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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force A. Popov 3 Internet-Draft M. Nystroem 4 Intended status: Standards Track Microsoft Corp. 5 Expires: January 8, 2017 D. Balfanz, Ed. 6 A. Langley 7 Google Inc. 8 J. Hodges 9 Paypal 10 July 7, 2016 12 Token Binding over HTTP 13 draft-ietf-tokbind-https-05 15 Abstract 17 This document describes a collection of mechanisms that allow HTTP 18 servers to cryptographically bind authentication tokens (such as 19 cookies and OAuth tokens) to TLS [RFC5246] connections. 21 We describe both _first-party_ and _federated_ scenarios. In a 22 first-party scenario, an HTTP server is able to cryptographically 23 bind the security tokens it issues to a client, and which the client 24 subsequently returns to the server, to the TLS connection between the 25 client and server. Such bound security tokens are protected from 26 misuse since the server can generally detect if they are replayed 27 inappropriately, e.g., over other TLS connections. 29 Federated token bindings, on the other hand, allow servers to 30 cryptographically bind security tokens to a TLS connection that the 31 client has with a _different_ server than the one issuing the token. 33 This Internet-Draft is a companion document to The Token Binding 34 Protocol [I-D.ietf-tokbind-protocol] 36 Status of This Memo 38 This Internet-Draft is submitted in full conformance with the 39 provisions of BCP 78 and BCP 79. 41 Internet-Drafts are working documents of the Internet Engineering 42 Task Force (IETF). Note that other groups may also distribute 43 working documents as Internet-Drafts. The list of current Internet- 44 Drafts is at http://datatracker.ietf.org/drafts/current/. 46 Internet-Drafts are draft documents valid for a maximum of six months 47 and may be updated, replaced, or obsoleted by other documents at any 48 time. It is inappropriate to use Internet-Drafts as reference 49 material or to cite them other than as "work in progress." 51 This Internet-Draft will expire on January 8, 2017. 53 Copyright Notice 55 Copyright (c) 2016 IETF Trust and the persons identified as the 56 document authors. All rights reserved. 58 This document is subject to BCP 78 and the IETF Trust's Legal 59 Provisions Relating to IETF Documents 60 (http://trustee.ietf.org/license-info) in effect on the date of 61 publication of this document. Please review these documents 62 carefully, as they describe your rights and restrictions with respect 63 to this document. Code Components extracted from this document must 64 include Simplified BSD License text as described in Section 4.e of 65 the Trust Legal Provisions and are provided without warranty as 66 described in the Simplified BSD License. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 71 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 72 2. The Sec-Token-Binding Header Field . . . . . . . . . . . . . 4 73 2.1. HTTPS Token Binding Key Pair Scoping . . . . . . . . . . 4 74 3. First-party Use Cases . . . . . . . . . . . . . . . . . . . . 5 75 4. Federation Use Cases . . . . . . . . . . . . . . . . . . . . 5 76 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 5 77 4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6 78 4.3. HTTP Redirects . . . . . . . . . . . . . . . . . . . . . 7 79 4.4. Negotiated Key Parameters . . . . . . . . . . . . . . . . 9 80 4.5. Federation Example . . . . . . . . . . . . . . . . . . . 9 81 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 82 5.1. Security Token Replay . . . . . . . . . . . . . . . . . . 12 83 5.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS 84 Versions . . . . . . . . . . . . . . . . . . . . . . . . 12 85 5.3. Sensitivity of the Sec-Token-Binding Header . . . . . . . 12 86 5.4. Securing Federated Sign-On Protocols . . . . . . . . . . 13 87 6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15 88 6.1. Scoping of Token Binding Keys . . . . . . . . . . . . . . 15 89 6.2. Life Time of Token Binding Keys . . . . . . . . . . . . . 16 90 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 91 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 92 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 93 9.1. Normative References . . . . . . . . . . . . . . . . . . 17 94 9.2. Informative References . . . . . . . . . . . . . . . . . 18 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 97 1. Introduction 99 The Token Binding Protocol [I-D.ietf-tokbind-protocol] defines a 100 Token Binding ID for a TLS connection between a client and a server. 101 The Token Binding ID of a TLS connection is related to a private key, 102 that the client proves possession of to the server, and is long-lived 103 (i.e., subsequent TLS connections between the same client and server 104 have the same Token Binding ID). When issuing a security token (e.g. 105 an HTTP cookie or an OAuth token) to a client, the server can include 106 the Token Binding ID in the token, thus cryptographically binding the 107 token to TLS connections between that particular client and server, 108 and inoculating the token against abuse (re-use, attempted 109 impersonation, etc.) by attackers. 