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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IPv6 maintenance Working Group (6man) F. Gont 3 Internet-Draft SI6 Networks / UTN-FRH 4 Updates: 3971, 4861 (if approved) December 10, 2012 5 Intended status: Standards Track 6 Expires: June 13, 2013 8 Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery 9 draft-ietf-6man-nd-extension-headers-02 11 Abstract 13 This document analyzes the security implications of using IPv6 14 Extension Headers with Neighbor Discovery (ND) messages. It updates 15 RFC 4861 such that use of the IPv6 Fragmentation Header is forbidden 16 in all Neighbor Discovery messages, thus allowing for simple and 17 effective counter-measures for Neighbor Discovery attacks. Finally, 18 it discusses the security implications of using IPv6 fragmentation 19 with SEcure Neighbor Discovery (SEND), and formally updates RFC 3971 20 to provide advice regarding how the aforementioned security 21 implications can be prevented. 23 Status of this Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at http://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on June 13, 2013. 40 Copyright Notice 42 Copyright (c) 2012 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (http://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Traditional Neighbor Discovery and IPv6 Fragmentation . . . . 5 59 3. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation . . . 6 60 4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 7 61 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 62 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 63 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 64 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 65 8.1. Normative References . . . . . . . . . . . . . . . . . . . 11 66 8.2. Informative References . . . . . . . . . . . . . . . . . . 11 67 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 69 1. Introduction 71 The Neighbor Discovery Protocol (NDP) is specified in RFC 4861 72 [RFC4861] and RFC 4862 [RFC4862]. It is used by both hosts and 73 routers. Its functions include Neighbor Discovery (ND), Router 74 Discovery (RD), Address Autoconfiguration, Address Resolution, 75 Neighbor Unreachability Detection (NUD), Duplicate Address Detection 76 (DAD), and Redirection. 78 Many of the possible attacks against the Neighbor Discovery Protocol 79 are discussed in detail in [RFC3756]. In order to mitigate the 80 aforementioned possible attacks, the SEcure Neighbor Discovery (SEND) 81 was standardized. SEND employs a number of mechanisms to certify the 82 origin of Neighbor Discovery packets and the authority of routers, 83 and to protect Neighbor Discovery packets from being the subject of 84 modification or replay attacks. 86 However, a number of factors, such as the use of trust anchors and 87 the unavailability of SEND implementations for many widely-deployed 88 operating systems, make SEND hard to deploy [Gont-DEEPSEC2011]. 89 Thus, in many general scenarios it may be necessary and/or convenient 90 to use other mitigation techniques for NDP-based attacks. The 91 following mitigations are currently available for NDP attacks: 93 o Layer-2 filtering of Neighbor Discovery packets (such as RA-Guard 94 [RFC6105]) 96 o Neighbor Discovery monitoring tools (e.g., such as NDPMon 97 [NDPMon]) 99 o Intrusion Prevention Systems (IPS) 101 IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique 102 for attack vectors based on ICMPv6 Router Advertisement messages. It 103 is meant to block attack packets at a layer-2 device before the 104 attack packets actually reach the target nodes. [RFC6104] describes 105 the problem statement of "Rogue IPv6 Router Advertisements", and 106 [RFC6105] specifies the "IPv6 Router Advertisement Guard" 107 functionality. 109 Tools such as NDPMon [NDPMon] and ramond [ramond] aim at monitoring 110 Neighbor Discovery traffic in the hopes of detecting possible attacks 111 when there are discrepancies between the information advertised in 112 Neighbor Discovery packets and the information stored on a local 113 database. 115 Some Intrusion Prevention Systems (IPS) can mitigate Neighbor 116 Discovery attacks. We recommend that Intrusion Prevention Systems 117 (IPS) implement mitigations for NDP attacks. 