Handling of Overlapping IPv6 Fragments
Forbid overlapping IPv6 fragments so reassembly cannot change what a firewall inspected.
Krishnan, S., "Handling of Overlapping IPv6 Fragments", RFC 5722, DOI 10.17487/RFC5722, December 2009, <https://www.rfc-editor.org/info/rfc5722>.
@misc{rfc5722,
series = {Request for Comments},
number = 5722,
howpublished = {RFC 5722},
publisher = {RFC Editor},
doi = {10.17487/RFC5722},
url = {https://www.rfc-editor.org/info/rfc5722},
author = {S. Krishnan},
title = {{Handling of Overlapping IPv6 Fragments}},
pagetotal = 6,
year = 2009,
month = dec,
} RFC text
Contents
Network Working Group S. Krishnan
Request for Comments: 5722 Ericsson
Updates: 2460 December 2009
Category: Standards Track
Handling of Overlapping IPv6 Fragments
Abstract
The fragmentation and reassembly algorithm specified in the base IPv6
specification allows fragments to overlap. This document
demonstrates the security issues associated with allowing overlapping
fragments and updates the IPv6 specification to explicitly forbid
overlapping fragments.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 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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the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. Overlapping Fragments ...........................................2
3. The Attack ......................................................3
4. Node Behavior ...................................................5
5. Security Considerations .........................................5
6. Acknowledgements ................................................5
7. References ......................................................6
7.1. Normative References .......................................6
7.2. Informative References .....................................6
1. Introduction
Fragmentation is used in IPv6 when the IPv6 packet will not fit
inside the path MTU to its destination. When fragmentation is
performed, an IPv6 node uses a fragment header, as specified in
Section 4.5 of the IPv6 base specification [RFC2460], to break down
the datagram into smaller fragments that will fit in the path MTU.
The destination node receives these fragments and reassembles them.
The algorithm specified for fragmentation in [RFC2460] does not
prevent the fragments from overlapping, and this can lead to some
security issues with firewalls [RFC4942]. This document explores the
issues that can be caused by overlapping fragments and updates the
IPv6 specification to explicitly forbid overlapping fragments.
1.1. Conventions Used in This Document
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 [RFC2119].
2. Overlapping Fragments
Commonly used firewalls use the algorithm specified in [RFC1858] to
weed out malicious packets that try to overwrite parts of the
transport-layer header in order to bypass inbound connection checks.
[RFC1858] prevents an overlapping fragment attack on an upper-layer
protocol (in this case, TCP) by recommending that packets with a
fragment offset of 1 be dropped. While this works well for IPv4
fragments, it will not work for IPv6 fragments. This is because the
fragmentable part of the IPv6 packet can contain extension headers
before the TCP header, making this check less effective.
3. The Attack
This attack describes how a malicious node can bypass a firewall
using overlapping fragments. Consider a sufficiently large IPv6
packet that needs to be fragmented.
+------------------+--------------------//-----------------------+
| Unfragmentable | Fragmentable |
| Part | Part |
+------------------+--------------------//-----------------------+
Figure 1: Large IPv6 Packet
This packet is split into several fragments by the sender so that the
packet can fit inside the path MTU. Let's say the packet is split
into two fragments.
+------------------+--------+--------------------+
| Unfragmentable |Fragment| first |
| Part | Header | fragment |
+------------------+--------+--------------------+
+------------------+--------+--------------------+
| Unfragmentable |Fragment| second |
| Part | Header | fragment |
+------------------+--------+--------------------+
Figure 2: Fragmented IPv6 Packet
Consider the first fragment. Let's say it contains a destination
options header (DOH) 80 octets long and is followed by a TCP header.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
|NextHdr=DOH(60)| Reserved | FragmentOffset = 0 |Res|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification=aaaabbbb |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==DOH
|NextHdr=TCP(6) | HdrExtLen = 9 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
. .
. Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset| Reserved |U|A|P|R|S|F| Window |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: First Fragment
The TCP header has the following values of the flags: S(YN)=1 and
A(CK)=1. This may make an inspecting stateful firewall think that it
is a response packet for a connection request initiated from the
trusted side of the firewall. Hence, it will allow the fragment to
pass. It will also allow the following fragments with the same
Fragment Identification value in the fragment header to pass through.
