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2 IPv6 Operations Working Group (v6ops) F. Gont
3 Internet-Draft UK CPNI
4 Intended status: Informational June 8, 2011
5 Expires: December 10, 2011
7 IPv6 Router Advertisement Guard (RA-Guard) Evasion
8 draft-gont-v6ops-ra-guard-evasion-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. This document describes possible ways in which current RA-
17 Guard implementations can be circumvented, and discusses possible
18 mitigations.
20 Status of this Memo
22 This Internet-Draft is submitted in full conformance with the
23 provisions of BCP 78 and BCP 79. This document may not be modified,
24 and derivative works of it may not be created, and it may not be
25 published except as an Internet-Draft.
27 Internet-Drafts are working documents of the Internet Engineering
28 Task Force (IETF). Note that other groups may also distribute
29 working documents as Internet-Drafts. The list of current Internet-
30 Drafts is at http://datatracker.ietf.org/drafts/current/.
32 Internet-Drafts are draft documents valid for a maximum of six months
33 and may be updated, replaced, or obsoleted by other documents at any
34 time. It is inappropriate to use Internet-Drafts as reference
35 material or to cite them other than as "work in progress."
37 This Internet-Draft will expire on December 10, 2011.
39 Copyright Notice
41 Copyright (c) 2011 IETF Trust and the persons identified as the
42 document authors. All rights reserved.
44 This document is subject to BCP 78 and the IETF Trust's Legal
45 Provisions Relating to IETF Documents
46 (http://trustee.ietf.org/license-info) in effect on the date of
47 publication of this document. Please review these documents
48 carefully, as they describe your rights and restrictions with respect
49 to this document. Code Components extracted from this document must
50 include Simplified BSD License text as described in Section 4.e of
51 the Trust Legal Provisions and are provided without warranty as
52 described in the Simplified BSD License.
54 Table of Contents
56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
57 2. Router Advertisement Guard (RA Guard) Evasion Vulnerability . 4
58 2.1. Attack Vector based on IPv6 Extension Headers . . . . . . 4
59 2.2. Attack vector based on IPv6 fragmentation . . . . . . . . 4
60 3. Mitigations . . . . . . . . . . . . . . . . . . . . . . . . . 8
61 4. Other Implications . . . . . . . . . . . . . . . . . . . . . . 9
62 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
63 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
64 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
65 7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
66 7.2. Informative References . . . . . . . . . . . . . . . . . . 12
67 Appendix A. Changes from previous versions of the draft (to
68 be removed by the RFC Editor before publication
69 of this document as a RFC . . . . . . . . . . . . . . 13
70 A.1. Changes from draft-gont-v6ops-ra-guard-evasion-00 . . . . 13
71 Appendix B. Assessment tools . . . . . . . . . . . . . . . . . . 14
72 Appendix C. Advice and guidance to vendors . . . . . . . . . . . 15
73 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
75 1. Introduction
77 IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique
78 for attack vectors based on ICMPv6 Router Advertisement messages.
79 [RFC6104] describes the problem statement of "Rogue IPv6 Router
80 Advertisements", and [RFC6105] specifies the "IPv6 Router
81 Advertisement Guard" functionality.
83 The basic concept behind RA-Guard is that a layer-2 device filters
84 ICMPv6 Router Advertisement messages, according to a number of
85 different criteria. The most basic filtering criterion is that
86 Router Advertisement messages are discarded by the layer-2 device
87 unless they are received on a specified port of the layer-2 device.
88 Clearly, the effectiveness of the RA Guard mitigation relies on the
89 ability of the layer-2 device to identify ICMPv6 Router Advertisement
90 messages.
92 As part of the project "Security Assessment of the Internet Protocol
93 version 6 (IPv6)" [CPNI-IPv6], we have devised two techniques for
94 circumventing the RA-Guard protection, which are described in the
95 following sections of this document. These techniques, and the
96 corresponding tools to assess their effectiveness, had so far been
97 made available only to vendors, in the hopes that they could
98 implement counter-measures before they were publicly disclosed.
99 However, since there has been some public discussion about these
100 issues, it was deemed as appropriate to publish the present document.
