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Versions: (draft-gont-6man-predictable-fragment-id) 00 01 02 03 04 05 06 07 08 09 10 RFC 7739

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Informational                         December 19, 2014
Expires: June 22, 2015


  Security Implications of Predictable Fragment Identification Values
               draft-ietf-6man-predictable-fragment-id-02

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 a 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 employed for any other fragmented
   packet sent recently with the same Source Address and Destination
   Address.  Some implementations use simple a global counter for
   setting the Identification field, thus leading to predictable values.
   This document analyzes the security implications of predictable
   Identification values, and provides implementation guidance for
   selecting the Identification field of the Fragment Header, 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 June 22, 2015.








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Copyright Notice

   Copyright (c) 2014 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
   Provisions Relating to IETF Documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Security Implications of Predictable Fragment Identification
       values  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Constraints for the selection of Fragment Identification
       Values  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Algorithms for Selecting Fragment Identification Values . . .   7
     4.1.  Per-destination counter (initialized to a random value) .   7
     4.2.  Randomized Identification values  . . . . . . . . . . . .   8
     4.3.  Hash-based Fragment Identification selection algorithm  .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Information leakage produced by vulnerable
                implementations  . . . . . . . . . . . . . . . . . .  13
   Appendix B.  Survey of Fragment Identification selection
                algorithms employed by popular IPv6 implementations   15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

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 a 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 employed for any other fragmented



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   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 transmitted.  However,
   this trivial algorithm leads to predictable Identification values,
   which can be leveraged to perform a variety of attacks.

   Section 2 of this document analyzes the security implications of
   predictable Identification values.  Section 3 discusses constraints
   in the possible algorithms for selecting Fragment Identification
   values.  Section 4 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.

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,

   o  perform Denial of Service (DoS) attacks, or,

   o  perform data injection attacks against transport or application
      protocols

   [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] explains how to exploit



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      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).  [Zalewski2003] originally envisioned the exploitation
      of IP fragmentation/reassembly for performing data injection
      attacks against upper-layer protocols.  [Herzberg2013] explores
      the use of IPv4/IPv6 fragmentation and predictable Identification
      values for performing DNS cache poisoning attacks in great detail.
      [RFC6274] covers the security implications of the IPv4 case 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 present only in those packets that carry a Fragment Header.
   As a result, successful exploitation of the IPv6 Fragment
   Identification field depends on two different factors:

   o  vulnerable IPv6 Fragment Identification generators, and,

   o  the ability of an attacker to trigger the use of IPv6
      fragmentation for packets sent from/to the victim node

   As noted in the previous section, some implementations have been
   known to use predictable Fragment Identification values.  For
   instance, Appendix B of this document shows that recent versions of a
   number of popular IPv6 implementations have been employing
   predictable values for the IPv6 Fragment Identification.

   Additionally, we note that [RFC2460] states that when an ICMPv6
   Packet Too Big error message advertising 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.  This triggers the use of the so-called IPv6 "atomic
   fragments" [RFC6946]: IPv6 fragments with a Fragment Offset equal to
   0, and the "M" ("More fragments") bit clear.
   [I-D.ietf-6man-deprecate-atomfrag-generation] aims at deprecating the
   generation of IPv6 atomic fragments.

   Thus, an attacker can usually cause a victim host to "fragment" its
   outgoing packets by sending it a forged ICMPv6 'Packet Too Big' (PTB)
   error message that advertises a Next-Hop MTU smaller than 1280 bytes.

   There are a number of aspects that should be considered, though:



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   o  All the implementations the author is aware of record the Path-MTU
      information on a per-destination basis.  Thus, an attacker can
      only cause the victim to enable fragmentation for those packets
      sent to the Source Address of IPv6 packet embedded in the payload
      of the ICMPv6 PTB message.  However, we note that Section 5.2 of
      [RFC1981] notes that an implementation could maintain a single
      system-wide PMTU value to be used for all packets sent to that
      node.  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.  However, many current
      implementations fail to enforce these validation checks.  For
      example, Linux 2.6.38-8 does not even require received ICMPv6
      error messages to correspond to an ongoing communication instance.

   Implementations that employ predictable Identification values and
   also fail to enforce validation checks on ICMPv6 error messages
   become vulnerable to the same type of attacks that can be exploited
   with IPv4 fragmentation, discussed earlier in this section.

   One possible way in which predictable Identification values could be
   leveraged for performing a Denial of Service (DoS) attack is as
   follows: Let us assume that Host A is communicating with Host B, and
   that an attacker wants to DoS such communication.  The attacker would
   learn the the Identification value currently in use by Host A,
   possibly by sending any packet that would elicit a fragmented
   response (e.g., an ICPMv6 echo request with a large payload).  The
   attacker would then send a forged ICMPv6 Packet Too Big error message
   to Host A (with the IPv6 Destination Address of the embedded IPv6
   packet set to the IPv6 address of a Host B), such that any subsequent
   packets sent by Host A to Host B include a Fragment Header.  Finally,
   the attacker would send forged IPv6 fragments to Host B, with their
   IPv6 Source Address set to that of Host A, and Identification values
   that would result in collisions with the Identification values
   employed for the legitimate traffic sent by Host A to Host B.  If
   Host B discards fragments that result in collisions of Identification
   values (e.g., such fragments overlap, and the host implements
   [RFC5722]), the attacker could simply trash the Identification space
   by sending multiple forged fragments with different Identification
   values, such that any subsequent packets from Host A to Host B are
   discarded at Host B as a result of the malicious fragments sent by
   the attacker.

