Internet Engineering Task Force P. Savola Internet Draft CSC/FUNET Expiration Date: August 2004 February 2004 IPv6 Transition/Co-existence Security Considerations draft-savola-v6ops-security-overview-02.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. 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. To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract The transition/co-existance from IPv4 to IPv4/IPv6 causes one to consider the security considerations of such a process. In this memo, I try to give an overview of different aspects relating to IPv6 grouped in three categories: issues due to IPv6 protocol itself, issues due to transition mechanisms, and issues due to IPv6 deployment. Savola [Expires August 2004] [Page 1] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 Table of Contents 1. Introduction ............................................... 2 2. Issues Due to IPv6 Protocol ................................ 3 2.1. IPv6 Protocol-specific Issues .......................... 3 2.2. IPv4-mapped IPv6 Addresses ............................. 4 2.3. Increased End-to-End Transparency ...................... 4 3. Issues Due to Transition Mechanisms ........................ 5 3.1. IPv6 Transition/Co-existence Mechanism-specific Issues . 5 3.2. Automatic Tunneling and Relays ......................... 5 3.3. Tunneling May Break Security Assumptions ............... 6 4. Issues Due to IPv6 Deployment .............................. 7 4.1. IPv6 Service Piloting Done Insecurely .................. 7 4.2. Enabling IPv6 by Default Brings the Usability Down ..... 8 4.3. Operational Factors when Enabling IPv6 in the Network .. 8 5. Acknowledgements ........................................... 9 6. Security Considerations .................................... 9 7. References ................................................. 9 7.1. Normative .............................................. 9 7.2. Informative ............................................ 9 Author's Address ............................................... 11 A. IPv6 Probing/Mapping Considerations ........................ 11 B. IPv6 Privacy Considerations ................................ 12 B.1. Exposing MAC Addresses ................................. 12 B.2. Exposing Multiple Devices .............................. 12 Intellectual Property Statement ................................ 13 Full Copyright Statement ....................................... 13 1. Introduction The transition/co-existance from IPv4 to IPv4/IPv6 causes one to consider the security considerations of such a process. In this memo, I try to give an overview of different aspects relating to IPv6 grouped in three categories: issues due to IPv6 protocol itself, issues due to transition mechanisms, and issues due to IPv6 deployment. A view of IPv6 transition has been presented in a separate document [TRANS]; it is important to read it at least cursorily to understand that the point is not about replacing IPv4 with IPv6 (in the short term), but adding IPv6 alongside IPv4. This document also (at the moment, may be removed in future versions) describes two "non-issues", in Appedix A and B: considerations about probing/mapping IPv6 addresses, and considerations wrt. privacy in Savola [Expires August 2004] [Page 2] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 IPv6. 2. Issues Due to IPv6 Protocol 2.1. IPv6 Protocol-specific Issues Some features of IPv6 are a bit different from IPv4, and may include some potential problems specification-wise. Some examples include at least: o how hosts should interact with routing headers (they must act as forwarders) [RHHOSTS] o how routing headers may be too generic contructs to be useful for e.g. MIPv6 purposes [RHHAOSEC] o how home address option was previously specified (fixed now) [RHHAOSEC] o how ICMPv6 messages, in some cases, may be generated in response to multicast packets (where in IPv4 they can't) [FW] o how the privacy IPv6 addresses may not actually provide all that much privacy (ie. the applicability is unclear) [3041HARM] o how IPv6 has been specified wrt. middleboxes such as firewalls (e.g. when new extension headers etc. are used) [FW] On the other hand, there are several aspects where IPv4 security is weak have been made stronger (at least by additional specifications), at least: o threats related to local links, comparable to different ARP spoofing techniques; the ND threats have been documented [SENDREQ] and fixes specified [SEND] o Mobile IPv6 depends on the return routability checks for its security; this seems relatively robust form of security; the design has been described in [RRSEC] Appendix A lists (typically bogus) considerations related to IPv6 network mapping or probing. Appendix B lists mainly unfound claims about the lack of privacy