6man Working Group S. Krishnan Internet-Draft Ericsson Updates: 2460 (if approved)
November 3, 2008March 8, 2009 Intended status: Standards Track Expires: May 7,September 9, 2009 Handling of overlapping IPv6 fragments draft-ietf-6man-overlap-fragment-01draft-ietf-6man-overlap-fragment-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or sheThis Internet-Draft is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed,submitted to IETF in accordancefull conformance with Section 6the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 7,September 9, 2009. 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract The fragmentation and reassembly algorithm specified in the base IPv6 specification allows fragments to overlap. This document 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 56 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 56 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. Normative References . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . . .7 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. 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 makes 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 would 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 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. IPv6 nodes that receive a fragment that overlaps with a previously received fragment MUST cease the reassembly process and MUST ignore further fragments with the same IPv6 Source Address, IPv6 Destination Address and Fragment Identification. It MUST alsodiscard the previously received fragments with the same IPv6 Source Address, IPv6 Destination Address and Fragment Identification. 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. IANA Considerations This document does not require any action from the IANA. 7. 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: email@example.com Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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