111 While the Token Binding Protocol [I-D.ietf-tokbind-protocol] defines 112 a message format for establishing a Token Binding ID, it does not 113 specify how this message is embedded in higher-level protocols. The 114 purpose of this specification is to define how TokenBindingMessages 115 are embedded in HTTP (both versions 1.1 [RFC7230] and 2 [RFC7540]). 116 Note that TokenBindingMessages are only defined if the underlying 117 transport uses TLS. This means that Token Binding over HTTP is only 118 defined when the HTTP protocol is layered on top of TLS (commonly 119 referred to as HTTPS). 121 HTTP clients establish a Token Binding ID with a server by including 122 a special HTTP header field in HTTP requests. The HTTP header field 123 value is a base64url-encoded TokenBindingMessage. 125 TokenBindingMessages allow clients to establish multiple Token 126 Binding IDs with the server, by including multiple TokenBinding 127 structures in the TokenBindingMessage. By default, a client will 128 establish a _provided_ Token Binding ID with the server, indicating a 129 Token Binding ID that the client will persistently use with the 130 server. Under certain conditions, the client can also include a 131 _referred_ Token Binding ID in the TokenBindingMessage, indicating a 132 Token Binding ID that the client is using with a _different_ server 133 than the one that the TokenBindingMessage is sent to. This is useful 134 in federation scenarios. 136 1.1. Requirements Language 138 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 139 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 140 document are to be interpreted as described in [RFC2119]. 142 2. The Sec-Token-Binding Header Field 144 Once a client and server have negotiated the Token Binding Protocol 145 with HTTP/1.1 or HTTP/2 (see [I-D.ietf-tokbind-protocol] and 146 [I-D.ietf-tokbind-negotiation]), clients MUST include the Sec-Token- 147 Binding header field in their HTTP requests. The ABNF of the Sec- 148 Token-Binding header field is (in [RFC7230] style, see also [RFC7231] 149 Section 8.3): 151 Sec-Token-Binding = EncodedTokenBindingMessage 153 The header field name is "Sec-Token-Binding", and 154 EncodedTokenBindingMessage is a base64url encoding (see [RFC4648] 155 Section 5) of the TokenBindingMessage as defined in 156 [I-D.ietf-tokbind-protocol]. 158 For example: 160 Sec-Token-Binding: 162 The TokenBindingMessage MUST contain one TokenBinding structure with 163 TokenBindingType of provided_token_binding, which MUST be signed with 164 the Token Binding private key used by the client for connections 165 between itself and the server that the HTTP request is sent to 166 (clients use different Token Binding keys for different servers, see 167 Section 2.1 below). The Token Binding ID established by this 168 TokenBinding is called a _Provided Token Binding ID_. 170 The TokenBindingMessage MAY also contain one TokenBinding structure 171 with TokenBindingType of referred_token_binding, as specified in 172 Section 4.3. In addition to the latter, or rather than the latter, 173 the TokenBindingMessage MAY contain other TokenBinding structures. 174 This is use case-specific, and such use cases are outside the scope 175 of this specification. 177 In HTTP/2, the client SHOULD use Header Compression [RFC7541] to 178 avoid the overhead of repeating the same header field in subsequent 179 HTTP requests. 181 2.1. HTTPS Token Binding Key Pair Scoping 183 HTTPS is used in conjunction with various application protocols, and 184 application contexts, in various ways. For example, general purpose 185 Web browsing is one such HTTP-based application context. Within the 186 latter context, HTTP cookies [RFC6265] are typically utilized for 187 state management, including client authentication. A related, though 188 distinct, example of other HTTP-based application contexts is where 189 OAuth tokens [RFC6749] are utilized to manage authorization for 190 third-party application access to resources. The token scoping rules 191 of these two examples can differ: the scoping rules for cookies are 192 concisely specified in [RFC6265], whereas OAuth is a framework and 193 defines various token types with various scopings, some of which are 194 determined by the encompassing application. 196 The Token Binding key pair scoping for those key pairs generated in 197 the context of the first-party and federation use cases defined in 198 this specification (below), and to be used for binding HTTP cookies 199 MUST be at the granularity of "effective top-level domain (public 200 suffix) + 1" (eTLD+1), i.e., at the same granularity at which cookies 201 can be set (see [RFC6265]). Key pairs used to bind other application 202 tokens, such as OAuth tokens, SHOULD adhere to the above eTLD+1 203 scoping requirement for those tokens being employed in first-party or 204 federation scenarios as described below, e.g., OAuth refresh tokens 205 or Open ID Connect "ID Tokens". See also Section 6.1, below. 