in IPv6. Savola [Expires August 2004] [Page 3] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 2.2. IPv4-mapped IPv6 Addresses Overloaded functionality is always a double-edged sword: it may yield some deployment benefits, but often also incurs the price which comes with ambiguity. One example of such is IPv4-mapped IPv6 addresses: a representation of IPv4 address as an IPv6 address inside an operating system. Since then, IPv4-mapped addresses have been extended to be used with a transition mechanism [SIIT], on the wire. Therefore, it becomes difficult to unambiguously discern whether an IPv4 mapped address is really an IPv4 address represented in the IPv6 address format *or* an IPv6 address received from the wire (which may be subject to address forgery, etc.). In addition, special cases like these, while giving deployment benefits in some arenas, require some amount of code complexity (e.g. in the implementations of bind() system calls) which we might be better off without [V4MAPPEDA] [V4MAPPEDW]. At least, the mapped addresses should be disallowed on the wire. Changing the application behavior would have significant impact on application porting methods, though. 2.3. Increased End-to-End Transparency With IPv6, increased end-to-end transparency in general can sometimes be seen as a threat. Some seem to want limited end-to-end capabilities, e.g. in the form of private, local addressing, even when it is not necessary. People have gotten used to the perceived, dubious security benefits of NATs and perimeter firewalls, and the bidirectionality and transparency that IPv6 can provide may seem undesirable at times. This is a really important issue especially for most enterprise network managers. It is worth noting that IPv6 does not *require* end-to-end connectivity. It merely provides end-to-end addressability; the connectivity can still be controlled using firewalls (or other mechanisms), and it is indeed wise to do so. Savola [Expires August 2004] [Page 4] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 3. Issues Due to Transition Mechanisms 3.1. IPv6 Transition/Co-existence Mechanism-specific Issues The more complicated the IPv6 transition/co-existence becomes, the more danger there is to introduce security issues in the mechanisms (which may or may not be readily apparent). Therefore it would be desirable to keep the mechanisms simple, in as small pieces as possible. One case where such security issues have been analyzed is [6TO4SEC]. As tunneling has been proposed as a model for quite a bit more cases than are currently being used, its security properties should be analyzed in more detail. There are some generic dangers to tunneling: o it may be easier to avoid ingress filtering checks o it is possible to attack the tunnel interface: several IPv6 security mechanisms depend on checking that Hop Limit equals 255 on receipt and that link-local addresses are used. Sending such packets to the tunnel interface is much easier than gaining access to a physical segment and sending them there. o automatic tunneling mechanisms are typically particularly dangerous as the other end-point is unspecified, and packets have to be accepted and decapsulated from everywhere. Therefore, special care should be observed when specifying automatic tunneling techniques. 3.2. Automatic Tunneling and Relays Two mechanisms have been (or are being) specified which use automatic tunneling over IPv4 or UDP/IPv4 between the nodes enabling the same mechanism for connectivity: 6to4 and Teredo (respectively). The first obvious issue (as mentioned above) in such approaches is that such nodes must allow decapsulation of traffic from anywhere in the Internet. That kind of decapsulation function must be extremely well secured as it's so wide open. Even more difficult problem is how these mechanisms are able to communicate with native IPv6 nodes or between the automatic tunneling mechanisms: such connectivity requires the use of some kind of "relays". These relays could be deployed e.g., in all native IPv6 nodes, native IPv6 sites, IPv6 ISPs, or just somewhere in the Internet. This has some obvious trust and scaling issues. As Savola [Expires August 2004] [Page 5] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 authentication of such a relay service is very difficult, and more so in some of those deployment models, relays provide a means to for address spoofing, (reflected) Denial-of-Service attacks, and other threats. Threats related to 6to4 are discussed in [6TO4SEC]. 