idnits 2.17.1 draft-ietf-ecrit-trustworthy-location-07.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (30 July 2013) is 3916 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 2818 (Obsoleted by RFC 9110) -- Obsolete informational reference (is this intentional?): RFC 4474 (Obsoleted by RFC 8224) -- Obsolete informational reference (is this intentional?): RFC 5246 (Obsoleted by RFC 8446) -- Obsolete informational reference (is this intentional?): RFC 5751 (Obsoleted by RFC 8551) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ECRIT Working Group H. Tschofenig 3 INTERNET-DRAFT Nokia Siemens Networks 4 Category: Informational H. Schulzrinne 5 Expires: February 14, 2014 Columbia University 6 B. Aboba (ed.) 7 Skype 8 30 July 2013 10 Trustworthy Location 11 draft-ietf-ecrit-trustworthy-location-07.txt 13 Abstract 15 For some location-based applications, such as emergency calling or 16 roadside assistance, the trustworthiness of location information is 17 critically important. 19 This document describes how to convey location in a manner that is 20 inherently secure and reliable. It also provides guidelines for 21 assessing the trustworthiness of location information. 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 February 14, 2014. 40 Copyright Notice 42 Copyright (c) 2013 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 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 59 2. Threats . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 60 2.1. Location Spoofing . . . . . . . . . . . . . . . . . . . . 6 61 2.2. Identity Spoofing . . . . . . . . . . . . . . . . . . . . 7 62 3. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 8 63 3.1. Signed Location by Value . . . . . . . . . . . . . . . . . 8 64 3.2. Location by Reference . . . . . . . . . . . . . . . . . . 11 65 3.3. Proxy Adding Location . . . . . . . . . . . . . . . . . . 14 66 4. Location Trust Assessment . . . . . . . . . . . . . . . . . . 16 67 5. Security Considerations . . . . . . . . . . . . . . . . . . . 18 68 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 69 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 70 7.1. Informative references . . . . . . . . . . . . . . . . . . 20 71 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22 72 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23 74 1. Introduction 76 Several public and commercial services depend upon location 77 information in their operations. This includes emergency services 78 (such as fire, ambulance and police) as well as commercial services 79 such as food delivery and roadside assistance. 81 Services that depend on location commonly experience security issues 82 today. While prank calls have been a problem for emergency services 83 dating back to the time of street corner call boxes, with the move to 84 IP-based emergency services, the ability to launch automated attacks 85 has increased. As the European Emergency Number Association (EENA) 86 has noted [EENA]: "False emergency calls divert emergency services 87 away from people who may be in life-threatening situations and who 88 need urgent help. This can mean the difference between life and 89 death for someone in trouble." 91 EENA [EENA] has attempted to define terminology and describe best 92 current practices for dealing with false emergency calls, which in 93 certain European countries can constitute as much as 70% of all 94 emergency calls. Reducing the number of prank calls represents a 95 challenge, since emergency services authorities in most countries are 96 required to answer every call (whenever possible). Where the caller 97 cannot be identified, the ability to prosecute is limited. 99 Since prank emergency calls can endanger bystanders or emergency 100 services personnel, or divert resources away from legitimate 101 emergencies, they can be life threatening. A particularly dangerous 102 form of prank call is "swatting" - an prank emergency call that draws 103 a response from law enforcement (e.g. a fake hostage situation that 104 results in dispatching of a "Special Weapons And Tactics" (SWAT) 105 team). In 2008 the FBI issued a warning [Swatting] about an increase 106 in the frequency and sophistication of these attacks. 108 Many documented cases of "swatting" involve not only the faking of an 109 emergency, but also the absence of accurate caller identification and 110 the delivery of misleading location data. Today these attacks are 111 often carried out by providing false caller identification, since for 112 circuit-switched calls from landlines, location provided to the PSAP 113 is determined from a lookup using the calling telephone number. With 114 IP-based emergency services, in addition to the potential for false 115 caller identification, it is also possible to attach misleading 116 location information to the emergency call. 118 Ideally, a call taker at a Public Service Answering Point (PSAP) 119 should be put in the position to assess, in real-time, the level of 120 trust that can be placed on the information provided within a call. 121 This includes automated location conveyed along with the call and 122 location information communicated by the caller, as well as identity 123 information about the caller. Where real-time assessment is not 124 possible, it is important to be able to determine the source of the 125 call in a post-mortem, so as to be able to enforce accountability. 127 This document defines terminology (including the meaning of 128 "trustworthy location") in Section 1.1, investigates security threats 129 in Section 2, outlines potential solutions in Section 3, covers trust 130 assessment in Section 4 and discusses security considerations in 131 Section 5. 133 1.1. Terminology 135 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 136 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 137 document are to be interpreted as described in [RFC2119]. 139 The definition for "Target" is taken from "Geopriv Requirements" 140 [RFC3693]. 