Internet Engineering Task Force (IETF) F. Gont

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 7739 Huawei Technologies
Category: Informational February 2016
ISSN: 2070-1721

Security Implications of Predictable Fragment Identification Values

Abstract

IPv6 specifies the Fragment Header, which is employed for the
fragmentation and reassembly mechanisms. The Fragment Header
contains an "Identification" field that, 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 by the receiving host.
The only requirement for setting the Identification field is that the
corresponding value must be different than that employed for any
other fragmented datagram sent recently with the same Source Address
and Destination Address. Some implementations use a simple global
counter for setting the Identification field, thus leading to
predictable Identification values. This document analyzes the
security implications of predictable Identification values, and
provides implementation guidance for setting the Identification field
of the Fragment Header, such that the aforementioned security
implications are mitigated.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7739.

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Copyright Notice

Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Implications of Predictable Fragment Identification
Values . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Constraints for the Selection of Fragment Identification
Values . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Algorithms for Selecting Fragment Identification Values . . . 8
5.1. Per-Destination Counter (Initialized to a Random Value) . 8
5.2. Randomized Identification Values . . . . . . . . . . . . 9
5.3. Hash-Based Fragment Identification Selection Algorithm . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Normative References . . . . . . . . . . . . . . . . . . 13
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Information Leakage Produced by Vulnerable
Implementations . . . . . . . . . . . . . . . . . . 16
Appendix B. Survey of Fragment Identification Selection
Algorithms Employed by Popular IPv6 Implementations 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20

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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 that, 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 by the receiving host.
The only requirement for setting the Identification field is that its
value must be different than that employed for any other fragmented
datagram sent recently with the same Source Address and Destination
Address.

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

Section 3 of this document analyzes the security implications of
predictable Identification values. Section 4 discusses constraints
in the possible algorithms for selecting Identification values.
Section 5 specifies a number of algorithms that could be used for
generating Identification values that mitigate the issues discussed
in this document. Finally, Appendix B contains a survey of the
algorithms employed by popular IPv6 implementations for generating
the Identification values.

2. Terminology

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].

3. 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

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

o perform data injection attacks against transport or application
protocols

The security implications introduced by predictable Identification
values in IPv6 are very similar to those of predictable
Identification values in IPv4.

NOTE:
[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] explained how the IPv4 Identification
field could 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
Identification field depends on two different factors:

o vulnerable Identification generators, and

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

The scenarios in which an attacker may successfully perform the
aforementioned attacks depend on the specific attack type. For
example, in order to perform a DoS attack on communications between
two hosts, an attacker would need to know the IPv6 addresses employed
by the aforementioned two nodes. Such knowledge may be readily
available if the target of the attack is the communication between

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two specific BGP peers, two specific SMTP servers, or one specific
primary DNS server and one of its secondary DNS servers, but may not
be easily available if the goal is a DoS attack on all communications
between arbitrary IPv6 hosts (e.g., the goal is to perform a DoS
attack on all communications involving one specific node with
arbitrary/unknown hosts). Other attacks, such as performing stealth
port scans to a third party or determining the packet rate at which a
given system is transmitting information, only require the attacker
to know the IPv6 address of a vulnerable implementation.

As noted in Section 1, some implementations have been known to use
predictable Identification values. For instance, Appendix B of this
document shows that recent versions of a number of popular IPv6
implementations employ predictable values for the Identification
field of the Fragment Header.

Additionally, we note that [RFC2460] states that when an ICMPv6
Packet Too Big (PTB) error message advertising a Maximum Transfer
Unit (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.
[DEPGEN] documents the motivation of deprecating the generation of
IPv6 atomic fragments in [RFC2460].

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 an MTU smaller than 1280 bytes.

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

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 Path MTU (PMTU) value to be used for all packets sent
to that node. Clearly, such implementations would exacerbate the
problem of any attacks based on Path MTU Discovery (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 employ
fragmentation for its outgoing packets. However, many current

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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.

o Some implementations (notably Linux) have already been updated
according to [DEPGEN] such that ICMPv6 PTB messages do not result
in the generation of IPv6 atomic fragments.

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 DoS attack is as follows: Let us assume
that Host A is communicating with Host B, and that an attacker wants
to perform a DoS attack such communication. The attacker would learn
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 PTB error message to Host A (with the IPv6
Source Address of the embedded IPv6 packet set to the IPv6 address of
Host A, and the 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.