207 Scoping rules for other HTTP-based application contexts are outside 208 the scope of this specification. 210 3. First-party Use Cases 212 In a first-party use case, an HTTP server issues a security token 213 such as a cookie (or similar) to a client, and expects the client to 214 return the security token at a later time, e.g., in order to 215 authenticate. Binding the security token to the TLS connection 216 between client and server protects the security token from misuse 217 since the server can detect if the security token is replayed 218 inappropriately, e.g., over other TLS connections. 220 See [I-D.ietf-tokbind-protocol] Section 6 for general guidance 221 regarding binding of security tokens and their subsequent validation. 223 4. Federation Use Cases 225 4.1. Introduction 227 For privacy reasons, clients use different private keys to establish 228 Provided Token Binding IDs with different servers. As a result, a 229 server cannot bind a security token (such as an OAuth token or an 230 OpenID Connect identity token) to a TLS connection that the client 231 has with a different server. This is, however, a common requirement 232 in federation scenarios: For example, an Identity Provider may wish 233 to issue an identity token to a client and cryptographically bind 234 that token to the TLS connection between the client and a Relying 235 Party. 237 In this section we describe mechanisms to achieve this. The common 238 idea among these mechanisms is that a server (called the _Token 239 Consumer_ in this document) signals to the client that it should 240 reveal the Provided Token Binding ID that is used between the client 241 and itself, to another server (called the _Token Provider_ in this 242 document). Also common across the mechanisms is how the Token 243 Binding ID is revealed to the Token Provider: The client uses the 244 Token Binding Protocol [I-D.ietf-tokbind-protocol], and includes a 245 TokenBinding structure in the Sec-Token-Binding HTTP header field 246 defined above. What differs between the various mechanisms is _how_ 247 the Token Consumer signals to the client that it should reveal the 248 Token Binding ID to the Token Provider. Below we specify one such 249 mechanism, which is suitable for redirect-based interactions between 250 Token Consumers and Token Providers. 252 4.2. Overview 254 In a Federated Sign-On protocol, an Identity Provider issues an 255 identity token to a client, which sends the identity token to a 256 Relying Party to authenticate itself. Examples of this include 257 OpenID Connect (where the identity token is called "ID Token") and 258 SAML (where the identity token is a SAML assertion). 260 To better protect the security of the identity token, the Identity 261 Provider may wish to bind the identity token to the TLS connection 262 between the client and the Relying Party, thus ensuring that only 263 said client can use the identity token: The Relying Party will 264 compare the Token Binding ID in the identity token with the Token 265 Binding ID of the TLS connection between it and the client. 267 This is an example of a federation scenario, which more generally can 268 be described as follows: 270 o A Token Consumer causes the client to issue a token request to the 271 Token Provider. The goal is for the client to obtain a token and 272 then use it with the Token Consumer. 274 o The client delivers the token request to the Token Provider. 276 o The Token Provider issues the token. The token is issued for the 277 specific Token Consumer who requested it (thus preventing 278 malicious Token Consumers from using tokens with other Token 279 Consumers). The token is, however, typically a bearer token, 280 meaning that any client can use it with the Token Consumer, not 281 just the client to which it was issued. 283 o Therefore, in the previous step, the Token Provider may want to 284 include in the token the Token-Binding public key that the client 285 uses when communicating with the Token Consumer, thus _binding_ 286 the token to client's Token-Binding keypair. The client proves 287 possession of the private key when communicating with the Token 288 Consumer through the Token Binding Protocol 289 [I-D.ietf-tokbind-protocol], and reveals the corresponding public 290 key of this keypair as part of the Token Binding ID. Comparing 291 the public key from the token with the public key from the Token 292 Binding ID allows the Token Consumer to verify that the token was 293 sent to it by the legitimate client. 