102 It should be noted that the aforementioned techniques could also be
103 exploited to evade network monitoring tools such as NDPMon [NDPMon],
104 ramond [ramond], and rafixd [rafixd], and could probably be exploited
105 to perform stealth DHCPv6 attacks.
107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
109 document are to be interpreted as described in RFC 2119 [RFC2119].
111 2. Router Advertisement Guard (RA Guard) Evasion Vulnerability
113 The following subsections describe two different vectors for evading
114 the RA-Guard protection. Section 2.1 describes an attack vector
115 based on the use of IPv6 Extension Headers with the ICMPv6 Router
116 Advertisement messages, which may be used to circumvent the RA-Guard
117 protection of those implementations that fail to process an entire
118 IPv6 header chain when trying to identify the ICMPv6 Router
119 Advertisement messages. Section 2.2 describes an attack method based
120 on the use of IPv6 fragmentation, possibly in conjunction with the
121 use of IPv6 Extension Headers. This later vector is expected to be
122 effective with all existing implementations of the RA-Guard
123 functionality.
125 2.1. Attack Vector based on IPv6 Extension Headers
127 While there is currently no legitimate use for IPv6 Extension Headers
128 in ICMPv6 Router Advertisement messages, Neighbor Discovery
129 implementations allow the use of Extension Headers with these
130 messages, by simply ignoring the received options. We believe that
131 some implementations may simply try to identify ICMPv6 Router
132 Advertisement messages by looking at the "Next Header" field of the
133 fixed IPv6 header, rather than following the entire header chain. As
134 a result, these implementations would fail to identify any ICMPv6
135 Router Advertisement messages that include any Extension Headers (for
136 example, Hop by Hop Options header, Destination Options Header,
137 etc.).
139 The following figure illustrates the structure of ICMPv6 Router
140 Advertisement messages that implement this RA-Guard evasion
141 technique:
143 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
144 |NH=60| |NH=58| | |
145 +-+-+-+ +-+-+-+ + +
146 | IPv6 header | Dst Opt Hdr | ICMPv6 Router Advertisement |
147 + + + +
148 | | | |
149 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
151 2.2. Attack vector based on IPv6 fragmentation
153 While the attack vector described in Section 2.1 may be effective
154 with implementations that fail to process the entire header chain, it
155 can easily be mitigated by an RA-Guard implementation, since all the
156 information needed to identify ICMPv6 Router Advertisement messages
157 is present in the attack packets.
159 This section presents a different attack vector, which aims at making
160 it virtually impossible for a layer-2 device to identify ICMPv6
161 Router Advertisements by leveraging the IPv6 Fragment Header. The
162 basic idea behind this attack vector is that if the forged ICMPv6
163 Router Advertisement is fragmented into at least two fragments, the
164 layer-2 device implementing "RA-Guard" would be unable to identify
165 the attack packet, and would thus fail to block it.
167 A first variant of this attack vector would be an original ICMPv6
168 Router Advertisement message preceded with a Destination Options
169 Header, that results in two fragments. The following figure
170 illustrates the "original" attack packet, prior to fragmentation, and
171 the two resulting fragments which are actually sent as part of the
172 attack.
174 Original packet:
176 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
177 |NH=60| |NH=58| | |
178 +-+-+-+ +-+-+-+ + +
179 | IPv6 header | Dst Opt Hdr | ICMPv6 RA |
180 + + + +
181 | | | |
182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
184 First fragment:
186 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
187 |NH=44| |NH=60| |NH=58| |
188 +-+-+-+ +-+-+-+ +-+-+-+ +
189 | IPv6 Header | Frag Hdr | Dst Opt Hdr |
190 + + + +
191 | | | |
192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
194 Second fragment:
196 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
197 |NH=44| |NH=60| | | |
198 +-+-+-+ +-+-+-+ + + +
199 | IPv6 header | Frag Hdr | Dst Opt Hdr | ICMPv6 RA |
200 + + + + +
201 | | | | |
202 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
204 It should be noted that the "Hdr Ext Len" field of the Destination
205 Options Header is present in the first fragment (rather than the
206 second). Therefore, it would be impossible for a device processing
207 only the second fragment to locate the ICMPv6 header contained in
208 that fragment, since it is unknown how many bytes should be "skipped"
209 to get to the next header following the Destination Options Header.
