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Gont 3 Internet-Draft UK CPNI 4 Intended status: BCP February 2, 2012 5 Expires: August 5, 2012 7 Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard) 8 draft-gont-v6ops-ra-guard-implementation-01 10 Abstract 12 The IPv6 Router Advertisement Guard (RA-Guard) mechanism is commonly 13 employed to mitigate attack vectors based on forged ICMPv6 Router 14 Advertisement messages. Many existing IPv6 deployments rely on RA- 15 Guard as the first line of defense against the aforementioned attack 16 vectors. However, some implementations of RA-Guard have been found 17 to be prone to circumvention by employing IPv6 Extension Headers. 18 This document describes the evasion techniques that affect the 19 aforementioned implementations, and provides advice on the 20 implementation of RA-Guard, such that the RA-Guard evasion vectors 21 are eliminated. 23 Status of this Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. This document may not be modified, 27 and derivative works of it may not be created, and it may not be 28 published except as an Internet-Draft. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on August 5, 2012. 42 Copyright Notice 44 Copyright (c) 2012 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2. Evasion techniques for some Router Advertisement Guard (RA 61 Guard) implementations . . . . . . . . . . . . . . . . . . . . 4 62 2.1. Attack Vector based on IPv6 Extension Headers . . . . . . 4 63 2.2. Attack vector based on IPv6 fragmentation . . . . . . . . 4 64 3. RA-Guard implementation advice . . . . . . . . . . . . . . . . 8 65 4. Other Implications . . . . . . . . . . . . . . . . . . . . . . 9 66 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 67 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 68 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 69 7.1. Normative References . . . . . . . . . . . . . . . . . . . 12 70 7.2. Informative References . . . . . . . . . . . . . . . . . . 12 71 Appendix A. Changes from previous versions of the draft (to 72 be removed by the RFC Editor before publication 73 of this document as a RFC . . . . . . . . . . . . . . 13 74 A.1. Changes from 75 draft-gont-v6ops-ra-guard-implementation-00 . . . . . . . 13 76 A.2. Changes from draft-gont-v6ops-ra-guard-evasion-01 . . . . 13 77 Appendix B. Assessment tools . . . . . . . . . . . . . . . . . . 14 78 Appendix C. Advice and guidance to vendors . . . . . . . . . . . 15 79 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 81 1. Introduction 83 IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique 84 for attack vectors based on ICMPv6 Router Advertisement messages. 85 [RFC6104] describes the problem statement of "Rogue IPv6 Router 86 Advertisements", and [RFC6105] specifies the "IPv6 Router 87 Advertisement Guard" functionality. 89 The basic concept behind RA-Guard is that a layer-2 device filters 90 ICMPv6 Router Advertisement messages, according to a number of 91 different criteria. The most basic filtering criterion is that 92 Router Advertisement messages are discarded by the layer-2 device 93 unless they are received on a specified port of the layer-2 device. 94 Clearly, the effectiveness of the RA Guard mitigation relies on the 95 ability of the layer-2 device to identify ICMPv6 Router Advertisement 96 messages. 98 Some popular RA-Guard implementations have been found to be easy to 99 circumvent by employing IPv6 extension headers [CPNI-IPv6]. This 100 document describes such evasion techniques, and provides advice to 101 RA-Guard implementers such that the aforementioned evasion vectors 102 can be eliminated. 104 It should be noted that the aforementioned techniques could also be 105 exploited to evade network monitoring tools such as NDPMon [NDPMon], 106 ramond [ramond], and rafixd [rafixd], and could probably be exploited 107 to perform stealth DHCPv6 attacks. 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in RFC 2119 [RFC2119]. 