295 o To allow the Token Provider to include the Token-Binding public 296 key in the token, the Token Binding ID (between client and Token 297 Consumer) must therefore be communicated to the Token Provider 298 along with the token request. Communicating a Token Binding ID 299 involves proving possession of a private key and is described in 300 the Token Binding Protocol [I-D.ietf-tokbind-protocol]. 302 The client will perform this last operation (proving possession of a 303 private key that corresponds to a Token Binding ID between the client 304 and the Token Consumer while delivering the token request to the 305 Token Provider) only if the Token Consumer requests the client to do 306 so. 308 Below, we specify how Token Consumers can signal this request in 309 redirect-based federation protocols. Note that this assumes that the 310 federated sign-on flow starts at the Token Consumer, or at the very 311 least include a redirect from Token Consumer to Token Provider. It 312 is outside the scope of this document to specify similar mechanisms 313 for flows that do not include such redirects. 315 4.3. HTTP Redirects 317 When a Token Consumer redirects the client to a Token Provider as a 318 means to deliver the token request, it SHOULD include a Include- 319 Referred-Token-Binding-ID HTTP response header field in its HTTP 320 response. The ABNF of the Include-Referred-Token-Binding-ID header 321 is (in [RFC7230] style, see also [RFC7231] Section 8.3): 323 Include-Referred-Token-Binding-ID = "true" 325 Where the header field name is "Include-Referred-Token-Binding-ID", 326 and the field-value of "true" is case-insensitive. For example: 328 Include-Referred-Token-Binding-ID: true 330 Including this response header field signals to the client that it 331 should reveal, to the Token Provider, the Token Binding ID used 332 between itself and the Token Consumer. In the absence of this 333 response header field, the client will not disclose any information 334 about the Token Binding used between the client and the Token 335 Consumer to the Token Provider. 337 As illustrated in Section 4.5, when a client receives this header 338 field, it should take the TokenBindingID of the provided TokenBinding 339 from the referrer and create a referred TokenBinding with it to 340 include in the TokenBindingMessage on the redirect request. In other 341 words, the Token Binding message in the redirect request to the Token 342 Provider now includes one provided binding and one referred binding, 343 the latter constructed from the binding between the client and the 344 Token Consumer. Note that that the referred token binding is sent 345 only on the request resulting from the redirect and not on any 346 subsequent requests to the Token Provider 348 If the Include-Referred-Token-Binding-ID header field is received in 349 response to a request that did not include the Token-Binding header 350 field, the client MUST ignore the Include-Referred-Token-Binding-ID 351 header field. 353 This header field has only meaning if the HTTP status code is 301, 354 302, 303, 307 or 308, and MUST be ignored by the client for any other 355 status codes. If the client supports the Token Binding Protocol, and 356 has negotiated the Token Binding Protocol with both the Token 357 Consumer and the Token Provider, it already sends the Sec-Token- 358 Binding header field to the Token Provider with each HTTP request 359 (see above). 361 The TokenBindingMessage SHOULD contain a TokenBinding with 362 TokenBindingType referred_token_binding. If included, this 363 TokenBinding MUST be signed with the Token Binding key used by the 364 client for connections between itself and the Token Consumer (more 365 specifically, the web origin that issued the Include-Referred-Token- 366 Binding-ID response header field). The Token Binding ID established 367 by this TokenBinding is called a _Referred Token Binding ID_. 369 As described above, the TokenBindingMessage MUST additionally contain 370 a Provided Token Binding ID, i.e., a TokenBinding structure with 371 TokenBindingType provided_token_binding, which MUST be signed with 372 the Token Binding key used by the client for connections between 373 itself and the Token Provider (more specifically, the web origin that 374 the token request is being sent to). 376 If for some deployment-specific reason the initial Token Provider 377 ("TP1") needs to redirect the client to another Token Provider 378 ("TP2"), rather than directly back to the Token Consumer, it can be 379 accomodated using the header fields defined in this specification in 380 the following fashion ("the redirect-chain approach"): 382 Initially, the client is redirected to TP1 by the Token Consumer 383 ("TC"), as described above. Upon receiving the client's request, 384 containing a TokenBindingMessage which contains both provided and 385 referred TokenBindings (for TP1 and TC, respectively), TP1 386 responds to the client with a redirect response containing the 387 Include-Referred-Token-Binding-ID header field and directing the 388 client to send a request to TP2. This causes the client to follow 389 the same pattern and send a request containing a 390 TokenBindingMessage which contains both provided and referred 391 TokenBindings (for TP2 and TP1, respectively) to TP2. Note that 392 this pattern can continue to further Token Providers. In this 393 case, TP2 issues a security token, bound to the client's 394 TokenBinding with TP1, and sends a redirect response to the client 395 pointing to TP1. TP1 in turn constructs a security token for the 396 Token Consumer, bound to the TC's referred TokenBinding which had 397 been conveyed earlier, and sends a redirect response pointing to 398 the TC, containing the bound security token, to the client. 