3.3. Tunneling May Break Security Assumptions NATs and firewalls have been deployed extensively in the IPv4 Internet, for the good or the bad. People who deploy them typically have some security/operational requirements in mind (e.g. a desire to block inbound connection attempts), whether misguided or not. Tunneling can change that model. IPv6-over-IPv4 tunneling is typically explicitly allowed or disallowed implicitly. Tunneling IPv6 over IPv4 with UDP, however, is often an entirely different thing: as UDP must usually be allowed through, at least in part and in a possibly stateful manner, one can "punch holes" in NAT's and firewalls using UDP. Actually, the mechanisms have been explicitly designed to traverse both NATs and firewalls in a similar fashion. One could say that tunneling is especially questionable in home/SOHO environments where the level of network administration is not that high; in these environments the hosts may not be as managed as in others (e.g., network services might be enabled unnecessarily), leading to possible security break-ins or other vulnerabilities. Holes can be punched both intentionally and unintentionally. In case it is a willing choice from the administrator/user, this is less of a problem (but e.g., enterprises might want to block IPv6 tunneling explicitly if some employees would do something like this willingly on their own). On the other hand, if a hole is punched transparently, without people understanding the consequences, it will very probably result in a serious threat sooner or later. When deploying tunneling solutions, especially tunneling solutions which are automatic and/or can be enabled easily by users not understanding the consequences, care should be taken not to compromise the security assumptions held by the users. For example, NAT traversal should not be performed by default unless there is a firewall producing a similar by-default security policy as IPv4 NAT provides. Protocol-41 tunneling is less of a problem, as it is easier to block if necessary; however, if the host is protected in IPv4, the IPv6 side should be protected as well. Savola [Expires August 2004] [Page 6] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 As has been shown in Appendix A, it is relatively easy to find out IPv6 address of an IPv4, so one should never rely on "security by obscurity" i.e., relying that nobody is able to guess or know the IPv6 address of the host. 4. Issues Due to IPv6 Deployment 4.1. IPv6 Service Piloting Done Insecurely In many cases, IPv6 service piloting is done in a manner which is considered to be less secure than as one would do with IPv4. For example, hosts and routers might not be protected by IPv6 firewalls, even if in IPv4 firewalls are being used. The other possible alternative, in some places, is that no service piloting is done at all because IPv6 firewalls may not be widely used -- and IPv6 deployment suffers (of course, this is also one of the nice excuses for not doing IPv6). This problem may be partially due to a slow speed of IPv6-capable firewall development and deployment. However, it is also a problem with a lack of information: actually, there are quite a few IPv6 packet filters and firewalls already, which could be used for sufficient access controls, but network administrators may not be aware of them yet. However, there appears to be a real lack in two areas: "personal firewalls" and enterprise firewalls; the same devices that support and are used for IPv4 today are often expected to also become IPv6-capable -- even though this is not really required. That is, IPv4 access could be filtered by one firewall, and when IPv6 access is added, it could be protected by another firewall; they don't have to be the same, and even their models don't have to be the same. Another, smaller factor may be that due to a few decisions on how IPv6 was built, it's more difficult for firewalls to be implemented and work under all the cases (e.g. when new extension headers etc. are used) [FW]: it's a bit more difficult for intermediate nodes to process the IPv6 header chains than IPv4 packets. A similar argument, stated to hinder IPv6 deployment, has been the lack of Intrusion Detection Systems (IDS). It's not clear whether this is more of an excuse than a real reason. Savola [Expires August 2004] [Page 7] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 4.2. Enabling IPv6 by Default Brings the Usability Down A practical disadvantage of enabling IPv6 as of this writing is that it typically brings the observed service level down a bit; that is, the usability suffers. This is due to at least three reasons: o global IPv6 routing is still rather unstable, leading to packet loss, lower throughput, and higher delay [6BMESS] o some applications can not properly handle both IPv4 and IPv6 or may have problems handling all the fallbacks and failure modes (and in some cases, e.g. if the TCP timeout kicks in, this may be very difficult) o some DNS server implementations have flaws that severely affect DNS queries for IPv6 addresses [DNSA4] Actually, some would be 100% ready to release IPv6 services (e.g. web) today, but that would mean trouble for many of their dual- stacked customers or users all over the world so they don't: these are often published under a separate domain or subdomain, and are practically not used that often. These issues are also described at some length in [ONBYDEF]. 