211 Thus, by leveraging the use of the Fragment Header together with the
212 use of the Destination Options header, the attacker is able to
213 conceal the type and contents of the ICMPv6 message he is sending (an
214 ICMPv6 Router Advertisement in this example). Unless the layer-2
215 device were to implement IPv6 fragment reassembly, it would be
216 impossible for the device to identify the ICMPv6 type of the message.
218 A layer-2 device could, however, at least detect that that an
219 ICMPv6 message (or some type) is being sent, since the "Next
220 Header" field of the Destination Options header contained in the
221 first fragment is set to "58" (ICMPv6).
223 It is possible to take this idea further, such that it is also
224 impossible for the layer-2 device to detect that the attacker is
225 sending an ICMPv6 message in the first place. This can be achieved
226 with an original ICMPv6 Router Advertisement message preceded with
227 two Destination Options Headers, that results in two fragments. The
228 following figure illustrates the "original" attack packet, prior to
229 fragmentation, and the two resulting packets which are actually sent
230 as part of the attack.
232 Original packet:
234 +-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
235 |NH=60| |NH=60| |NH=58| | |
236 +-+-+-+ +-+-+-+ +-+-+-+ + +
237 | IPv6 header | Dst Opt Hdr | Dst Opt Hdr | ICMPv6 RA |
238 + + + + +
239 | | | | |
240 +-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
242 First fragment:
244 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
245 |NH=44| |NH=60| |NH=60| |
246 +-+-+-+ +-+-+-+ +-+-+-+ +
247 | IPv6 header | Frag Hdr | Dst Opt Hdr |
248 + + + +
249 | | | |
250 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
252 Second fragment:
254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
255 |NH=44| |NH=60| | |NH=58| | |
256 +-+-+-+ +-+-+-+ + +-+-+-+ + +
257 | IPv6 header | Frag Hdr | Dst O Hdr | Dst Opt Hdr | ICMPv6 RA |
258 + + + + + +
259 | | | | | |
260 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
262 In this variant, the "Next Header" field of the Destination Options
263 header contained in the first fragment is set "60" (Destination
264 Options header), and thus it is impossible for a device processing
265 only the first fragment to detect that an ICMPv6 message is being
266 sent in the first place.
268 The second fragment presents the same challenges as the second
269 fragment of the previous variant. That is, it would be impossible
270 for a device processing only the second fragment to locate the second
271 Destination Options header (and hence the ICMPv6 header), since the
272 "Hdr Ext Len" field of the first Destination Options header is
273 present in the first fragment (rather than the second).
275 3. Mitigations
277 The most effective and efficient mitigation for the RA-Guard evasion
278 vulnerability discussed in this document would be to prohibit the use
279 of IPv6 Extension Headers in Neighbor Discovery messages, as proposed
280 in [I-D.gont-6man-nd-extension-headers].
282 Nevertheless, an administrator might want to mitigate these
283 vulnerabilities by deploying more advanced filtering. The following
284 filtering rules could be implemented as part of an "RA-Guard"
285 implementation, such that the vulnerabilities discussed in this
286 document can be mitigated:
288 o When trying to identify an ICMPv6 Router Advertisement message,
289 follow the IPv6 header chain, enforcing a limit on the maximum
290 number of Extension Headers that is allowed for each packet. If
291 such limit is exceeded, block the packet.
293 o If the layer-2 device is unable to identify whether the packet is
294 an ICMPv6 Router Advertisement message or not (i.e., the packet is
295 a fragment, and the necessary information is missing), and the
296 IPv6 Source Address of the packet is a link-local address or the
297 unspecified address (::), block the packet.
299 o In all other cases, pass the packet as usual.
301 This filtering policy assumes that host implementations require that
302 the IPv6 Source Address of ICMPv6 Router Advertisement messages be a
303 link-local address, and that they discard the packet if this check
304 fails, as required by the current IETF specifications [RFC4861].
305 Unfortunately, it should be noted that the aforementioned filtering
306 policy might be inefficient to implement (if at all possible), and
307 might also result (at least in theory) in false positives.