119 A key challenge that these mitigation or monitoring techniques face 120 is that introduced by IPv6 fragmentation, since it is trivial for an 121 attacker to conceal his attack by fragmenting his packets into 122 multiple fragments. This may limit or even eliminate the 123 effectiveness of the aforementioned mitigation or monitoring 124 techniques. Recent work [CPNI-IPv6] indicates that current 125 implementations of the aforementioned mitigations for NDP attacks can 126 be trivially evaded. For example, as noted in 127 [I-D.ietf-v6ops-ra-guard-implementation], current RA-Guard 128 implementations can be trivially evaded by fragmenting the attack 129 packets into multiple fragments, such that the layer-2 device cannot 130 find all the necessary information to perform packet filtering in the 131 same packet. While Neighbor Discovery monitoring tools could (in 132 theory implement IPv6 fragment reassembly, this is usually an arms- 133 race with the attacker (an attacker generate lots of forged fragments 134 to "confuse" the monitoring tools), and therefore the aforementioned 135 tools are unreliable for the detection of such attacks. 137 Section 2 analyzes the use of IPv6 fragmentation in traditional 138 Neighbor discovery. Section 3 analyzes the use of IPv6 fragmentation 139 in SEcure Neighbor Discovery (SEND). Section 4 formally updates RFC 140 4861 such that use of the IPv6 Fragment Header with traditional 141 Neighbor Discovery is forbidden, and also formally updates RFC 3971 142 providing advice on the use of IPv6 fragmentation with SEND. 144 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 145 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 146 document are to be interpreted as described in RFC 2119 [RFC2119]. 148 2. Traditional Neighbor Discovery and IPv6 Fragmentation 150 The only potential use case for IPv6 fragmentation with traditional 151 (i.e., non-SEND) IPv6 Neighbor Discovery would be that in which a 152 Router Advertisement must include a large number of options (Prefix 153 Information Options, Route Information Options, etc.). However, this 154 could still be achieved without employing fragmentation, by splitting 155 the aforementioned information into multiple Router Advertisement 156 messages. 158 Some Neighbor Discovery implementations are known to silently 159 ignore Router Advertisement messages that employ fragmentation. 160 Therefore, splitting the necessary information into multiple RA 161 messages (rather than sending a large RA message that is 162 fragmented into multiple IPv6 fragments) is probably desirable 163 even from an interoperability point of view. 165 As a result of the aforementioned considerations, and since avoiding 166 the use of IPv6 fragmentation in traditional Neighbor Discovery would 167 greatly simplify and improve the effectiveness of monitoring and 168 filtering ND, Section 4 specifies that hosts silently ignore 169 traditional Neighbor Discovery messages (i.e., those specified in 170 [RFC4861]) that employ IPv6 fragmentation. 172 3. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation 174 SEND packets typically carry more information than traditional 175 Neighbor Discovery packets: for example, they include additional 176 options such as the CGA option and the RSA signature option. 178 When SEND nodes employ any of the Neighbor Discovery messages 179 specified in [RFC4861], the situation is roughly the same: if more 180 information than would fit in a non-fragmented Neighbor Discovery 181 packet needs to be sent, it should be split into multiple Neighbor 182 Discovery messages (such that IPv6 fragmentation is avoided). 184 However, Certification Path Advertisement messages (specified in 185 [RFC3971]) pose a different situation, since the Certificate Option 186 they include typically contains much more information than any other 187 Neighbor Discovery option. For example, Appendix C of [RFC3971] 188 reports Certification Path Advertisement messages from 1050 to 1066 189 bytes on an Ethernet link layer. Since the size of CPA messages 190 could potentially exceed the MTU of the local link, Section 4 191 recommends that fragmented CPA messages be normally processed, but 192 discourages the use of keys that would result in fragmented CPA 193 messages. 195 It should be noted that relying on fragmentation opens the door to a 196 variety of IPv6 fragmentation-based attacks. In particular, if an 197 attacker is located on the same broadcast domain as the victim host, 198 and Certification Path Advertisement messages employ IPv6 199 fragmentation, it would be trivial for the attacker to forge IPv6 200 fragments such that they result in "Fragment ID collisions", causing 201 both the attack fragments and the legitimate fragments to be 202 discarded by the victim node. This would eventually cause the 203 Authorization Delegation Discovery to fail, thus leading the host to 204 fall back (depending on local configuration) either to unsecured 205 mode, or to reject the corresponding Router Advertisement messages 206 (possibly resulting in a Denial of Service). 