142 The term "location determination method" refers to the mechanism used 143 to determine the location of a Target. This may be something 144 employed by a location information server (LIS), or by the Target 145 itself. It specifically does not refer to the location configuration 146 protocol (LCP) used to deliver location information either to the 147 Target or the Recipient. This term is re-used from "GEOPRIV PIDF-LO 148 Usage Clarification, Considerations, and Recommendations" [RFC5491]. 150 The term "source" is used to refer to the LIS, node, or device from 151 which a Recipient (Target or Third-Party) obtains location 152 information. 154 Additionally, the terms Location-by-Value (LbyV), Location-by- 155 Reference (LbyR), Location Configuration Protocol, Location 156 Dereference Protocol, and Location URI are re-used from "Requirements 157 for a Location-by-Reference Mechanism" [RFC5808]. 159 "Trustworthy Location" is defined as location information that can be 160 attributed to a trusted source, has been protected against 161 modification in transmit, and has been assessed as trustworthy. 163 "Location Trust Assessment" refers to the process by which the 164 reliability of location information can be assessed. This topic is 165 discussed in Section 4. 167 The following additional terms apply to location spoofing: 169 "Place Shifting" is where the attacker constructs a PIDF-LO for a 170 location other than where they are currently located. In some cases, 171 place shifting can be limited in range (e.g., within the coverage 172 area of a particular cell tower). 174 "Time Shifting" is where the attacker uses or re-uses location 175 information that was valid in the past, but is no longer valid 176 because the attacker has moved. 178 "Location Theft" is where the attacker captures a Target's location 179 information and presents it as their own. Location theft can occur 180 on a one-off basis, or may be continuous (e.g., where the attacker 181 has gained control over the victim's device). Location theft may 182 also be combined with time shifting to present someone else's 183 location information after the original Target has moved. Where the 184 Target and attacker collude, the term "location swapping" is used. 186 2. Threats 188 While previous IETF documents have analyzed aspects of the security 189 of emergency services or threats to geographic location privacy, 190 those documents do not cover the threats arising from unreliable 191 location information. 193 A threat analysis of the emergency services system is provided in 194 "Security Threats and Requirements for Emergency Call Marking and 195 Mapping" [RFC5069]. RFC 5069 describes attacks on the emergency 196 services system, such as attempting to deny system services to all 197 users in a given area, to gain fraudulent use of services and to 198 divert emergency calls to non-emergency sites. [RFC5069] also 199 describes attacks against individuals, including attempts to prevent 200 an individual from receiving aid, or to gain information about an 201 emergency. "Threat Analysis of the Geopriv Protocol" [RFC3694] 202 describes threats against geographic location privacy, including 203 protocol threats, threats resulting from the storage of geographic 204 location data, and threats posed by the abuse of information. 206 This document focuses on threats from attackers providing false 207 location information within emergency calls. Since we do not focus 208 on attackers gaining control of infrastructure elements (e.g., 209 location servers, call route servers) or the emergency services IP 210 network, the threats arise from end hosts. In addition to threats 211 arising from the intentional forging of caller identification or 212 location information, end hosts may be induced to provide 213 untrustworthy location information. For example, end hosts may 214 obtain location from civilian GPS, which is vulnerable to spoofing 215 [GPSCounter] or from third party Location Service Providers (LSPs) 216 which may be vulnerable to attack or may not provide location 217 accuracy suitable for emergency purposes. 219 To provide a structured analysis we distinguish between three 220 adversary models: 222 External adversary model: The end host, e.g., an emergency caller 223 whose location is going to be communicated, is honest and the 224 adversary may be located between the end host and the location 225 server or between the end host and the PSAP. None of the 226 emergency service infrastructure elements act maliciously. 228 Malicious infrastructure adversary model: The emergency call routing 229 elements, such as the LIS, the LoST infrastructure, used for 230 mapping locations to PSAP address, or call routing elements, may 231 act maliciously. 233 Malicious end host adversary model: The end host itself acts 234 maliciously, whether the owner is aware of this or whether it is 235 acting under the control of a third party. 237 In this document, we focus only on the malicious end host adversary 238 model. 240 2.1. Location Spoofing 242 An adversary can provide false location information in an emergency 243 call in order to misdirect emergency resources. For calls 244 originating within the PSTN or via a fixed Voice over IP service, 245 this attack can be carried out via caller-id spoofing. For example, 246 where a Voice Service Provider enables setting of the outbound caller 247 identification without checking it against the authenticated 248 identity, forging caller identification is trivial. Where an 249 attacker can gain entry to a PBX, they can then subsequently use that 250 access to launch a denial of service attack against the PSAP, or to 251 make fraudulent emergency calls. 253 Where location is attached to the emergency call by an end host, 254 several avenues are available to provide false location information: 256 1. The end host could fabricate a PIDF-LO and convey it within an 257 emergency call; 259 2. The VSP (and indirectly a LIS) could be fooled into using the 260 wrong identity (such as an IP address) for location lookup, 261 thereby providing the end host with misleading location 262 information; 264 3. Inaccurate or out-of-date information (such as spoofed GPS 265 signals, a stale wiremap or an inaccurate access point location 266 database) could be utilized by the LIS or the end host in its 267 location determination, thereby leading to an inaccurate 268 determination of location. 