   NOTES:



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

      [RFC6946] 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 previous attack scenario is simply included to illustrate the
   problem of employing predictable fragment Identification values.  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 attacks.

3.  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.

      NOTE: [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 low-order 16 bits
   of the IPv6 Fragment Identification are of use to the destination
   system.

   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 this requirement could 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
   impact should be considered when choosing an algorithm for the
   selection of Fragment Identification values.




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4.  Algorithms for Selecting Fragment Identification Values

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

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

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

   2.  If such an entry exists, it contains the last Fragment
       Identification value used for that Destination Address.
       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 will be lost.  Thus, subsequent traffic to
      that destination would cause that entry to be re-created and re-




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      initialized to random value, thus possibly leading to Fragment
      Identification "collisions".

   o  Since the Fragment Identification values are predictable by the
      destination host, a vulnerable host might 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 one possible scenario for such leakage
      in detail.

4.2.  Randomized Identification values

   Clearly, use of a Pseudo-Random Number Generator for selecting the
   Fragment Identification would 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 that 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.

4.3.  Hash-based Fragment Identification selection algorithm

   Another alternative is to implement a hash-based algorithm similar to
   that specified in [RFC6056] 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:

   Identification:



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      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.

   secret2:
      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 the
      computed "index"/argument for counter[].

         NOTE: 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* be lost.  This is an
         improvement over the algorithm specified in Section 4.1.

   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 possible 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 same counter to be incremented).

   This algorithm might be preferable (over the one specified in
   Section 4.1) 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.

      NOTE: In such scenarios, if the algorithm specified in Section 4.1
      were implemented, entries from the Destinations Cache might need
      to be pruned frequently, thus increasing the risk of fragment
      Identification collisions.

5.  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.

6.  Security Considerations

   This document discusses the security implications of predictable
   Fragment Identification values, and provides implementation guidance
   such that the aforementioned security implications can be mitigated.



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   A number of possible algorithms are described, to provide some
   implementation alternatives 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.).

7.  Acknowledgements

   The author would like to thank Ivan Arce for proposing the attack
   scenario described in Appendix A.

   The author would like to thank Ivan Arce, Ron Bonica, Tassos
   Chatzithomaoglou, Brian Haberman, Bob Hinden, Tatuya Jinmei, Merike
   Kaeo, Will Liu, Juan Antonio Matos, Simon Perreault, Hosnieh Rafiee,
   Mark Smith, and 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).

8.  References

8.1.  Normative References

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

   [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.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056, January
              2011.

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

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
              6946, May 2013.




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8.2.  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-deprecate-atomfrag-generation]
              Gont, F., Will, W., and t. tore, "Deprecating the
              Generation of IPv6 Atomic Fragments", draft-ietf-6man-
              deprecate-atomfrag-generation-00 (work in progress),
              November 2014.

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

   [Herzberg2013]
              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", Technical Report 13-03, March 2013,
              <http://u.cs.biu.ac.il/~herzbea/security/13-03-frag.pdf>.

   [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>.



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   [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>.

   [Zalewski2003]
              Zalewski, M., "A new TCP/IP blind data injection
              technique?", Post to Bugtraq mailing-list, Thu, 11 Dec
              2003 00:28:28 +0100 (CET), 2003,
              <http://lcamtuf.coredump.cx/ipfrag.txt>.

Appendix A.  Information leakage produced by vulnerable implementations

   Section 2 provides a number of references describing a number of ways
   in which a vulnerable implementation may reveal the Fragment
   Identification values to be used in subsequent packets, thus opening
   the door to a number of attacks.  In all of those scenarios, a
   vulnerable implementation leaks/reveals its own Identification
   number.

   This section presents a attack scenario, in which a vulnerable
   implementation leaks/reveals the Identification number of a non-
   vulnerable implementation.  That is, a vulnerable implementation
   (Host A) leaks the current Fragment Identification value in use by a
   third-party host (Host B) to send fragmented datagrams from Host B to
   Host A.

      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 non-vulnerable
      implementation.

   The following scenarios assume:

   Host A:
      Is an IPv6 host that implements the the algorithm specified in
      Section 4.1, implements [RFC5722], but does not implement
      [RFC6946].

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

   Host C:



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

   In the following scenarios, large ICMPv6 Echo Request packets are
   employed to "sample" the Fragment Identification value of a host.  We
   note that while the figures show only one packet for the ICMPv6 Echo
   Request and the ICMPv6 Echo Response, each of those packets will
   typically comprise two fragments, such that the corresponding
   unfragmented datagram is larger than the MTU of the networks to which
   Host B and Host C are attached.

   In lines #1-#2 (and lines #8-#9), the attacker samples the current
   Fragment Identification value.  In line #3, the attacker sends a
   forged TCP SYN segment to Host A.  If corresponding TCP port is
   closed, and the attacker 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 --->

   The two RST segments are elicited by the SYN/ACK segment from line
   #4, and the (illegitimately elicited by the SYN in line #3) SYN/ACK
   segment from line #5.  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 -->





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Internet-Draft  Implications of Predictable Fragment IDs   December 2014


   Clearly, the Fragment Identification value sampled 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, even when Host A itself has a non-vulnerable
   implementation.

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].
































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Internet-Draft  Implications of Predictable Fragment IDs   December 2014


   +-----------------------+-------------------------------------------+
   |    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".

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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