IPv6 maintenance Working Group (6man) F. Gont Internet-Draft UK CPNI Updates: 2460, 5722 (if approved) December 15, 2011 Intended status: Standards Track Expires: June 17, 2012 Processing of IPv6 "atomic" fragments draft-gont-6man-ipv6-atomic-fragments-00 Abstract IPv6 allows packets to contain a Fragment Header, without the packet being actually fragmented into multiple pieces. Such packets typically result from hosts that have received an ICMPv6 "Packet Too Big" error message that advertises a "Next-Hop MTU" smaller than 1280 bytes, and are currently processed by hosts as "fragmented traffic". By forging ICMPv6 "Packet Too Big" error messages an attacker can cause hosts to employ "atomic fragments", and the launch any fragmentation-based attacks against such traffic. This document discusses the generation of the aforementioned "atomic fragments", the corresponding security implications, and formally updates RFC 2460 and RFC 5722 such that the attack vector based on "atomic fragments" is completely eliminated. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. This document may not be modified, and derivative works of it may not be created, and it may not be published except as an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on June 17, 2012. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. Gont Expires June 17, 2012 [Page 1]
Internet-Draft IPv6 atomic fragments December 2011 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 Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Generation of IPv6 'atomic fragments' . . . . . . . . . . . . 5 3. Updating RFC 2460 and RFC 5722 . . . . . . . . . . . . . . . . 7 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7.1. Normative References . . . . . . . . . . . . . . . . . . . 11 7.2. Informative References . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 Gont Expires June 17, 2012 [Page 2]
Internet-Draft IPv6 atomic fragments December 2011 1. Introduction [RFC2460] specifies the IPv6 fragmentation mechanism, which allows IPv6 packets to be fragmented into smaller pieces such that they fit in the Path-MTU to the intended destination(s). [RFC2460] allowed fragments to overlap, and hence allowed for ambiguity in the reassembly process, which could be leveraged by attackers to bypass firewall rules and/or evade Network Intrusion Detection Systems (NIDs) [RFC5722]. [RFC5722] forbid overlapping fragments, specifying that when overlapping fragments are detected, all the overlapping fragments should be silently discarded. As specified in Section 5 of [RFC2460], when a host receives an ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller than 1280 (the minimum IPv6 MTU), it is not required to reduce the assumed Path-MTU, but must simply include a Fragment Header. The resulting packets will thus *not* be actually fragmented into several pieces, but only include a Fragment Header with both the "Fragment Offset" and the "M" bit set to 0. While these packets are really "atomic fragments" (they can be processed by the IPv6 module and handed to the upper-layer protocol without waiting for any other fragments), most IPv6 implementations process them as regular fragments. Namely, they try to perform IPv6 reassembly with the "atomic fragment" and any other fragments already queued with the same set {IPv6 Source Address, IPv6 Destination Address, Fragment Identification}. For example, in the case of IPv6 implementations that have been updated to support [RFC5722], if a fragment with the same {IPv6 Source Address, IPv6 Destination Address, Fragment Identification} is already queued for reassembly at a host when an "atomic fragment" is received with the same set {IPv6 Source Address, IPv6 Destination Address, Fragment Identification}, and both fragments "overlap", all the fragments are silently discarded. Processing an "atomic fragment" as regular fragmented packet clearly provides an unnecessary vector to perform fragmentation-based attacks against non-fragmented traffic (i.e., IPv6 datagrams that are not really split into multiple pieces, but that just include a Fragment Header). IPv6 fragmentation attacks have been discussed in great detail in [PREDICTABLE-ID] and [CPNI-IPv6], and [RFC5722] describes a specific firewall-circumvention attack that could be performed by leveraging overlapping fragments. The possible IPv6 fragmentation-based attacks are, in most cases, "ports" of the IPv4 fragmentation attacks Gont Expires June 17, 2012 [Page 3]
Internet-Draft IPv6 atomic fragments December 2011 discussed in [RFC6274]. Section 2 describes the generation of IPv6 "atomic fragments", and how they can be remotely "triggered" by a remote attacker. Section 3 formally updates [RFC2460] and [RFC5722] such that the aforementioned attack vector is eliminated. 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 RFC 2119 [RFC2119]. Gont Expires June 17, 2012 [Page 4]