113 2. Evasion techniques for some Router Advertisement Guard (RA Guard) 114 implementations 116 The following subsections describe two different vectors that have 117 been found to be effective for the evasion of popular implementations 118 of the RA-Guard protection. Section 2.1 describes an attack vector 119 based on the use of IPv6 Extension Headers with the ICMPv6 Router 120 Advertisement messages, which may be used to circumvent the RA-Guard 121 protection of those implementations that fail to process an entire 122 IPv6 header chain when trying to identify the ICMPv6 Router 123 Advertisement messages. Section 2.2 describes an attack method based 124 on the use of IPv6 fragmentation, possibly in conjunction with the 125 use of IPv6 Extension Headers. This later vector has been found to 126 be effective with all existing implementations of the RA-Guard 127 mechanism. 129 2.1. Attack Vector based on IPv6 Extension Headers 131 While there is currently no legitimate use for IPv6 Extension Headers 132 in ICMPv6 Router Advertisement messages, Neighbor Discovery 133 implementations allow the use of Extension Headers with these 134 messages, by simply ignoring the received options. Some RA-Guard 135 implementations try to identify ICMPv6 Router Advertisement messages 136 by simply looking at the "Next Header" field of the fixed IPv6 137 header, rather than following the entire header chain. As a result, 138 such implementations fail to identify any ICMPv6 Router Advertisement 139 messages that include any Extension Headers (for example, a Hop by 140 Hop Options header, a Destination Options Header, etc.), and can be 141 easily circumvented. 143 The following figure illustrates the structure of ICMPv6 Router 144 Advertisement messages that implement this evasion technique: 146 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 147 |NH=60| |NH=58| | | 148 +-+-+-+ +-+-+-+ + + 149 | IPv6 header | Dst Opt Hdr | ICMPv6 Router Advertisement | 150 + + + + 151 | | | | 152 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 154 2.2. Attack vector based on IPv6 fragmentation 156 This section presents a different attack vector, which has been found 157 to be effective against all implementations of RA-Guard. The basic 158 idea behind this attack vector is that if the forged ICMPv6 Router 159 Advertisement is fragmented into at least two fragments, the layer-2 160 device implementing "RA-Guard" would be unable to identify the attack 161 packet, and would thus fail to block it. 163 A first variant of this attack vector would be an original ICMPv6 164 Router Advertisement message preceded with a Destination Options 165 Header, that results in two fragments. The following figure 166 illustrates the "original" attack packet, prior to fragmentation, and 167 the two resulting fragments which are actually sent as part of the 168 attack. 170 Original packet: 172 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 173 |NH=60| |NH=58| | | 174 +-+-+-+ +-+-+-+ + + 175 | IPv6 header | Dst Opt Hdr | ICMPv6 RA | 176 + + + + 177 | | | | 178 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 180 First fragment: 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 |NH=44| |NH=60| |NH=58| | 184 +-+-+-+ +-+-+-+ +-+-+-+ + 185 | IPv6 Header | Frag Hdr | Dst Opt Hdr | 186 + + + + 187 | | | | 188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 190 Second fragment: 192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 193 |NH=44| |NH=60| | | | 194 +-+-+-+ +-+-+-+ + + + 195 | IPv6 header | Frag Hdr | Dst Opt Hdr | ICMPv6 RA | 196 + + + + + 197 | | | | | 198 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 200 It should be noted that the "Hdr Ext Len" field of the Destination 201 Options Header is present in the first fragment (rather than the 202 second). Therefore, it is impossible for a device processing only 203 the second fragment to locate the ICMPv6 header contained in that 204 fragment, since it is unknown how many bytes should be "skipped" to 205 get to the next header following the Destination Options Header. 207 Thus, by leveraging the use of the Fragment Header together with the 208 use of the Destination Options header, the attacker is able to 209 conceal the type and contents of the ICMPv6 message he is sending (an 210 ICMPv6 Router Advertisement in this example). Unless the layer-2 211 device were to implement IPv6 fragment reassembly, it would be 212 impossible for the device to identify the ICMPv6 type of the message. 214 A layer-2 device could, however, at least detect that that an 215 ICMPv6 message (or some type) is being sent, since the "Next 216 Header" field of the Destination Options header contained in the 217 first fragment is set to "58" (ICMPv6). 