.

   The possible drawbacks of this algorithm are:

   o  Since the Fragment Identification values are predictable by the
      destination host, a vulnerable host could possibly leak to third-
      parties the Fragment Identification values used by other hosts to
      send traffic to it (i.e., Host B could leak to Host C the Fragment
      Identification values that Host A is using to send packets to Host
      B).

         Appendix A describes a scenario in which that information
         leakage could take place.  We note, however, that this
         algorithm makes the aforementioned attack less reliable for the
         attacker, since each counter could be possibly shared by
         multiple traffic flows (i.e., packets destined to other
         destinations might cause the counter to be incremented).

   This algorithm might be preferable (over the one specified in
   Section 5.1) in those scenarios in which a node is expected to
   communicate with a large number of destinations, and thus it is
   desirable to limit the amount of information to be maintained in
   memory.

      In such scenarios, if the algorithm specified in Section 5.1 were
      implemented, entries from the Destinations Cache might need to be
      pruned frequently, thus increasing the risk of fragment
      Identification collisions.












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6.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.














































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7.  Security Considerations

   This document discusses the security implications of predictable
   Fragment Identification values, and updates RFC 2460 such that
   Fragment Identification values are required to be unpredictable by
   off-path attackers, hence mitigating the aforementioned security
   implications.

   A number of possible algorithms are specified, to provide some
   implementation alternatives to implementers.  However, the selection
   of an specific algorithm that complies with Section 3 is left to
   implementers.  We note that the selection of such an algorithm
   usually implies a number of trade-offs (security, performance,
   implementation complexity, interoperability properties, etc.).





































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8.  Acknowledgements

   The author would like to thank Ivan Arce for proposing the attack
   scenario described in Appendix A, and for providing valuable comments
   on earlier versions of this document.

   The author would like to thank Dave Thaler for providing valuable
   comments on earlier versions of this document.

   This document is based on the technical report "Security Assessment
   of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
   Fernando Gont on behalf of the UK Centre for the Protection of
   National Infrastructure (CPNI).

   Fernando Gont would like to thank the UK CPNI
   (http://www.cpni.gov.uk) for their continued support.



































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9.  References

9.1.  Normative References

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, December 2009.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

9.2.  Informative References

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol
              Version 4", RFC 6274, July 2011.

   [I-D.ietf-6man-ipv6-atomic-fragments]
              Gont, F., "Processing of IPv6 "atomic" fragments",
              draft-ietf-6man-ipv6-atomic-fragments-03 (work in
              progress), December 2012.

   [Bellovin2002]
              Bellovin, S., "A Technique for Counting NATted Hosts",
              IMW'02 Nov. 6-8, 2002, Marseille, France, 2002.

   [CPNI-IPv6]
              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

   [Fyodor2004]
              Fyodor, "Idle scanning and related IP ID games", 2004,



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              <http://www.insecure.org/nmap/idlescan.html>.

   [Klein2007]
              Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
              Predictable IP ID Vulnerability", 2007, <http://
              www.trusteer.com/files/
              OpenBSD_DNS_Cache_Poisoning_and_Multiple_OS_Predictable_IP
              _ID_Vulnerability.pdf>.

   [Sanfilippo1998a]
              Sanfilippo, S., "about the ip header id", Post to Bugtraq
              mailing-list, Mon Dec 14 1998,
              <http://www.kyuzz.org/antirez/papers/ipid.html>.

   [Sanfilippo1998b]
              Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list,
              1998, <http://www.kyuzz.org/antirez/papers/dumbscan.html>.

   [Sanfilippo1999]
              Sanfilippo, S., "more ip id", Post to Bugtraq mailing-
              list, 1999,
              <http://www.kyuzz.org/antirez/papers/moreipid.html>.

   [SI6-IPv6]
              "SI6 Networks' IPv6 toolkit",
              <http://www.si6networks.com/tools/ipv6toolkit>.

























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Appendix A.  Information leakage produced by vulnerable implementations

   Section 2 provides a number of references describing a number of ways
   in which the information leakage produced by a vulnerable
   implementation could be leveraged by an attacker.  This section
   describes a specific network scenario in which a vulnerable
   implementation could possibly leak the current Fragment
   Identification value in use by a third-party host to send fragmented
   datagrams to the vulnerable implementation.

