Handling of Overlapping IPv6 Fragments
RFC 5722
| Document | Type |
RFC - Proposed Standard
(December 2009; Errata)
Updated by RFC 6946
Updates RFC 2460
|
|
|---|---|---|---|
| Last updated | 2015-10-14 | ||
| Replaces | draft-krishnan-6man-overlap-fragment | ||
| Stream | IETF | ||
| Formats | plain text pdf html bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 5722 (Proposed Standard) | |
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Jari Arkko | ||
| Send notices to | (None) | ||
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
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 BSD License.
Krishnan Standards Track [Page 1]
RFC 5722 Handling of Overlapping IPv6 Fragments December 2009
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.
Krishnan Standards Track [Page 2]
RFC 5722 Handling of Overlapping IPv6 Fragments December 2009
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.
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