270 The following represent examples of location spoofing: 272 Place shifting: Trudy, the adversary, pretends to be at an 273 arbitrary location. 275 Time shifting: Trudy pretends to be at a location she was a 276 while ago. 278 Location theft: Trudy observes Alice's location and replays 279 it as her own. 281 Location swapping: Trudy and Malory collude and swap location 282 information, pretending to be in each other's location. 284 2.2. Identity Spoofing 286 With calls originating on an IP network, at least two forms of 287 identity are relevant, with the distinction created by the split 288 between the AIP and the VSP: 290 (a) network access identity such as might be determined via 291 authentication (e.g., using the Extensible Authentication Protocol 292 (EAP) [RFC3748]); 294 (b) caller identity, such as might be determined from authentication 295 of the emergency caller at the VoIP application layer. 297 If the adversary did not authenticate itself to the VSP, then 298 accountability may depend on verification of the network access 299 identity. However, this also may not have been authenticated, such 300 as in the case where an open IEEE 802.11 Access Point is used to 301 initiate a prank emergency call. Although endpoint information such 302 as the IP or MAC address may have been logged, tying this back to the 303 device owner may be challenging. 305 Unlike the existing telephone system, VoIP emergency calls can 306 provide a strong identity that need not necessarily be coupled to a 307 business relationship with the AIP, ISP or VSP. However, due to the 308 time-critical nature of emergency calls, multi-layer authentication 309 is undesirable, so that in most cases, only the device placing the 310 call will be able to be identified, making the system vulnerable to 311 bot-net attacks. Furthermore, deploying additional credentials for 312 emergency service purposes (such as certificates) increases costs, 313 introduces a significant administrative overhead and is only useful 314 if widely deployed. 316 3. Solutions 318 This section presents three mechanisms which can be used to convey 319 location securely: signed location by value (Section 3.1), location 320 by reference (Section 3.2) and proxy added location (Section 3.3). 322 In order to provide authentication and integrity protection for the 323 SIP messages conveying location, several security approaches are 324 available. It is possible to ensure that modification of the 325 identity and location in transit can be detected by the location 326 recipient (e.g., the PSAP), using cryptographic mechanisms, as 327 described in "Enhancements for Authenticated Identity Management in 328 the Session Initiation Protocol" [RFC4474]. However, compatibility 329 with Session Border Controllers (SBCs) that modify integrity- 330 protected headers has proven to be an issue in practice. As a 331 result, SIP over TLS is currently a more deployable mechanism to 332 provide per-message authentication and integrity protection hop-by- 333 hop. 335 3.1. Signed Location by Value 337 With location signing, a location server signs the location 338 information before it is sent to the end host, (the entity subject to 339 the location determination process). The signed location information 340 is then verified by the location recipient and not by the target. A 341 straw-man proposal for location signing is provided in "Digital 342 Signature Methods for Location Dependability" [I-D.thomson-geopriv- 343 location-dependability]. 345 Figure 1 shows the communication model with the target requesting 346 signed location in step (a), the location server returns it in step 347 (b) and it is then conveyed to the location recipient in step (c) who 348 verifies it. For SIP, the procedures described in "Location 349 Conveyance for the Session Initiation Protocol" [RFC6442] are 350 applicable for location conveyance. 352 +-----------+ +-----------+ 353 | | | Location | 354 | LIS | | Recipient | 355 | | | | 356 +-+-------+-+ +----+------+ 357 ^ | --^ 358 | | -- 359 Geopriv |Req. | -- 360 Location |Signed |Signed -- Geopriv 361 Configuration |Loc. |Loc. -- Using Protocol 362 Protocol |(a) |(b) -- (e.g., SIP) 363 | v -- (c) 364 +-+-------+-+ -- 365 | Target / | -- 366 | End Host + 367 | | 368 +-----------+ 370 Figure 1: Location Signing 372 In order to limit replay attacks, [I.D.thomson-geopriv-location- 373 dependability] proposes the addition of a "validity" element to the 374 PIDF-LO, including a "from" sub-element containing the time that 375 location information was validated by the signer, as well as an 376 "until" sub-element containing the last time that the signature can 377 be considered valid. 379 One of the consequences of including an "until" element is that even 380 a stationary target would need to periodically obtain a fresh PIDF- 381 LO, or incur the additional delay of querying during an emergency 382 call. 384 Although privacy-preserving procedures may be disabled for emergency 385 calls, by design, PIDF-LO objects limit the information available for 386 real-time attribution. As noted in [RFC5985] Section 6.6: 388 The LIS MUST NOT include any means of identifying the Device in 389 the PIDF-LO unless it is able to verify that the identifier is 390 correct and inclusion of identity is expressly permitted by a Rule 391 Maker. Therefore, PIDF parameters that contain identity are 392 either omitted or contain unlinked pseudonyms [RFC3693]. A 393 unique, unlinked presentity URI SHOULD be generated by the LIS for 394 the mandatory presence "entity" attribute of the PIDF document. 395 Optional parameters such as the "contact" and "deviceID" elements 396 [RFC4479] are not used. 398 Also, the device referred to in the PIDF-LO may not necessarily be 399 the same entity conveying the PIDF-LO to the PSAP. As noted in 401 [RFC6442] Section 1: 403 In no way does this document assume that the SIP user agent client 404 that sends a request containing a location object is necessarily 405 the Target. The location of a Target conveyed within SIP 406 typically corresponds to that of a device controlled by the 407 Target, for example, a mobile phone, but such devices can be 408 separated from their owners, and moreover, in some cases, the user 409 agent may not know its own location. 