NOTE:
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 aforementioned attack scenario is simply included to illustrate
the problem of employing predictable Identification values. We note
that regardless of the attacker's ability to cause a victim host to

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employ fragmentation when communicating with third parties, use of
predictable Identification values makes communication flows that
employ fragmentation vulnerable to any fragmentation-based attacks.

4. Constraints for the Selection of Fragment Identification Values

The Identification field of the Fragment Header is 32-bits long.
However, when translators (e.g. [RFC6145]) are employed, the high-
order 16 bits of the Identification field are effectively ignored.

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".

Additionally, Section 3.3 of [RFC6052] encourages operators to use
a Network-Specific Prefix (NSP) that maps the IPv4 address space
into IPv6. Thus, when an NSP is being used, IPv6 addresses
representing IPv4 nodes (reached through a stateless translator)
are indistinguishable from native IPv6 addresses.

Thus, when translators are employed, the "effective" length of the
Identification field is 16 bits and, 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 Identification field
are of use to the destination system.

Regarding the selection of Identification values, the only
requirement specified in [RFC2460] is that the Identification value
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 "reuse"
requirements specified in [RFC2460], that specifies that an
Identification value must be different than that employed for any
other fragmented packet sent recently with the same Source Address
and Destination Address.

Finally, since 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
Identification values.

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5. Algorithms for Selecting Fragment Identification Values

There are a number of algorithms that may be used for setting the
Identification field such that the security issues discussed in this
document are avoided. This section presents three of those.

The algorithm in Section 5.1 typically leads to a low Identification
reuse frequency at the expense of keeping per-destination state; this
algorithm only uses a Pseudorandom Number Generator (PNRG) when the
host communicates with a new destination. The algorithm in
Section 5.2 may result in a higher Identification reuse frequency.
It also uses a PRNG for each datagram that needs to be fragmented.
Hence, the algorithm in Section 5.1 will likely result in better
performance properties. Finally, the algorithm in Section 5.3
achieves a similar Identification reuse frequency to that of the
algorithm in Section 5.1 without the need of keeping state, but
possibly at the expense of lower per-packet performance.

NOTE:
Since the specific algorithm to be employed for the PRNGs in
Section 5.1 and Section 5.2, and the specific algorithms to be
employed for the hash functions in Section 5.3 have not been
specified, it is impossible to provide a quantitative performance
comparison of the algorithms described in this section.

5.1. Per-Destination Counter (Initialized to a Random Value)

This algorithm consists of the following steps:

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

2. If such an entry exists, it contains the last Identification
value used for that Destination Address. Therefore, such a value
should be incremented by 1 and used for setting the
Identification field of the outgoing packet. Additionally, the
updated value should be recorded in the corresponding entry of
the Destination Cache [RFC4861].

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 fragmented datagram.

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The advantages of this algorithm are:

o It is simple to implement, with the only complexity residing in
the PRNG used to initialize the Identification value contained in
each entry of the Destination Cache.

o The Identification reuse 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 Destination Cache has been created and initialized, 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
Destination Cache must be removed, the last Identification value
used for that Destination will be lost. Thus, subsequent traffic
to that destination would cause that entry to be recreated and
reinitialized to random value, thus possibly leading to
Identification "collisions".

o Since the Identification values are predictable by the destination
host, a vulnerable host might possibly leak to third parties the
Identification values used by other hosts to send traffic to it
(i.e., Host B could leak to Host C the Identification values that
Host A is using to send packets to Host B). Appendix A describes
one possible scenario for such leakage in detail.

5.2. Randomized Identification Values

Clearly, use of a Pseudorandom Number Generator for selecting the
Identification would be desirable from a security standpoint. With
such a scheme, the 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 employed. For example, some PRNGs may result in higher
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.

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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 [RFC6056] for the selection of transport port
numbers. With such a scheme, the Identification value of each
fragmented datagram would be selected with the expression:

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

where:

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. This value can be
initialized to a pseudo-random value during the system
bootstrapping sequence. It should remain constant at least while
there could be previously sent fragments still in the network or
at the fragment reassembly buffer of the corresponding destination
system(s).

counter[]:
System-wide array of 32-bit counters (e.g. with 8K elements or
more). Each counter should be initialized to a pseudo-random
value during the system bootstrapping sequence.