A malicious node can form a second fragment with a TCP header that
changes the flags and sets S(YN)=1 and A(CK)=0. This can change the
packet on the receiving end to consider the packet as a connection
request instead of a response. By doing this, the malicious node has
bypassed the firewall's access control to initiate a connection
request to a node protected by a firewall.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
|NextHdr=DOH(60)| Reserved | FragmentOffset = 10 |Res|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification=aaaabbbb |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset| Reserved |U|A|P|R|S|F| Window |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Second Fragment
Note that this attack is much more serious in IPv6 than in IPv4. In
IPv4, the overlapping part of the TCP header does not include the
source and destination ports. In IPv6, the attack can easily work to
replace the source or destination port with an overlapping fragment.
4. Node Behavior
IPv6 nodes transmitting datagrams that need to be fragmented MUST NOT
create overlapping fragments. When reassembling an IPv6 datagram, if
one or more its constituent fragments is determined to be an
overlapping fragment, the entire datagram (and any constituent
fragments, including those not yet received) MUST be silently
discarded.
Nodes MAY also provide mechanisms to track the reception of such
packets, for instance, by implementing counters or alarms relating to
these events.
5. Security Considerations
This document discusses an attack that can be used to bypass IPv6
firewalls using overlapping fragments. It recommends disallowing
overlapping fragments in order to prevent this attack.
6. Acknowledgements
The author would like to thank Thomas Narten, Doug Montgomery,
Gabriel Montenegro, Remi Denis-Courmont, Marla Azinger, Arnaud
Ebalard, Seiichi Kawamura, Behcet Sarikaya, Vishwas Manral, Christian
Vogt, Bob Hinden, Carl Wallace, Jari Arkko, Pasi Eronen, Francis
Dupont, Neville Brownlee, Dan Romascanu, Lars Eggert, Cullen
Jennings, and Alfred Hoenes for their reviews and suggestions that
made this document better.
7. References
7.1. Normative References
[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.
7.2. Informative References
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6
Transition/Co-existence Security Considerations", RFC
4942, September 2007.
Author's Address
Suresh Krishnan
Ericsson
8400 Blvd Decarie
Town of Mount Royal, Quebec
Canada
EMail: suresh.krishnan@ericsson.com
ISO Plain Language
On this page
§1Why IPv6 Fragments Need a Rule
IPv6 uses fragmentation when a packet will not fit within the path's maximum transmission unit. A sender divides the datagram, and the destination reassembles the fragments.
The base IPv6 specification did not prevent fragments from overlapping. This RFC explains the security consequences and updates the IPv6 rules to prohibit overlap.
A sender may split one IPv6 datagram into pieces, and the destination joins those pieces again. The pieces must not cover the same data, because that can make the final result ambiguous.
§2Why a Firewall Can Miss the Attack
Some firewalls inspect fragments to detect attempts to overwrite transport headers and bypass inbound connection checks. An IPv4 defense recommends dropping fragments with offset 1.
That check is less effective for IPv6 because extension headers can occur before the TCP header in the fragmentable part. The TCP header may therefore appear at a different offset.
A firewall may inspect only part of a fragmented packet. IPv6 extension headers can move the transport header, so a fixed offset check may not find the important fields.
§3How Overlap Can Change a Packet
The RFC describes a first fragment containing an 80-octet Destination Options Header followed by a TCP header with SYN and ACK set. A stateful firewall may interpret it as a response to a connection begun from the trusted side and allow related fragments with the same Fragment Identification.
A second, overlapping fragment can replace part of the TCP header and set SYN while clearing ACK. After reassembly, the destination sees a connection request rather than the reply the firewall inspected. In IPv6, the overlap can also replace source or destination ports.
The first fragment can appear to be an allowed reply, while another fragment changes the TCP fields before reassembly. The destination may then receive a packet that the firewall did not actually approve.
§4Required Sender and Receiver Behavior
An IPv6 node that sends a datagram requiring fragmentation MUST NOT create overlapping fragments. This is a requirement on the sender.
During reassembly, if any fragment overlaps another, the node MUST silently discard the entire datagram. This includes fragments already received and fragments that have not yet arrived. A node MAY count or raise alarms for these events.
Senders must make fragment ranges that do not overlap. If a receiver finds any overlap, it discards the whole datagram without replying.
- SenderMust not create overlapping IPv6 fragments.
- ReceiverSilently discards the complete datagram when fragments overlap.
§3The Two-Fragment Example
The example uses two fragments with the same Identification value, aaaabbbb. The first fragment has offset 0 and the more-fragments bit set; it carries an 80-octet Destination Options Header before TCP.
The second fragment has offset 10 and the more-fragments bit cleared. Its data begins within the first fragment's TCP header, creating the overlap that can alter the flags seen after reassembly.
In the example, both fragments share an identification value, and their data ranges overlap. The receiver must reject the datagram rather than choose which fragment's bytes to keep.