208 4. Specification 210 Nodes MUST NOT employ IPv6 fragmentation for sending any of the 211 following Neighbor Discovery and SEcure Neighbor Discovery messages: 213 o Neighbor Solicitation 215 o Neighbor Advertisement 217 o Router Solicitation 219 o Router Advertisement 221 o Redirect 223 o Certification Path Solicitation 225 Nodes SHOULD NOT employ IPv6 fragmentation for sending the following 226 messages: 228 o Certification Path Advertisement messages 230 Nodes MUST silently ignore the following Neighbor Discovery and 231 SEcure Neighbor Discovery messages if the packets carrying them 232 include an IPv6 Fragmentation Header: 234 o Neighbor Solicitation 236 o Neighbor Advertisement 238 o Router Solicitation 240 o Router Advertisement 242 o Redirect 244 o Certification Path Solicitation 246 Nodes SHOULD normally process the following messages when the packets 247 carrying them include an IPv6 Fragmentation Header: 249 o Certification Path Advertisement 251 SEND nodes SHOULD NOT employ keys that would result in fragmented CPA 252 messages. 254 5. IANA Considerations 256 There are no IANA registries within this document. The RFC-Editor 257 can remove this section before publication of this document as an 258 RFC. 260 6. Security Considerations 262 The IPv6 Fragmentation Header can be leveraged to circumvent network 263 monitoring tools and current implementations of mechanisms such as 264 RA-Guard [I-D.ietf-v6ops-ra-guard-implementation]. By updating the 265 relevant specifications such that the IPv6 Fragment Header is not 266 allowed in any Neighbor Discovery messages except "Certification Path 267 Advertisement", protection of local nodes against Neighbor Discovery 268 attacks, and monitoring of Neighbor Discovery traffic is greatly 269 simplified. 271 [I-D.ietf-v6ops-ra-guard-implementation] discusses an improvement to 272 the RA-Guard mechanism that can mitigate Neighbor Discovery attacks 273 that employ IPv6 Fragmentation. However, it should be noted that 274 unless [RFC4861] is updated (as proposed in this document), Neighbor 275 Discovery monitoring tools (such as NDPMon [NDPMon]) would remain 276 unreliable and trivial to circumvent by a skilled attacker. 278 As noted in Section 3, use of SEND could potentially result in 279 fragmented "Certification Path Advertisement" messages, thus allowing 280 an attacker to employ IPv6 fragmentation-based attacks against such 281 messages. Therefore, to the extent that is possible, such use of 282 fragmentation should be avoided. 284 7. Acknowledgements 286 The author would like to thank (in alphabetical order) Mikael 287 Abrahamsson, Ran Atkinson, Ron Bonica, Jean-Michel Combes, David 288 Farmer, Roque Gagliano, Bob Hinden, Philip Homburg, Ray Hunter, 289 Arturo Servin, and Mark Smith, for providing valuable comments on 290 earlier versions of this document. 292 This document resulted from the project "Security Assessment of the 293 Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by 294 Fernando Gont on behalf of the UK Centre for the Protection of 295 National Infrastructure (CPNI). The author would like to thank the 296 UK CPNI, for their continued support. 298 8. References 300 8.1. Normative References 302 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 303 Requirement Levels", BCP 14, RFC 2119, March 1997. 305 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure 306 Neighbor Discovery (SEND)", RFC 3971, March 2005. 308 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 309 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 310 September 2007. 312 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 313 Address Autoconfiguration", RFC 4862, September 2007. 315 8.2. Informative References 317 [RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor 318 Discovery (ND) Trust Models and Threats", RFC 3756, 319 May 2004. 321 [RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement 322 Problem Statement", RFC 6104, February 2011. 324 [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J. 325 Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, 326 February 2011. 328 [NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor", 329 . 331 [ramond] "ramond", . 333 [I-D.ietf-v6ops-ra-guard-implementation] 334 Gont, F., "Implementation Advice for IPv6 Router 335 Advertisement Guard (RA-Guard)", 336 draft-ietf-v6ops-ra-guard-implementation-07 (work in 337 progress), November 2012. 339 [CPNI-IPv6] 340 Gont, F., "Security Assessment of the Internet Protocol 341 version 6 (IPv6)", UK Centre for the Protection of 342 National Infrastructure, (available on request). 344 [Gont-DEEPSEC2011] 345 Gont, "Results of a Security Assessment of the Internet 346 Protocol version 6 (IPv6)", DEEPSEC 2011 Conference, 347 Vienna, Austria, November 2011, . 351 Author's Address 353 Fernando Gont 354 SI6 Networks / UTN-FRH 355 Evaristo Carriego 2644 356 Haedo, Provincia de Buenos Aires 1706 357 Argentina 359 Phone: +54 11 4650 8472 360 Email: fgont@si6networks.com 361 URI: http://www.si6networks.com