4.3. Operational Factors when Enabling IPv6 in the Network You have to be careful when enabling IPv6 in the network gear for multiple reasons: IPv6-enabled router software may be unstable(r) yet; either IPv6 is unstable, or the software you have to run to be able to run IPv6 is different (from non-IPv6 parts) from the one you would run otherwise, making the software in practice more unstable -- and raising the bar for IPv6 adoption. IPv6 processing may not happen at (near) line speed (or in the same level as IPv4). A high amount of IPv6 traffic (even legitimate, e.g. NNTP) could easily overload the software-based IPv6 processing and cause harm also to IPv4 processing, affecting availability. That is, if people don't feel confident enough in the IPv6 support, they will be hesitant to enable it in their "production" networks. Sometimes required features may be missing from the vendors' software releases; an example is a software enabling IPv6 telnet/SSH access, but having no ability to turn it off or limit access to it! Savola [Expires August 2004] [Page 8] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 Sometimes the default IPv6 configuration is insecure. For example, in one vendor, if you've restricted IPv4 telnet to only a few hosts in the configuration, you need to be aware that IPv6 telnet will be automatically enabled, that the configuration commands used previously do not block IPv6 telnet, IPv6 telnet is open to the world by default, and that you have to use a separate command to also lock down the IPv6 telnet access. Many operator networks have to run interior routing protocols for both IPv4 and IPv6. It's possible to run the both in one routing protocol, or have two separate routing protocols; either approach has its tradeoffs. If multiple routing protocols are used, one should note that this causes double the number of processing when links flap or recalculation is otherwise needed -- which might more easily overload the routers' CPU, causing slightly slower convergence time. 5. Acknowledgements Alain Durand, Alain Baudot, Luc Beloeil, and Andras Kis-Szabo provided feedback to improve this memo. Michael Wittsend and Michael Cole discussed issues relating to probing/mapping and privacy. 6. Security Considerations This memo tries to give an overview of security considerations of the different aspects of IPv6. 7. References 7.1. Normative [TRANS] Savola, P., "A View on IPv6 Transition Architecture", draft-savola-v6ops-transarch-03.txt, Jan 2004. 7.2. Informative [3041HARM] Dupont, F., Savola, P., "RFC 3041 Considered Harmful", draft-dupont-ipv6-rfc3041harmful-04.txt, Feb 2004. [6BMESS] Savola, P., "Moving from 6bone to IPv6 Internet", draft-savola-v6ops-6bone-mess-01.txt, Nov 2002. [6TO4SEC] Savola, P., Patel, C., "Security Considerations for 6to4", draft-ietf-v6ops-6to4-security-01.txt, Feb 2004. [DNSA4] Morishita., Y., Jinmei, T., "Common Misbehavior against DNS Queries for IPv6 Addresses", draft-morishita-dnsop- misbehavior-against-aaaa-00.txt, Jun 2003. Savola [Expires August 2004] [Page 9] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 [FNAT] Bellovin, S., "A Technique for Counting NATted Hosts", Second Internet Measurement Workshop, http://www.research.att.com/~smb/papers/fnat.pdf, Nov 2003. [FW] Savola, P. "Firewalling Considerations for IPv6", draft-savola-v6ops-firewalling-02.txt, Oct 2003. [MAPPING] Schild, C., Strauf, C., "Guide to Mapping IPv4 to IPv6 Subnets", draft-schild-v6ops-guide-v4mapping-00.txt, Dec 2003. [ONBYDEF] Roy, S., et al., "Dual Stack IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-00.txt, Jun 2003. [PORTSCAN] Chown, T., "IPv6 Implications for TCP/UDP Port Scanning", draft-chown-v6ops-port-scanning-implications-00.txt, Oct 2003. [RHHAOSEC] Savola, P. "Security of IPv6 Routing Header and Home Address Options", draft-savola-ipv6-rh-ha-security-03.txt, Dec 2002. [RHHOSTS] Savola, P. "Note about Routing Header Processing on IPv6 Hosts", draft-savola-ipv6-rh-hosts-00.txt, Feb 2002. [RRSEC] Nikander, P, et al., "Mobile IP version 6 Route Optimization Security Design Background", draft-nikander-mobileip-v6-ro-sec-02.txt, Dec 2003. [SEND] Arkko, J., et al., "SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-03.txt, Jan 2004. [SENDREQ] Nikander, P., et al., "IPv6 Neighbor Discovery trust models and threats", draft-ietf-send-psreq-04.txt, Oct 2003. [SIIT] Nordmark, E., "Stateless IP/ICMP Translation Algorithm", RFC276, February 