Handling of Overlapping IPv6 Fragments
RFC 5722
Document | Type |
RFC - Proposed Standard
(December 2009; Errata)
Updated by RFC 6946
Updates RFC 2460
|
|
---|---|---|---|
Author | Suresh Krishnan | ||
Last updated | 2020-01-21 | ||
Replaces | draft-krishnan-6man-overlap-fragment | ||
Stream | IETF | ||
Formats | plain text html pdf htmlized with errata bibtex | ||
Reviews | |||
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. +------------------+--------------------//-----------------------+ | Unfragmentable | Fragmentable | | Part | Part | +------------------+--------------------//-----------------------+ Figure 1: Large IPv6 Packet This packet is split into several fragments by the sender so that theShow full document text