Handling of Overlapping IPv6 Fragments
draft-ietf-6man-overlap-fragment-03
The information below is for an old version of the document that is already published as an RFC.
| Document | Type | RFC Internet-Draft (6man WG) | |
|---|---|---|---|
| Author | Suresh Krishnan | ||
| Last updated | 2020-01-21 (Latest revision 2009-07-03) | ||
| Replaces | draft-krishnan-6man-overlap-fragment | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Reviews | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | RFC 5722 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Jari Arkko | ||
| Send notices to | (None) |
draft-ietf-6man-overlap-fragment-03
6man Working Group S. Krishnan
Internet-Draft Ericsson
Updates: 2460 (if approved) July 2, 2009
Intended status: Standards Track
Expires: January 3, 2010
Handling of overlapping IPv6 fragments
draft-ietf-6man-overlap-fragment-03
Status of this Memo
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Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Abstract
The fragmentation and reassembly algorithm specified in the base IPv6
specification allows fragments to overlap. This document
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demonstrates the security issues with allowing overlapping fragments
and updates the IPv6 specification to explicitly forbid overlapping
fragments.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . 3
2. Overlapping Fragments . . . . . . . . . . . . . . . . . . . . . 3
3. The attack . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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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.
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 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
fragment offset 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.
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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.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==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.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==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 did 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. Recommendation
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.
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, and Alfred Hoenes for their reviews and suggestions that made
this document better.
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7. IANA Considerations
This document does not require any action from the IANA.
8. Normative References
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[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.
[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
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