219 This idea can be taken further, such that it is also impossible for 220 the layer-2 device to detect that the attacker is sending an ICMPv6 221 message in the first place. This can be achieved with an original 222 ICMPv6 Router Advertisement message preceded with two Destination 223 Options Headers, that results in two fragments. The following figure 224 illustrates the "original" attack packet, prior to fragmentation, and 225 the two resulting packets which are actually sent as part of the 226 attack. 228 Original packet: 230 +-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 231 |NH=60| |NH=60| |NH=58| | | 232 +-+-+-+ +-+-+-+ +-+-+-+ + + 233 | IPv6 header | Dst Opt Hdr | Dst Opt Hdr | ICMPv6 RA | 234 + + + + + 235 | | | | | 236 +-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 238 First fragment: 240 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 241 |NH=44| |NH=60| |NH=60| | 242 +-+-+-+ +-+-+-+ +-+-+-+ + 243 | IPv6 header | Frag Hdr | Dst Opt Hdr | 244 + + + + 245 | | | | 246 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 248 Second fragment: 250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 251 |NH=44| |NH=60| | |NH=58| | | 252 +-+-+-+ +-+-+-+ + +-+-+-+ + + 253 | IPv6 header | Frag Hdr | Dst O Hdr | Dst Opt Hdr | ICMPv6 RA | 254 + + + + + + 255 | | | | | | 256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 258 In this variant, the "Next Header" field of the Destination Options 259 header contained in the first fragment is set "60" (Destination 260 Options header), and thus it is impossible for a device processing 261 only the first fragment to detect that an ICMPv6 message is being 262 sent in the first place. 264 The second fragment presents the same challenges as the second 265 fragment of the previous variant. That is, it would be impossible 266 for a device processing only the second fragment to locate the second 267 Destination Options header (and hence the ICMPv6 header), since the 268 "Hdr Ext Len" field of the first Destination Options header is 269 present in the first fragment (rather than the second). 271 3. RA-Guard implementation advice 273 The following filtering rules MUST be implemented as part of an "RA- 274 Guard" implementation on those ports that are not allowed to send 275 ICMPv6 Router Advertisement messages, such that the vulnerabilities 276 discussed in this document are eliminated: 278 1. When trying to identify an ICMPv6 Router Advertisement message, 279 follow the IPv6 header chain, enforcing a limit on the maximum 280 number of Extension Headers that is allowed for each packet. If 281 such limit is hit before the upper-layer protocol is can be 282 determined, silently drop the packet. 284 2. If the packet is identified to be an ICMPv6 Router Advertisement 285 message, silently drop the packet. 287 3. If the layer-2 device is unable to identify whether the packet is 288 an ICMPv6 Router Advertisement message or not (i.e., the packet 289 is a fragment, and the necessary information is missing), the 290 IPv6 Source Address of the packet is a link-local address or the 291 unspecified address (::), and the Hop Limit is 255, silently drop 292 the packet. 294 4. In all other cases, pass the packet as usual. 296 This filtering policy assumes that host implementations require that 297 the IPv6 Source Address of ICMPv6 Router Advertisement messages be a 298 link-local address, and that they discard the packet if this check 299 fails, as required by the current IETF specifications [RFC4861]. 300 Additionally, it assumes that hosts require the Hop Limit of Neighbor 301 Discovery messages to be 255, and discard those packets otherwise. 303 Note that the aforementioned filtering rules implicitly handle the 304 case of fragmented packets: if the RA-Guard device fails to identify 305 the upper-layer protocol as a result of the use of fragmentation, the 306 corresponding packets would be silently dropped. 308 4. Other Implications 310 A similar concept to that of "RA-Guard" has been implemented for 311 protecting against forged DHCPv6 messages. Such protection can be 312 circumvented with the same techniques discussed in this document, and 313 the counter-measures for such evasion attack are analogous to those 314 described in Section 3 of this document. 