400 The above is intended as only a non-normative example. Details are 401 specific to deployment contexts. Other approaches are possible, but 402 are outside the scope of this specification. 404 4.4. Negotiated Key Parameters 406 The TLS Extension for Token Binding Protocol Negotiation 407 [I-D.ietf-tokbind-negotiation] allows the server and client to 408 negotiate the parameters (signature algorithm, length) of the Token 409 Binding key. It is possible that the Token Binding ID used between 410 the client and the Token Consumer, and the Token Binding ID used 411 between the client and Token Provider, use different key parameters. 412 The client MUST use the key parameters negotiated with the Token 413 Consumer in the referred_token_binding TokenBinding of the 414 TokenBindingMessage, even if those key parameters are different from 415 the ones negotiated with the origin that the header field is sent to. 417 Token Providers SHOULD support all the Token Binding key parameters 418 specified in the [I-D.ietf-tokbind-protocol]. If a token provider 419 does not support the key parameters specified in the 420 referred_token_binding TokenBinding in the TokenBindingMessage, it 421 MUST issue an unbound token. 423 4.5. Federation Example 425 The diagram below shows a typical HTTP Redirect-based Web Browser SSO 426 Profile (no artifact, no callbacks), featuring binding of, e.g., a 427 TLS Token Binding ID into an OpenID Connect "ID Token". 429 Legend: 431 +------------+------------------------------------------------------+ 432 | EKM: | TLS Exported Keying Material [RFC5705] | 433 | {EKMn}Ksm: | EKM for server "n", signed by private key of TBID | 434 | | "m", where "n" must represent server receiving the | 435 | | ETBMSG, if a conveyed TB's type is | 436 | | provided_token_binding, then m = n, else if TB's | 437 | | type is referred_token_binding, then m != n. E.g., | 438 | | see step 1b in diagram below. | 439 | ETBMSG: | "Sec-Token-Binding" HTTP header field conveying an | 440 | | EncodedTokenBindingMessage, in turn conveying | 441 | | TokenBinding (TB)struct(s), e.g.: ETBMSG[[TB]] or | 442 | | ETBMSG[[TB1],[TB2]] | 443 | ID Token: | the "ID Token" in OIDC, it is the semantic | 444 | | equivalent of a SAML "authentication assertion". "ID | 445 | | Token w/TBIDn" denotes a "token bound" ID Token | 446 | | containing TBIDn. | 447 | Ks & Kp: | private (aka secret) key, and public key, | 448 | | respectively, of client-side Token Binding key pair | 449 | OIDC: | Open ID Connect | 450 | TB: | TokenBinding struct containing signed EKM, TBID, and | 451 | | TB type, e.g.: | 452 | | [{EKM1}Ks1,TBID1,provided_token_binding] | 453 | TBIDn: | Token Binding ID for client and server n's token- | 454 | | bound TLS association. TBIDn contains Kpn. | 455 +------------+------------------------------------------------------+ 457 Client, Token Consumer, Token Provider, 458 aka: aka: aka: 459 User Agent OpenID Client, OpenID Provider, 460 OIDC Relying Party, OIDC Provider, 461 SAML Relying Party SAML Identity Provider 462 [ server "1" ] [ server "2" ] 463 +--------+ +----+ +-----+ 464 | Client | | TC | | TP | 465 +--------+ +----+ +-----+ 466 | | | 467 | | | 468 | | | 469 | 0. Client interacts w/TC | | 470 | over HTTPS, establishes Ks1 & Kp1, TBID1 | 471 | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] | 472 |------------------------------>| | 473 | | | 474 | | | 475 | | | 476 | 1a. OIDC ID Token request, aka| | 477 | "Authentication Request", conveyed with | 478 | HTTP response header field of: | 479 | Include-Referred-Token-Binding-ID:true | 480 | any security-relevant cookies | | 481 | should contain TBID1 | | 482 +<- - - - - - - - - - - - - - - - | | 483 . | (redirect to TP via 301, 302, | | 484 . | 303, 307, or 308) | | 485 . | | | 486 +------------------------------------------------------->| 487 | 1b. opens HTTPS w/TP, | 488 | establishes Ks2, Kp2, TBID2; | 489 | sends GET or POST with | 490 | ETBMSG[[{EKM2}Ks2,TBID2,provided_token_binding], | 491 | [{EKM2}Ks1,TBID1,referred_token_binding]] | 492 | as well as the ID Token request | 493 | | | 494 | | | 495 | | | 496 | 2. user authentication (if applicable, | 497 | methods vary, particulars are out of scope) | 498 |<====================================================>| 