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

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Dst Pref:
IPv6 "Destination Prefix" of the 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. This value can be
initialized to a pseudo-random value during the system
bootstrapping sequence. It should remain constant at least while
there could be previously sent fragments still in the network or
at the fragment reassembly buffer of the corresponding destination
system(s).

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

The output of F() will be constant for each (Src IP, Dst IP) pair.
Similarly, the output of G() will be constant for each (Src IP, Dst
Pref) pair. Thus, the resulting Identification value will be the
result of a random offset plus a linear function (provided by
counter[]), therefore resulting in a monotonically increasing
sequence of Identification values for each (src IP, Dst IP) pair.

NOTE:
F() essentially provides the unpredictability (by off-path
attackers) of the resulting Identification values, while counter[]
provides a linear function such that the Identification values are
different for each fragmented packet while the Identification
reuse frequency is minimized.

The advantages of this algorithm are:

o The Identification reuse 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 reuse frequency will be lower depends 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 datagram that is
fragmented).

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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 Destination 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 Destination Cache must be removed as a result of resource
management, the last Identification value used for that
Destination will *not* be lost. This is an improvement over
the algorithm specified in Section 5.1.

The possible drawbacks of this algorithm are:

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

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

6. Security Considerations

This document discusses the security implications of predictable
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. References

7.1. Normative References

[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>.

[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, DOI 10.17487/RFC5722, December 2009,
<http://www.rfc-editor.org/info/rfc5722>.

[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
DOI 10.17487/RFC6052, October 2010,
<http://www.rfc-editor.org/info/rfc6052>.

[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<http://www.rfc-editor.org/info/rfc6056>.

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[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
<http://www.rfc-editor.org/info/rfc6145>.

[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments",
RFC 6946, DOI 10.17487/RFC6946, May 2013,
<http://www.rfc-editor.org/info/rfc6946>.

7.2. Informative References

[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<http://www.rfc-editor.org/info/rfc4963>.

[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<http://www.rfc-editor.org/info/rfc5927>.

[RFC6274] Gont, F., "Security Assessment of the Internet Protocol
Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
<http://www.rfc-editor.org/info/rfc6274>.

[DEPGEN] Gont, F., Liu, S., and T. Anderson, "Generation of IPv6
Atomic Fragments Considered Harmful", Work in Progress,
draft-ietf-6man-deprecate-atomfrag-generation-05, January
2016.

[Bellovin2002]
Bellovin, S., "A Technique for Counting NATted Hosts",
IMW'02 Nov. 6-8, 2002, Marseille, France,
DOI 10.1145/637201.637243, 2002.

[Fyodor2004]
Lyon, G., "TCP Idle Scan", from Chapter 5 of "Nmap Network
Scanning", 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>.

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[Sanfilippo1998a]
Sanfilippo, S., "Subject: about the ip header id", message
to Bugtraq mailing list, 14 December 1998,
<http://diswww.mit.edu/menelaus.mit.edu/bt/8704>.

[Sanfilippo1998b]
Sanfilippo, S., "Subject: new tcp scan method", message
to Bugtraq mailing list, 18 December 1998,
<http://diswww.mit.edu/menelaus.mit.edu/bt/8736>.

[Sanfilippo1999]
Sanfilippo, S., "Subject: more about IP ID", message
to Bugtraq mailing list, 20 November 1999,
<http://diswww.mit.edu/menelaus.mit.edu/bt/12686>.

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

[Zalewski2003]
Zalewski, M., "Subject: A new TCP/IP blind data injection
technique?", message to Bugtraq mailing list, 11 December
2003, <http://lcamtuf.coredump.cx/ipfrag.txt>.

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Appendix A. Information Leakage Produced by Vulnerable Implementations

Section 3 provides a number of references describing a number of ways
in which a vulnerable implementation may reveal the 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 different 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 Identification value in use
by a third-party host (Host B) to send fragmented datagrams from Host
B to Host A.

NOTE:
For the most part, this section is included to illustrate how a
vulnerable implementation might be leveraged to leak out the
Identification value of an otherwise non-vulnerable
implementation.

The following scenarios assume:

Host A:
An IPv6 host that implements the algorithm specified in
Section 5.1, implements [RFC5722], but does not implement
[RFC6946].