316 5. Security Considerations 318 This document describes a number of techniques that have been found 319 to be effective to circumvent popular RA-Guard implementations. 321 The most effective and efficient mitigation for these attacks would 322 be to prohibit the use of some IPv6 extension headers with Router 323 Advertisement messages (as proposed by 324 [I-D.gont-6man-nd-extension-headers]), such that the RA-Guard 325 functionality is easier to implement. However, since such mitigation 326 would require an update to existing implementations, it cannot be 327 relied upon in the short or near term. 329 6. Acknowledgements 331 The author would like to thank Ran Atkinson, Karl Auer, Robert 332 Downie, David Farmer, Marc Heuse, Simon Perreault, Arturo Servin, and 333 Gunter van de Velde, for providing valuable comments on earlier 334 versions of this document. 336 This document resulted from the project "Security Assessment of the 337 Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by 338 Fernando Gont on behalf of the UK Centre for the Protection of 339 National Infrastructure (CPNI). The author would like to thank the 340 UK CPNI, for their continued support. 342 7. References 344 7.1. Normative References 346 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 347 Requirement Levels", BCP 14, RFC 2119, March 1997. 349 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 350 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 351 September 2007. 353 7.2. Informative References 355 [RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement 356 Problem Statement", RFC 6104, February 2011. 358 [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J. 359 Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, 360 February 2011. 362 [I-D.gont-6man-nd-extension-headers] 363 Gont, F., "Security Implications of the Use of IPv6 364 Extension Headers with IPv6 Neighbor Discovery", 365 draft-gont-6man-nd-extension-headers-02 (work in 366 progress), January 2012. 368 [CPNI-IPv6] 369 Gont, F., "Security Assessment of the Internet Protocol 370 version 6 (IPv6)", UK Centre for the Protection of 371 National Infrastructure, (available on request). 373 [NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor", 374 . 376 [rafixd] "rafixd", . 379 [ramond] "ramond", . 381 [THC-IPV6] 382 "THC-IPV6", . 384 Appendix A. Changes from previous versions of the draft (to be removed 385 by the RFC Editor before publication of this document as a 386 RFC 388 A.1. Changes from draft-gont-v6ops-ra-guard-implementation-00 390 o Miscellaneous (minor) editorial changes. 392 o The filtering rules in Section 3 have been polished. 394 A.2. Changes from draft-gont-v6ops-ra-guard-evasion-01 396 o The contents were updated to reflect that the evasion 397 vulnerabilities are based on implementation flaws, rather than on 398 the RA-Guard "concept" itself. 400 o The I-D now focuses on providing advice to RA-Guard implementers. 402 Appendix B. Assessment tools 404 CPNI has produced assessment tools (which have not yet been made 405 publicly available) to assess RA-Guard implementations with respect 406 to the issues described in this document. If you think that you 407 would benefit from these tools, we might be able to provide a copy of 408 the tools (please contact Fernando Gont at fernando@gont.com.ar). 410 [THC-IPV6] is a publicly-available set of tools that implements some 411 of the techniques described in this document. 413 Appendix C. Advice and guidance to vendors 415 Vendors are urged to contact CSIRTUK (csirt@cpni.gsi.gov.uk) if they 416 think they may be affected by the issues described in this document. 417 As the lead coordination centre for these issues, CPNI is well placed 418 to give advice and guidance as required. 420 CPNI works extensively with government departments and agencies, 421 commercial organisations and the academic community to research 422 vulnerabilities and potential threats to IT systems especially where 423 they may have an impact on Critical National Infrastructure's (CNI). 425 Other ways to contact CPNI, plus CPNI's PGP public key, are available 426 at http://www.cpni.gov.uk. 428 Author's Address 430 Fernando Gont 431 Centre for the Protection of National Infrastructure 433 Email: fgont@si6networks.com 434 URI: http://www.cpni.gov.uk