411 Without the ability to tie the target identity to the identity 412 asserted in the SIP message, it is possible for an attacker to cut 413 and paste a PIDF-LO obtained by a different device or user into a SIP 414 INVITE and send this to the PSAP. This cut and paste attack could 415 succeed even when a PIDF-LO is signed, or [RFC4474] is implemented. 417 To address location-swapping attacks, [I-D.thomson-geopriv-location- 418 dependability] proposes addition of an "identity" element which could 419 include a SIP URI (enabling comparison against the identity asserted 420 in the SIP headers) or an X.509v3 certificate. If the target was 421 authenticated by the LIS, an "authenticated" attribute is added. 422 However, inclusion of an "identity" attribute could enable location 423 tracking, so that a "hash" element is also proposed which could 424 contain a hash of the content of the "identity" element instead. In 425 practice, such a hash would not be much better for real-time 426 validation than a pseudonym. 428 Location signing is unlikely to deter attacks launched by bot-nets, 429 since the work required to verify the location signature is 430 considerable. However, while bot-nets are unlikely to be deterred by 431 location signing, accurate location information would limit the 432 subset of the bot-net that could be used for an attack, as only hosts 433 within the PSAP serving area would be useful in placing emergency 434 calls. 436 Location signing is also difficult when the host obtains location via 437 mechanisms such as GPS, unless trusted computing approaches, with 438 tamper-proof GPS modules, can be applied. Otherwise, an end host can 439 pretend to have a GPS device, and the recipient will need to rely on 440 its ability to assess the level of trust that should be placed in the 441 end host location claim. 443 [NENA-i2] Section 3.7 includes operational recommendations relating 444 to location signing: 446 Location determination is out of scope for NENA, but we can offer 447 guidance on what should be considered when designing mechanisms to 448 report location: 450 1. The location object should be digitally signed. 452 2. The certificate for the signer (LIS operator) should be 453 rooted in VESA. For this purpose, VPC and ERDB operators 454 should issue certs to LIS operators. 456 3. The signature should include a timestamp. 458 4. Where possible, the Location Object should be refreshed 459 periodically, with the signature (and thus the timestamp) 460 being refreshed as a consequence. 462 5. Anti-spoofing mechanisms should be applied to the Location 463 Reporting method. 465 [Note: The term Valid Emergency Services Authority (VESA) refers 466 to the root certificate authority.] 468 As noted above, signing of location objects implies the development 469 of a trust hierarchy that would enable a certificate chain provided 470 by the LIS operator to be verified by the PSAP. Rooting the trust 471 hierarchy in VESA can be accomplished either by having the VESA 472 directly sign the LIS certificates, or by the creation of 473 intermediate CAs certified by the VESA, which will then issue 474 certificates to the LIS. In terms of the workload imposed on the 475 VESA, the latter approach is highly preferable. However, this raises 476 the question of who would operate the intermediate CAs and what the 477 expectations would be. 479 In particular, the question arises as to the requirements for LIS 480 certificate issuance, and how they would compare to requirements for 481 issuance of other certificates such as an SSL/TLS web certificate. 483 3.2. Location by Reference 485 Location-by-reference was developed so that end hosts can avoid 486 having to periodically query the location server for up- to-date 487 location information in a mobile environment. Additionally, if 488 operators do not want to disclose location information to the end 489 host without charging them, location-by-reference provides a 490 reasonable alternative. As noted in "A Location Dereference Protocol 491 Using HTTP-Enabled Location Delivery (HELD)" [RFC6753], a location 492 reference can be obtained via HTTP-Enabled Location Delivery (HELD) 493 [RFC5985] or the Dynamic Host Configuration Protocol (DHCP) location 494 URI option [DHCP-URI-OPT]. 496 Figure 2 shows the communication model with the target requesting a 497 location reference in step (a), the location server returns the 498 reference in step (b), and it is then conveyed to the location 499 recipient in step (c). The location recipient needs to resolve the 500 reference with a request in step (d). Finally, location information 501 is returned to the Location Recipient afterwards. For location 502 conveyance in SIP, the procedures described in [RFC6442] are 503 applicable. 505 +-----------+ Geopriv +-----------+ 506 | | Location | Location | 507 | LIS +<------------->+ Recipient | 508 | | Dereferencing | | 509 +-+-------+-+ Protocol (d) +----+------+ 510 ^ | --^ 511 | | -- 512 Geopriv |Req. | -- 513 Location |LbyR |LbyR -- Geopriv 514 Configuration |(a) |(b) -- Using Protocol 515 Protocol | | -- (e.g., SIP) 516 | V -- (c) 517 +-+-------+-+ -- 518 | Target / | -- 519 | End Host + 520 | | 521 +-----------+ 523 Figure 2: Location by Reference 525 Where location by reference is provided, the recipient needs to 526 deference the LbyR in order to obtain location. The details for the 527 dereferencing operations vary with the type of reference, such as a 528 HTTP, HTTPS, SIP, SIPS URI or a SIP presence URI. 530 For location-by-reference, the location server needs to maintain one 531 or several URIs for each target, timing out these URIs after a 532 certain amount of time. References need to expire to prevent the 533 recipient of such a URL from being able to permanently track a host 534 and to offer garbage collection functionality for the location 535 server. 537 Off-path adversaries must be prevented from obtaining the target's 538 location. The reference contains a randomized component that 539 prevents third parties from guessing it. When the location recipient 540 fetches up-to-date location information from the location server, it 541 can also be assured that the location information is fresh and not 542 replayed. However, this does not address location swapping. 