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

Host C:
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 Identification value of a host. We note
that while the figures show only one packet for the ICMPv6 Echo
Request and the ICMPv6 Echo Reply packets, 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. Additionally, the following
scenarios assume that Host A employs a Fragment Header when sending
traffic to Host B (typically the so-called "IPv6 atomic fragments"
[RFC6946]): this behavior may be triggered by forged ICMPv6 PTB
messages that advertise an MTU smaller than 1280 bytes (assuming the
victim still generates atomic fragments [DEPGEN]).

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In lines #1-#2 (and lines #7-#8), the attacker samples the current
Identification value at Host B. In line #3, the attacker sends a
forged TCP SYN segment to Host A. In line 4, the attacker sends a
forged TCP segment to Host B as an incomplete IPv6 fragmented
datagram (e.g., a single fragment with Fragment Offset=0, More
fragments=1). If corresponding TCP port is closed, and the attacker
fails when trying to produce a collision of Identification values
(see line #4), the following packet exchange might take place:

A B C

#1 <------ Echo Req #1 -----------
#2 --- Echo Repl #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 <-------- Echo Req #2 ---------
#8 --- Echo Repl #2, FID=5002 --->

The RST segment in line #6 is elicited by the SYN/ACK segment from
line #5 (illegitimately elicited by the SYN segment from line #3).
The packet from line #4, sent as an incomplete IPv6 datagram,
eventually times out.

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

A B C

#1 <------- Echo Req #1 ----------
#2 --- Echo Repl #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 Repl #2, FID=5001 -->

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

As a result, the attacker could employ this technique to learn the
current Identification value used by host A to send packets to host
B, even when Host A itself has a non-vulnerable implementation.

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Appendix B. Survey of Fragment Identification Selection Algorithms
Employed by Popular IPv6 Implementations

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

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

+------------------------------+------------------------------------+
| Operating System | Algorithm |
+------------------------------+------------------------------------+
| Cisco IOS 15.3 | Predictable (Global Counter, |
| | Init=0, Incr=1) |
+------------------------------+------------------------------------+
| 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 | Unpredictable (Random, SKIP32) |
+------------------------------+------------------------------------+
| Solaris 10 | Predictable (Per-dst Counter, |
| | Init=0, Incr=1) |
+------------------------------+------------------------------------+
| Windows XP SP2 | Predictable (Global Counter, |
| | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows XP Professional | Predictable (Global Counter, |
| 32bit, SP3 | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows Vista (Build 6000) | Predictable (Global Counter, |
| | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows Vista Business | Predictable (Global Counter, |
| 64bit, SP1 | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows 7 Home Premium | Predictable (Global Counter, |
| | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows Server 2003 R2 | Predictable (Global Counter, |
| Standard 64bit, SP2 | Init=0, Incr=2) |

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RFC 7739 Implications of Predictable Fragment IDs February 2016

+------------------------------+------------------------------------+
| Windows Server 2008 Standard | Predictable (Global Counter, |
| 32bit, SP1 | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows Server 2008 R2 | Predictable (Global Counter, |
| Standard 64bit, SP1 | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows Server 2012 Standard | Predictable (Global Counter, |
| 64bit | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows 7 Home Premium | Predictable (Global Counter, |
| 32bit, SP1 | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows 7 Ultimate 32bit, | Predictable (Global Counter, |
| SP1 | Init=0, Incr=2) |
+------------------------------+------------------------------------+
| Windows 8 Enterprise 32 bit | Unpredictable (Alg. from Section |
| | 5.3) |
+------------------------------+------------------------------------+

Table 1: Fragment Identification algorithms employed by different OSs

NOTE:
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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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, Stephen Bensley, Ron
Bonica, Tassos Chatzithomaoglou, Guillermo Gont, Brian Haberman, Bob
Hinden, Sheng Jiang, Tatuya Jinmei, Merike Kaeo, Will Liu, Juan
Antonio Matos, Simon Perreault, Hosnieh Rafiee, Meral Shirazipour,
Mark Smith, Dave Thaler, and Klaas Wierenga, for providing valuable
comments on earlier draft versions of this document.

This document is based on work performed by Fernando Gont on behalf
of the UK Centre for the Protection of National Infrastructure
(CPNI).

The author would like to thank Buffy for her love and support.

Author's Address

Fernando Gont
Huawei Technologies
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina

Phone: +54 11 4650 8472
Email: [email protected]
URI: http://www.si6networks.com

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