      For the most part, this section is included to illustrate how a
      vulnerable implementation might be leveraged to leak-out the
      Fragment Identification value of an otherwise secure
      implementation.  This section might be removed in future revisions
      of this document.

   The following scenarios assume:

   A:
      Is an IPv6 host that implements the recommended Fragment
      Identification algorithm (Section 5.1), implements [RFC5722], but
      does not implement [I-D.ietf-6man-ipv6-atomic-fragments].

   B:
      Victim node.  Selected the Fragment Identification values from a
      global counter.

   C:
      Attacker.  Can forge the IPv6 Source Address of his packets at
      will.

   If the attacker sends forged SYN packets to a closed TCP port, and
   then fails when trying to produce a collision of Fragment
   Identifications (see line #4), the following packet exchange might
   take place:

          A                         B                               C

      #1                              <------ Echo Req #1 ----------
      #2                              --- Echo Resp #1, FID=5000 --->
      #3  <------------------- SYN #1, src= B -----------------------
      #4                              <---- SYN/ACK, FID=42 src = A---
      #5  ---- SYN/ACK, FID=9000 --->
      #6  <---- RST, FID= 5001 -----
      #7  <---- RST, FID= 5002 -----
      #8                             <-------- Echo Req #2 ----------
      #9                             ---- Echo Resp #2, FID= 5003 -->




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   On the other hand, if the attacker succeeds to produce a collision of
   Fragment Identification values, the following packet exchange could
   take place:

          A                           B                             C

      #1                              <------- Echo Req #1 ----------
      #2                              ---- Echo Resp #1, FID=5000 --->
      #3  <------------------- SYN #1, src= B -----------------------
      #4                              <-- SYN/ACK, FID=9000 src=A ---
      #5  ---- SYN/ACK, FID=9000 --->
                              ... (RFC5722) ...
      #6                             <-------- Echo Req #2 ----------
      #7                             ---- Echo Resp #2, FID= 5001 -->

   Clearly, the Fragment Identification value sampled by from the second
   ICMPv6 Echo Response packet ("Echo Resp #2") implicitly indicates
   whether the Fragment Identification in the forged SYN/ACK (see line
   #4 in both figures) was the current Fragment Identification in use by
   Host A.

   As a result, the attacker could employ this technique to learn the
   current Fragment Identification value used by host A to send packets
   to host B.



























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Appendix B.  Survey of Fragment Identification selection algorithms
             employed by popular IPv6 implementations

   This section includes a survey of the Fragment Identification
   selection algorithms employed in some popular operating systems.

      The survey was produced with the SI6 Networks IPv6 toolkit
      [SI6-IPv6].

   +-----------------------+-------------------------------------------+
   |    Operating System   |                 Algorithm                 |
   +-----------------------+-------------------------------------------+
   |      FreeBSD 9.0      |           Unpredictable (Random)          |
   +-----------------------+-------------------------------------------+
   |     Linux 3.0.0-15    |    Predictable (Global Counter, Init=0,   |
   |                       |                  Incr=1)                  |
   +-----------------------+-------------------------------------------+
   |     Linux-current     |      Unpredictable (Per-dest Counter,     |
   |                       |            Init=random, Incr=1)           |
   +-----------------------+-------------------------------------------+
   |       NetBSD 5.1      |           Unpredictable (Random)          |
   +-----------------------+-------------------------------------------+
   |    OpenBSD-current    |              Random (SKIP32)              |
   +-----------------------+-------------------------------------------+
   |       Solaris 10      |   Predictable (Per-dst Counter, Init=0,   |
   |                       |                  Incr=1)                  |
   +-----------------------+-------------------------------------------+
   |     Windows XP SP2    |    Predictable (Global Counter, Init=0,   |
   |                       |                  Incr=2)                  |
   +-----------------------+-------------------------------------------+
   |  Windows Vista (Build |    Predictable (Global Counter, Init=0,   |
   |         6000)         |                  Incr=2)                  |
   +-----------------------+-------------------------------------------+
   |     Windows 7 Home    |    Predictable (Global Counter, Init=0,   |
   |        Premium        |                  Incr=2)                  |
   +-----------------------+-------------------------------------------+

     Table 1: Fragment Identification algorithms employed by different
                                   OSes

      In the text above, "predictable" should be taken as "easily
      guessable by an off-path attacker, by sending a few probe
      packets".








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Author's Address

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com








































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