544 With respect to the security of the de-reference operation, [RFC6753] 545 Section 6 states: 547 TLS MUST be used for dereferencing location URIs unless 548 confidentiality and integrity are provided by some other 549 mechanism, as discussed in Section 3. Location Recipients MUST 550 authenticate the host identity using the domain name included in 551 the location URI, using the procedure described in Section 3.1 of 552 [RFC2818]. Local policy determines what a Location Recipient does 553 if authentication fails or cannot be attempted. 555 The authorization by possession model (Section 4.1) further relies 556 on TLS when transmitting the location URI to protect the secrecy 557 of the URI. Possession of such a URI implies the same privacy 558 considerations as possession of the PIDF-LO document that the URI 559 references. 561 Location URIs MUST only be disclosed to authorized Location 562 Recipients. The GEOPRIV architecture [RFC6280] designates the 563 Rule Maker to authorize disclosure of the URI. 565 Protection of the location URI is necessary, since the policy 566 attached to such a location URI permits anyone who has the URI to 567 view the associated location information. This aspect of security 568 is covered in more detail in the specification of location 569 conveyance protocols, such as [RFC6442]. 571 For authorizing access to location-by-reference, two authorization 572 models were developed: "Authorization by Possession" and 573 "Authorization via Access Control Lists". With respect to 574 "Authorization by Possession" [RFC6753] Section 4.1 notes: 576 In this model, possession -- or knowledge -- of the location URI 577 is used to control access to location information. A location URI 578 might be constructed such that it is hard to guess (see C8 of 579 [RFC5808]), and the set of entities that it is disclosed to can be 580 limited. The only authentication this would require by the LS is 581 evidence of possession of the URI. The LS could immediately 582 authorize any request that indicates this URI. 584 Authorization by possession does not require direct interaction 585 with Rule Maker; it is assumed that the Rule Maker is able to 586 exert control over the distribution of the location URI. 587 Therefore, the LIS can operate with limited policy input from a 588 Rule Maker. 590 Limited disclosure is an important aspect of this authorization 591 model. The location URI is a secret; therefore, ensuring that 592 adversaries are not able to acquire this information is paramount. 593 Encryption, such as might be offered by TLS [RFC5246] or S/MIME 594 [RFC5751], protects the information from eavesdroppers. 596 Using possession as a basis for authorization means that, once 597 granted, authorization cannot be easily revoked. Cancellation of 598 a location URI ensures that legitimate users are also affected; 599 application of additional policy is theoretically possible but 600 could be technically infeasible. Expiration of location URIs 601 limits the usable time for a location URI, requiring that an 602 attacker continue o learn new location URIs to retain access to 603 current location information. 605 In situations where "Authorization by Possession" is not suitable 606 (such as where location hiding [RFC6444] is required), the 607 "Authorization via Access Control Lists" model may be preferred. 609 Without the introduction of hierarchy, it would be necessary for the 610 PSAP to obtain client certificates or Digest credentials for all the 611 LISes in its coverage area, to enable it to successfully dereference 612 LbyRs. In situations with more than a few LISes per PSAP, this would 613 present operational challenges. 615 A certificate hierarchy providing PSAPs with client certificates 616 chaining to the VESA could be used to enable the LIS to authenticate 617 and authorize PSAPs for dereferencing. Note that unlike PIDF-LO 618 signing (which mitigates against modification of PIDF-LOs), this 619 merely provides the PSAP with access to a (potentially unsigned) 620 PIDF-LO, albeit over a protected TLS channel. 622 Another approach would be for the local LIS to upload location 623 information to a location aggregation point who would in turn manage 624 the relationships with the PSAP. This would shift the management 625 burden from the PSAPs to the location aggregation points. 627 3.3. Proxy Adding Location 629 Instead of relying upon the end host to provide location, is possible 630 for a proxy that has the ability to determine the location of the end 631 point (e.g., based on the end host IP or MAC address) to retrieve and 632 add or override location information. 634 The use of proxy-added location is primarily applicable in scenarios 635 where the end host does not provide location. As noted in [RFC6442] 636 Section 4.1: 638 A SIP intermediary SHOULD NOT add location to a SIP request that 639 already contains location. This will quite often lead to 640 confusion within LRs. However, if a SIP intermediary adds 641 location, even if location was not previously present in a SIP 642 request, that SIP intermediary is fully responsible for addressing 643 the concerns of any 424 (Bad Location Information) SIP response it 644 receives about this location addition and MUST NOT pass on 645 (upstream) the 424 response. A SIP intermediary that adds a 646 locationValue MUST position the new locationValue as the last 647 locationValue within the Geolocation header field of the SIP 648 request. 650 A SIP intermediary MAY add a Geolocation header field if one is 651 not present -- for example, when a user agent does not support the 652 Geolocation mechanism but their outbound proxy does and knows the 653 Target's location, or any of a number of other use cases (see 654 Section 3). 656 As noted in [RFC6442] Section 3.3: 658 This document takes a "you break it, you bought it" approach to 659 dealing with second locations placed into a SIP request by an 660 intermediary entity. That entity becomes completely responsible 661 for all location within that SIP request (more on this in Section 662 4). 