Handling of Overlapping IPv6 Fragments
IPv6 fragments must not overlap, because overlap lets an attacker slip past firewalls.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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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 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
AI ELI5
On this page
§1The Problem with Overlapping Fragments
IPv6 uses fragmentation when a packet will not fit inside the path MTU. The sender uses a fragment header, defined in Section 4.5 of RFC 2460, to split the datagram into smaller fragments, and the destination reassembles them.
The algorithm in RFC 2460 does not prevent fragments from overlapping. That can cause security problems with firewalls. RFC 5722 explores the issue and updates the IPv6 specification to forbid overlapping fragments explicitly.
A big parcel is split into smaller boxes and put back together at the end. Nothing said the boxes could not overlap.
§2Why the IPv4 Defence Falls Short
Common firewalls use the algorithm in RFC 1858 to catch malicious packets that overwrite parts of the transport-layer header to dodge inbound connection checks. For TCP, RFC 1858 recommends dropping packets with a fragment offset of 1.
That works for IPv4 but not for IPv6. The fragmentable part of an IPv6 packet can have extension headers in front of the TCP header, so the offset check is less effective.
The old guard watched one particular door. In IPv6 there are more doors, so watching just that one is not enough.
§3The Attack
The RFC walks through a packet split in two. The first fragment holds an 80-octet destination options header followed by a TCP header with the SYN and ACK flags set. A stateful firewall may take this for a reply to a connection started on the trusted side and let it through, along with later fragments that share the same Fragment Identification value.
The attacker then sends a second fragment that overlaps the TCP header and sets SYN=1 and ACK=0. After reassembly the receiver sees a new connection request instead of a reply, so the firewall's access control has been bypassed.
The RFC says this is more serious in IPv6 than in IPv4. In IPv4 the overlapping part of the TCP header does not include the source and destination ports, but in IPv6 an overlapping fragment can easily replace either port.
The attacker shows the guard a harmless-looking first piece, then slips in a second piece that covers part of it and changes what it says.
- First fragment: SYN=1, ACK=1Looks like a reply to a connection started from the trusted side.
- Fragment allowedLater fragments with the same Identification also pass.
- Second fragment: SYN=1, ACK=0Overlaps the TCP header.
- Fragment allowed
§4Required Node Behavior
Nodes sending datagrams that need fragmenting MUST NOT create overlapping fragments.
When reassembling, if one or more fragments is found to overlap, the entire datagram MUST be silently discarded. That includes any fragments already received and any that have not yet arrived. Nodes MAY also track these events, for instance with counters or alarms.
If any two pieces overlap, throw the whole puzzle away without saying anything, and keep count if you like.
§5The Example Fragments in Detail
The RFC draws both fragments of its example. Each carries a fragment header whose Next Header value is DOH(60) and whose Identification is aaaabbbb. The first has Fragment Offset 0 and the more-fragments bit set to 1; the second has Fragment Offset 10 and that bit set to 0.
The first fragment holds the destination options header with Next Header TCP(6) and HdrExtLen 9, then the TCP header. The second fragment begins with its own TCP header at its offset, which is why the two overlap and the later bytes can replace the flags the firewall saw.
Both pieces carry the same label, so they get glued together, but the second piece sits on top of part of the first and changes it.