2000. [V4MAPPEDA] Metz, C., Hagino, J., "IPv4-Mapped address API considered harmful", draft-cmetz-v6ops-v4mapped-api-harmful-01.txt, Oct 2003. [V4MAPPEDW] Metz, C., Hagino, J., "IPv4-Mapped Addresses on the Wire Considered Harmful", draft-itojun-v6ops-v4mapped-harmful-02.txt, Oct 2003. Savola [Expires August 2004] [Page 10] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 Author's Address Pekka Savola CSC/FUNET Espoo, Finland EMail: psavola@funet.fi A. IPv6 Probing/Mapping Considerations Some want the IPv6 numbering topology (either at network or node level) [MAPPING] match IPv4 as exactly as possible, the others see this as a security threat because IPv6 could have different security properties than IPv4. That is, if an attacker knows the IPv4 address of the node, he might want to try to probe the corresponding IPv6 address, based on the assumption that the security defenses might be lower. This might be the case particularly for nodes which are behind a NAT in IPv4, but globally addressable in IPv6. Naturally, this is not a concern if the similar security policies are in place. On the other hand, brute-force scanning or probing is unfeasible [PORTSCAN]. For example, automatic tunneling mechanisms use rather deterministic methods for generating IPv6 addresses, so probing/port-scanning an IPv6 node is simplified. The IPv4 address is embedded at least in 6to4, Teredo and ISATAP address. Further than that, it's possible (in the case of 6to4 in particular) to learn the address behind the prefix; for example, Microsoft 6to4 implementation uses the address 2002:V4ADDR::V4ADDR while Linux and BSD implementations default to 2002:V4ADDR::1. This could also be used as one way to identify an implementation. One proposal has been to randomize the addresses or Subnet identifier in the address of the 6to4 router. This doesn't really help, as the 6to4 router (whether a host or a router) will return an ICMPv6 Hop Limit Exceeded message, revealing the IP address. Hosts behind the 6to4 router can use methods such as RFC 3041 addresses to conceal themselves, though. To conclude, it seems that with an IPv4 address, the respective IPv6 address, when automatic tunneling mechanism is being used, could possibly be guessed with relative ease. This has significant implications if the IPv6 security policy isn't the same as IPv4. Savola [Expires August 2004] [Page 11] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 B. IPv6 Privacy Considerations It has been claimed that IPv6 harms the privacy of the user, either by exposing the MAC address, or by exposing the number of nodes connected to a site. B.1. Exposing MAC Addresses The MAC address, which with stateless address autoconfiguration, results in an EUI64, exposes the model of network card. The concern has been that a user might not want to expose the details of the system to outsiders, e.g., in the fear of a resulting burglary (e.g., if a crook identifies expensive equipment from the MAC addresses). In most cases, this seems completely unfounded. First, such an address must be learned somehow -- this is a non-trivial process; the addresses are visible e.g., in web site access logs, but the chances that a random web site owner is collecting this kind of information (or whether it would be of any use) are quite slim. Being able to eavesdrop the traffic to learn such addresses (e.g., by the compromise of DSL or Cable modem physical media) seems also quite far-fetched. Further, using RFC 3041 addresses for such purposes is straightforward if worried about the risk. Second, the burglar would have to be able to map the IP address to the physical location; this is typically only available in the private customer database of the ISP. B.2. Exposing Multiple Devices Another presented concern is whether the user wants to show off as having a lot of computers or other devices at a network; NAT "hides" everything behind an address, but is not perfect either [FNAT]. One practical reason why some may find this desirable is being able to thwart certain ISPs' business models, where one should pay extra for additional computers (and not the connectivity as a whole). Similar feasibility issues as described above apply. To a degree, the counting avoidance could be performed by the sufficiently frequent re-use of RFC 3041 addresses -- that is, if during a short period, dozens of generated addresses seem to be in use, it's difficult to estimate whether they are generated by just one host or multiple hosts. Savola [Expires August 2004] [Page 12] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF Savola [Expires August 2004] [Page 13] Internet Draft draft-savola-v6ops-security-overview-01.txt February 2004 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Savola [Expires August 2004] [Page 14]