664 While it is possible for the proxy to override location included by 665 the end host, [RFC6442] Section 3.4 notes the operational 666 limitations: 668 Overriding location information provided by the user requires a 669 deployment where an intermediary necessarily knows better than an 670 end user -- after all, it could be that Alice has an on-board GPS, 671 and the SIP intermediary only knows her nearest cell tower. Which 672 is more accurate location information? Currently, there is no way 673 to tell which entity is more accurate or which is wrong, for that 674 matter. This document will not specify how to indicate which 675 location is more accurate than another. 677 The disadvantage of this approach is the need to deploy application 678 layer entities, such as SIP proxies, at AIPs or associated with AIPs. 679 This requires a standardized VoIP profile to be deployed at every end 680 device and at every AIP. This might impose interoperability 681 challenges. 683 Additionally, the AIP needs to take responsibility for emergency 684 calls, even for customers they have no direct or indirect 685 relationship with. To provide identity information about the 686 emergency caller from the VSP it would be necessary to let the AIP 687 and the VSP to interact for authentication (see, for example, 688 [RFC4740]). This interaction along the Authentication, Authorization 689 and Accounting infrastructure is often based on business 690 relationships between the involved entities. The AIP and the VSP are 691 very likely to have no such business relationship, particularly when 692 talking about an arbitrary VSP somewhere on the Internet. In case 693 that the interaction between the AIP and the VSP fails due to the 694 lack of a business relationship then typically a fall-back would be 695 provided where no emergency caller identity information is made 696 available to the PSAP and the emergency call still has to be 697 completed. 699 4. Location Trust Assessment 701 The ability to assess the level of trustworthiness of conveyed 702 location information is important, since this makes it possible to 703 understand how much value should be placed on location information, 704 as part of the decision making process. As an example, if automated 705 location information is understood to be highly suspect, a call taker 706 can put more effort into obtaining location information from the 707 caller. 709 Location trust assessment has value regardless of whether the 710 location has been conveyed securely (via signed location, location- 711 by-reference or proxy-added location) or not (via location-by-value 712 without location signing), since secure conveyance does not provide 713 assurance relating to the validity or provenance of location data. 715 To prevent location-swapping attacks, the "entity" element of the 716 PIDF-LO is of limited value if an unlinked pseudonym is provided in 717 this field. However, if the LIS authenticates the target, then the 718 linkage between the pseudonym and the target identity can be 719 recovered post-mortem. 721 As noted in [I.D.thomson-geopriv-location-dependability], if the 722 location object was signed, the location recipient has additional 723 information on which to base their trust assessment, such as the 724 validity of the signature, the identity of the target, the identity 725 of the LIS, whether the LIS authenticated the target, and the 726 identifier included in the "entity" field. 728 Caller accountability is also an important aspect of trust 729 assessment. Can the individual purchasing the device or activating 730 service be identified or did the call originate from a non-service 731 initialized (NSI) device whose owner cannot be determined? Prior to 732 the call, was the caller authenticated at the network or application 733 layer? In the event of a prank call, can audit logs be made 734 available to an investigator, or can information relating to the 735 owner of an unlinked pseudonym be provided, enabling investigators to 736 unravel the chain of events that lead to the attack? In practice, 737 the ability to identify a caller may decrease the likelihood of 738 caller misbehavior. For example, where emergency calls have been 739 allowed from handsets lacking a SIM card, or where ownership of the 740 SIM card cannot be determined, the frequency of nuisance calls has 741 often been unacceptably high [TASMANIA][UK][SA]. 743 In practice, the source of the location data is important for 744 location trust assessment. For example, location provided by a 745 Location Information Server (LIS) whose administrator has an 746 established history of meeting emergency location accuracy 747 requirements (e.g. Phase II) may be considered more reliable than 748 location information provided by a third party Location Service 749 Provider (LSP) that disclaims use of location information for 750 emergency purposes. 752 However, even where an LSP does not attempt to meet the accuracy 753 requirements for emergency location, it still may be able to provide 754 information useful in assessing about how reliable location 755 information is likely to be. For example, was location determined 756 based on the nearest cell tower or 802.11 Access Point (AP), or was a 757 triangulation method used? If based on cell tower or AP location 758 data, was the information obtained from an authoritative source (e.g. 759 the tower or AP owner) and when was the last time that the location 760 of the tower or access point was verified? 762 For real-time validation, information in the signaling and media 763 packets can be cross checked against location information. For 764 example, it may be possible to determine the city, state, country or 765 continent associated with the IP address included within SIP Via: or 766 Contact: headers, or the media source address, and compare this 767 against the location information reported by the caller or conveyed 768 in the PIDF-LO. However, in some situations only entities close to 769 the caller may be able to verify the correctness of location 770 information. 