309 4. Other Implications
311 A similar concept to that of "RA-Guard" has been implemented for
312 protecting against forged DHCPv6 messages. Such protection can be
313 circumvented with the same techniques discussed in this document, and
314 the counter-measures for such evasion attack are analogous to those
315 described in Section 3 of this document.
317 5. Security Considerations
319 This document describes a number of techniques to circumvent a
320 mechanism known as "RA-Guard", which many organizations deploy as a
321 "first line of defense" against attacks based on forged Router
322 Advertisements.
324 The most effective and efficient mitigation for these attacks would
325 be to prohibit the use of IPv6 extension headers (as proposed by
326 [I-D.gont-6man-nd-extension-headers]), such that the RA-Guard
327 protection cannot be easily circumvented. However, since this
328 mitigation requires an update to existing implementations, in the
329 short term some network administrators might want to mitigate these
330 issues by implemented the more advanced filtering policy described in
331 Section 3.
333 6. Acknowledgements
335 The author would like to thank Karl Auer, Robert Downie, David
336 Farmer, Marc Heuse, and Arturo Servin, for providing valuable
337 comments on earlier versions of this document.
339 This document resulted from the project "Security Assessment of the
340 Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by
341 Fernando Gont on behalf of the UK Centre for the Protection of
342 National Infrastructure (CPNI). The author would like to thank the
343 UK CPNI, for their continued support.
345 7. References
347 7.1. Normative References
349 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
350 Requirement Levels", BCP 14, RFC 2119, March 1997.
352 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
353 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
354 September 2007.
356 7.2. Informative References
358 [RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
359 Problem Statement", RFC 6104, February 2011.
361 [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
362 Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
363 February 2011.
365 [I-D.gont-6man-nd-extension-headers]
366 Gont, F. and U. CPNI, "Security Implications of the Use of
367 IPv6 Extension Headers with IPv6 Neighbor Discovery",
368 draft-gont-6man-nd-extension-headers-00 (work in
369 progress), May 2011.
371 [CPNI-IPv6]
372 Gont, F., "Security Assessment of the Internet Protocol
373 version 6 (IPv6)", UK Centre for the Protection of
374 National Infrastructure, (to be published).
376 [NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor",
377 .
379 [rafixd] "rafixd", .
382 [ramond] "ramond", .
384 [THC-IPV6]
385 "THC-IPV6", .
387 Appendix A. Changes from previous versions of the draft (to be removed
388 by the RFC Editor before publication of this document as a
389 RFC
391 A.1. Changes from draft-gont-v6ops-ra-guard-evasion-00
393 o Minor editorial changes
395 o The discussion of the challenge represented by a combination of
396 fragmentation and Destination Options headers was improved/
397 clarified.
399 o In Section 2.2, in the illustration of the second variant of the
400 attack (fragmentation combined with two Destination Optios
401 headers), the figure corresponding to the "first fragment" was
402 corrected.
404 o Clarified the filtering rules in Section 3.
406 Appendix B. Assessment tools
408 CPNI has produced assessment tools, which have not yet been made
409 publicly available. If you think that you would benefit from these
410 tools to assess the security of your network or of your RA-Guard
411 implementation, we might be able to provide a copy of the tools
412 (please contact Fernando Gont at fernando@gont.com.ar).
414 [THC-IPV6] is a publicly-available set of tools that implements some
415 of the techniques described in this document.
417 Appendix C. Advice and guidance to vendors
419 Vendors are urged to contact CSIRTUK (csirt@cpni.gsi.gov.uk) if they
420 think they may be affected by the issues described in this document.
421 As the lead coordination centre for these issues, CPNI is well placed
422 to give advice and guidance as required.
424 CPNI works extensively with government departments and agencies,
425 commercial organisations and the academic community to research
426 vulnerabilities and potential threats to IT systems especially where
427 they may have an impact on Critical National Infrastructure's (CNI).
429 Other ways to contact CPNI, plus CPNI's PGP public key, are available
430 at http://www.cpni.gov.uk.
432 Author's Address
434 Fernando Gont
435 Centre for the Protection of National Infrastructure
437 Email: fernando@gont.com.ar
438 URI: http://www.gont.com.ar