Internet-Draft IPv6 atomic fragments December 2011 2. Generation of IPv6 'atomic fragments' Section 5 of [RFC2460] states: In response to an IPv6 packet that is sent to an IPv4 destination (i.e., a packet that undergoes translation from IPv6 to IPv4), the originating IPv6 node may receive an ICMP Packet Too Big message reporting a Next-Hop MTU less than 1280. In that case, the IPv6 node is not required to reduce the size of subsequent packets to less than 1280, but must include a Fragment header in those packets so that the IPv6-to-IPv4 translating router can obtain a suitable Identification value to use in resulting IPv4 fragments. Note that this means the payload may have to be reduced to 1232 octets (1280 minus 40 for the IPv6 header and 8 for the Fragment header), and smaller still if additional extension headers are used. This means that any ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller than 1280 could trigger the generation of the so-called "atomic fragments" (i.e., IPv6 datagrams that include a Fragment Header, but that are composed of a single fragment, with both the "Fragment Offset" and the "M" fields of the Fragment Header set to 0). This can be leveraged to perform a variety of fragmentation-based attacks [PREDICTABLE-ID] [CPNI-IPv6]. From a security standpoint, this situation is exacerbated by the following factors: Many implementations fail to perform validation checks on the received ICMPv6 error messages, as recommended in Section 5.2 of [RFC4443] and [RFC5927]. In some cases, such as when an ICMPv6 error message has (supposedly) been elicited by a connection-less transport protocol (or some other connection-less protocol being encapsulated in IPv6), it may be virtually impossible to perform validation checks on the received ICMPv6 error messages. Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big" error messages, the Destinations Cache is usually updated to reflect that any further packets to such destination should include a Fragment Header. This means that a single ICMPv6 "Packet Too Big" error message may affect multiple communication instances (e.g., TCP connections) with such destination. Gont Expires June 17, 2012 [Page 5]
Internet-Draft IPv6 atomic fragments December 2011 Some implementations employ Fragment Identification values that are predictable by remote attackers, greatly improving the chances of an attacker of successfully performing a fragmentation-based attack [PREDICTABLE-ID]. Gont Expires June 17, 2012 [Page 6]
Internet-Draft IPv6 atomic fragments December 2011 3. Updating RFC 2460 and RFC 5722 This document updates [RFC2460] and [RFC5722] as follows: A host that receives an IPv6 packet which includes a Fragment Header with the "Fragment Offset" equal to 0 and the "M" bit equal to 0 MUST process such packet in isolation from any other packets/ fragments, even if such packets/fragments contain the same set {IPV6 Source Address, IPv6 Destination Address, Fragment Identification}. In other words, the Fragment Header of "atomic fragments" should be ignored by the receiving host. Gont Expires June 17, 2012 [Page 7]
Internet-Draft IPv6 atomic fragments December 2011 4. IANA Considerations There are no IANA registries within this document. The RFC-Editor can remove this section before publication of this document as an RFC. Gont Expires June 17, 2012 [Page 8]
Internet-Draft IPv6 atomic fragments December 2011 5. Security Considerations This document describes how an attacker can exploit ICMPv6 "Packet Too Big" error messages to cause further IPv6 packets to include a Fragment Header, such that he can perform any fragmentation-based attack against otherwise non-fragmented traffic. This document updates [RFC2460] and [RFC5722], such that the aforementioned attack vector is completely eliminated. Gont Expires June 17, 2012 [Page 9]
Internet-Draft IPv6 atomic fragments December 2011 6. Acknowledgements This document is based on the technical report "Security Assessment of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by Fernando Gont on behalf of the UK Centre for the Protection of National Infrastructure (CPNI). Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for their continued support. Gont Expires June 17, 2012 [Page 10]
Internet-Draft IPv6 atomic fragments December 2011 7. References 7.1. Normative References [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. [RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments", RFC 5722, December 2009. 7.2. Informative References [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010. [RFC6274] Gont, F., "Security Assessment of the Internet Protocol Version 4", RFC 6274, July 2011. [CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol version 6 (IPv6)", UK Centre for the Protection of National Infrastructure, (available on request). [PREDICTABLE-ID] Gont, F., "Security Implications of Predictable Fragment Identification Values", Work in Progress, December 2011, < http://tools.ietf.org/html/ draft-gont-6man-predictable-fragment-id>. Gont Expires June 17, 2012 [Page 11]
Internet-Draft IPv6 atomic fragments December 2011 Author's Address Fernando Gont UK CPNI Email: fgont@si6networks.com URI: http://www.cpni.gov.uk Gont Expires June 17, 2012 [Page 12]