772 Real-time validation of the timestamp contained within PIDF-LO 773 objects (reflecting the time at which the location was determined) is 774 also challenging. To address time-shifting attacks, the "timestamp" 775 element of the PIDF-LO, defined in [RFC3863], can be examined and 776 compared against timestamps included within the enclosing SIP 777 message, to determine whether the location data is sufficiently 778 fresh. However, the timestamp only represents an assertion by the 779 LIS, which may or may not be trustworthy. For example, the recipient 780 of the signed PIDF-LO may not know whether the LIS supports time 781 synchronization, or whether it is possible to reset the LIS clock 782 manually without detection. Even if the timestamp was valid at the 783 time location was determined, a time period may elapse between when 784 the PIDF-LO was provided and when it is conveyed to the recipient. 785 Periodically refreshing location information to renew the timestamp 786 even though the location information itself is unchanged puts 787 additional load on LISes. As a result, recipients need to validate 788 the timestamp in order to determine whether it is credible. 790 While this document focuses on the discussion of real-time 791 determination of suspicious emergency calls, the use of audit logs 792 may help in enforcing accountability among emergency callers. For 793 example, in the event of a prank call, information relating to the 794 owner of the unlinked pseudonym could be provided to investigators, 795 enabling them to unravel the chain of events that lead to the attack. 796 However, while auditability is an important deterrent, it is likely 797 to be of most benefit in situations where attacks on the emergency 798 services system are likely to be relatively infrequent, since the 799 resources required to pursue an investigation are likely to be 800 considerable. However, although real-time validation based on PIDF- 801 LO elements is challenging, where LIS audit logs are available (such 802 as where a law enforcement agency can present a subpoena), linking of 803 a pseudonym to the device obtaining location can be accomplished in a 804 post-mortem. 806 Where attacks are frequent and continuous, automated mechanisms are 807 required. For example, it might be valuable to develop mechanisms to 808 exchange audit trails information in a standardized format between 809 ISPs and PSAPs / VSPs and PSAPs or heuristics to distinguish 810 potentially fraudulent emergency calls from real emergencies. While 811 a CAPTCHA-style test may be applied to suspicious calls to lower the 812 risk from bot-nets, this is quite controversial for emergency 813 services, due to the risk of delaying or rejecting valid calls. 815 5. Security Considerations 817 IP-based emergency services face a number of security threats that do 818 not exist within the legacy system. In order to limit prank calls, 819 legacy emergency services rely on the ability to identify callers, as 820 well as on the difficulty of location spoofing for normal users. The 821 ability to ascertain identity is important, since the threat of 822 punishment reduces prank calls; as an example, calls from pay phones 823 are subject to greater scrutiny by the call taker. 825 Mechanically placing a large number of emergency calls that appear to 826 come from different locations is difficult in a legacy environment. 827 Also, in the current system, it would be very difficult for an 828 attacker from country 'Foo' to attack the emergency services 829 infrastructure located in country 'Bar'. 831 However, within an IP-based emergency services a number of these 832 attacks become much easier to mount. Emergency services have three 833 finite resources subject to denial of service attacks: the network 834 and server infrastructure, call takers and dispatchers, and the first 835 responders, such as fire fighters and police officers. Protecting 836 the network infrastructure is similar to protecting other high-value 837 service providers, except that location information may be used to 838 filter call setup requests, to weed out requests that are out of 839 area. PSAPs even for large cities may only have a handful of PSAP 840 call takers on duty, so even if they can, by questioning the caller, 841 eliminate a lot of prank calls, they are quickly overwhelmed by even 842 a small-scale attack. Finally, first responder resources are scarce, 843 particularly during mass-casualty events. 845 Attackers may want to modify, prevent or delay emergency calls. In 846 some cases, this will lead the PSAP to dispatch emergency personnel 847 to an emergency that does not exist and, hence, the personnel might 848 not be available to other callers. It might also be possible for an 849 attacker to impede the users from reaching an appropriate PSAP by 850 modifying the location of an end host or the information returned 851 from the mapping protocol. In some countries, regulators may not 852 require the authenticated identity of the emergency caller, as is 853 true for PSTN-based emergency calls placed from pay phones or SIM- 854 less cell phones today. Furthermore, if identities can easily be 855 crafted (as it is the case with many VoIP offerings today), then the 856 value of emergency caller authentication itself might be limited. As 857 a consequence, an attacker can forge emergency call information 858 without the chance of being held accountable for its own actions. 860 The above-mentioned attacks are mostly targeting individual emergency 861 callers or a very small fraction of them. If attacks are, however, 862 launched against the mapping architecture (see [RFC5582] or against 863 the emergency services IP network (including PSAPs), a larger region 864 and a large number of potential emergency callers are affected. The 865 call takers themselves are a particularly scarce resource and if 866 human interaction by these call takers is required then this can very 867 quickly have severe consequences. 869 Although it is important to ensure that location information cannot 870 be faked there will be many GPS-enabled devices that will find it 871 difficult to utilize any of the solutions described in Section 3. It 872 is also unlikely that users will be willing to upload their location 873 information for "verification" to a nearby location server located in 874 the access network. 876 Nevertheless, it should be understood that mounting several of the 877 attacks described in this document is non-trivial. Location theft 878 requires the attacker to be in proximity to the location to spoofed, 879 and location swapping requires the attacker to collude with someone 880 who was at the spoofed location. Time shifting attacks require that 881 the attacker visit the location and submit it before the location 882 information is considered stale, while travelling rapidly away from 883 that location to avoid apprehension. Obtaining a PIDF-LO from a 884 spoofed IP address requires that the attacker be on the path between 885 the HELD requester and the LIS. 887 6. IANA Considerations 889 This document does not require actions by IANA. 891 7. References 893 7.1. Informative References 895 [DHCP-URI-OPT] 896 Polk, J., "Dynamic Host Configuration Protocol (DHCP) IPv4 and 897 IPv6 Option for a Location Uniform Resource Identifier (URI)", 898 Internet draft (work in progress), draft-ietf-geopriv-dhcp- 899 lbyr-uri-option-19, February 2013. 901 [EENA] EENA, "False Emergency Calls", EENA Operations Document, 902 Version 1.0, March 2011, 903 http://www.eena.org/ressource/static/files/ 904 2011_03_15_3.1.2.fc_v1.0.pdf 906 [GPSCounter] 907 Warner, J. S. and R. G. Johnston, "GPS Spoofing 908 Countermeasures", Los Alamos research paper LAUR-03-6163, 909 December 2003. 911 [NENA-i2] "08-001 NENA Interim VoIP Architecture for Enhanced 9-1-1 912 Services (i2)", December 2005. 914 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 915 Requirement Levels", BCP 14, RFC 2119, March 1997. 917 [RFC2818] Rescorla, E., "HTTP over TLS", RFC 2818, May 2000. 919 [RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J. 920 Polk, "Geopriv Requirements", RFC 3693, February 2004. 922 [RFC3694] Danley, M., Mulligan, D., Morris, J. and J. Peterson, "Threat 923 Analysis of the Geopriv Protocol", RFC 3694, February 2004. 925 [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. 926 Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 927 3748, June 2004. 929 [RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W. and 930 J. Peterson, "Presence Information Data Format (PIDF)", RFC 931 3863, August 2004. 933 [RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated 934 Identity Management in the Session Initiation Protocol (SIP)", 935 RFC 4474, August 2006. 937 [RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479, July 938 2006. 940 [RFC4740] Garcia-Martin, M., Belinchon, M., Pallares-Lopez, M., Canales- 941 Valenzuela, C., and K. Tammi, "Diameter Session Initiation 942 Protocol (SIP) Application", RFC 4740, November 2006. 944 [RFC5069] Taylor, T., Tschofenig, H., Schulzrinne, H. and M. Shanmugam, 945 "Security Threats and Requirements for Emergency Call Marking 946 and Mapping", RFC 5069, January 2008. 948 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Level Security 949 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 951 [RFC5491] Winterbottom, J., Thomson, M. and H. Tschofenig, "GEOPRIV 952 Presence Information Data Format Location Object (PIDF-LO) 953 Usage Clarification, Considerations, and Recommendations", RFC 954 5491, March 2009. 956 [RFC5582] Schulzrinne, H., "Location-to-URL Mapping Architecture and 957 Framework", RFC 5582, September 2009. 959 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail 960 Extensions (S/MIME) Version 3.2 Message Specification", RFC 961 5751, January 2010. 963 [RFC5808] Marshall, R., "Requirements for a Location-by-Reference 964 Mechanism", RFC 5808, May 2010. 966 [RFC5985] Barnes, M., "HTTP Enabled Location Delivery (HELD)", RFC 5985, 967 September 2010. 969 [RFC6280] Barnes, R., et. al, "An Architecture for Location and Location 970 Privacy in Internet Applications", RFC 6280, July 2011. 972 [RFC6442] Polk, J., Rosen, B. and J. Peterson, "Location Conveyance for 973 the Session Initiation Protocol", RFC 6442, December 2011. 975 [RFC6444] Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and A. 976 Kuett, "Location Hiding: Problem Statement and Requirements", 977 RFC 6444, January 2012. 979 [RFC6753] Winterbottom, J., Tschofenig. H., Schulzrinne, H. and M. 980 Thomson, "A Location Dereference Protocol Using HTTP-Enabled 981 Location Delivery (HELD)", RFC 6753, October 2012. 983 [SA] "Saudi Arabia - Illegal sale of SIMs blamed for surge in prank 984 calls", Arab News, May 4, 2010, 985 http://www.menafn.com/qn_news_story_s.asp?StoryId=1093319384 987 [Swatting] 988 "Don't Make the Call: The New Phenomenon of 'Swatting', 989 Federal Bureau of Investigation, February 4, 2008, 990 http://www.fbi.gov/news/stories/2008/february/swatting020408 992 [TASMANIA] 993 "Emergency services seek SIM-less calls block", ABC News 994 Online, August 18, 2006, 995 http://www.abc.net.au/news/newsitems/200608/s1717956.htm 997 [UK] "Rapper makes thousands of prank 999 emergency calls to UK 998 police", Digital Journal, June 24, 2010, 999 http://www.digitaljournal.com/article/293796?tp=1 1001 Acknowledgments 1003 We would like to thank the members of the IETF ECRIT working group, 1004 including Marc Linsner, Henning Schulzrinne and Brian Rosen, for 1005 their input at IETF 85 that helped get this documented pointed in the 1006 right direction. We would also like to thank members of the IETF 1007 GEOPRIV WG, including Andrew Newton, Murugaraj Shanmugam, Martin 1008 Thomson, Richard Barnes and Matt Lepinski for their feedback to 1009 previous versions of this document. 1011 Authors' Addresses 1013 Hannes Tschofenig 1014 Nokia Siemens Networks 1015 Linnoitustie 6 1016 Espoo 02600 1017 Finland 1019 Phone: +358 (50) 4871445 1020 Email: Hannes.Tschofenig@gmx.net 1021 URI: http://www.tschofenig.priv.at 1023 Henning Schulzrinne 1024 Columbia University 1025 Department of Computer Science 1026 450 Computer Science Building, New York, NY 10027 1027 US 1029 Phone: +1 212 939 7004 1030 Email: hgs@cs.columbia.edu 1031 URI: http://www.cs.columbia.edu 1033 Bernard Aboba 1034 Skype 1035 Redmond, WA 98052 1036 US 1038 Email: bernard_aboba@hotmail.com