499 | (TP generates ID Token for TC containing TBID1, may | 500 | also set cookie(s) containing TBID2 and/or TBID1, | 501 | details vary, particulars are out of scope) | 502 | | | 503 | | | 504 | | | 505 | 3a. ID Token containing Kp1, issued for TC, | 506 | conveyed via OIDC "Authentication Response" | 507 +<- - - - - - - - - - - - - - - - - - - - - - - - - - - -| 508 . | (redirect to TC) | | 509 . | | | 510 . | | | 511 +-------------------------------->| | 512 | 3b. HTTPS GET or POST with | 513 | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]] | 514 | conveying Authn Reponse containing | 515 | ID Token w/TBID1, issued for TC | 516 | | | 517 | | | 518 | | | 519 | 4. user is signed-on, any security-relevant cookie(s)| 520 | that are set SHOULD contain TBID1 | 521 |<------------------------------| | 522 | | | 523 | | | 525 5. Security Considerations 527 5.1. Security Token Replay 529 The goal of the Federated Token Binding mechanisms is to prevent 530 attackers from exporting and replaying tokens used in protocols 531 between the client and Token Consumer, thereby impersonating 532 legitimate users and gaining access to protected resources. Bound 533 tokens can still be replayed by malware present in the client. In 534 order to export the token to another machine and successfully replay 535 it, the attacker also needs to export the corresponding private key. 536 The Token Binding private key is therefore a high-value asset and 537 MUST be strongly protected, ideally by generating it in a hardware 538 security module that prevents key export. 540 5.2. Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions 542 The Token Binding protocol relies on the exported key material (EKM) 543 value [RFC5705] to associate a TLS connection with a TLS Token 544 Binding. The triple handshake attack [TRIPLE-HS] is a known 545 vulnerability in TLS 1.2 and older TLS versions, allowing the 546 attacker to synchronize keying material between TLS connections. The 547 attacker can then successfully replay bound tokens. For this reason, 548 the Token Binding protocol MUST NOT be negotiated with these TLS 549 versions, unless the Extended Master Secret [RFC7627] and 550 Renegotiation Indication [RFC5746] TLS extensions have also been 551 negotiated. 553 5.3. Sensitivity of the Sec-Token-Binding Header 555 The purpose of the Token Binding protocol is to convince the server 556 that the client that initiated the TLS connection controls a certain 557 key pair. For the server to correctly draw this conclusion after 558 processing the Sec-Token-Binding header field, certain secrecy and 559 integrity requirements must be met. 561 For example, the client's private Token Binding key must be kept 562 secret by the client. If the private key is not secret, then another 563 actor in the system could create a valid Token Binding header field, 564 impersonating the client. This can render the main purpose of the 565 protocol - to bind bearer tokens to certain clients - moot: Consider, 566 for example, an attacker who obtained (perhaps through a network 567 intrusion) an authentication cookie that a client uses with a certain 568 server. Consider further that the server bound that cookie to the 569 client's Token Binding ID precisely to thwart misuse of the cookie. 570 If the attacker were to come into possession of the client's private 571 key, he could then establish a TLS connection with the server and 572 craft a Sec-Token-Binding header field that matches the binding 573 present in the cookie, thus successfully authenticating as the 574 client, and gaining access to the client's data at the server. The 575 Token Binding protocol, in this case, did not successfully bind the 576 cookie to the client. 578 Likewise, we need integrity protection of the Sec-Token-Binding 579 header field: A client should not be tricked into sending a Sec- 580 Token-Binding header field to a server that contains Token Binding 581 messages about key pairs that the client does not control. Consider 582 an attacker A that somehow has knowledge of the exported keying 583 material (EKM) for a TLS connection between a client C and a server 584 S. (While that is somewhat unlikely, it is also not entirely out of 585 the question, since the client might not treat the EKM as a secret - 586 after all, a pre-image-resistant hash function has been applied to 587 the TLS master secret, making it impossible for someone knowing the 588 EKM to recover the TLS master secret. Such considerations might lead 589 some clients to not treat the EKM as a secret.) Such an attacker A 590 could craft a Sec-Token-Binding header field with A's key pair over 591 C's EKM. If the attacker could now trick C to send such a header 592 field to S, it would appear to S as if C controls a certain key pair 593 when in fact it does not (the attacker A controls the key pair). 595 If A has a pre-existing relationship with S (perhaps has an account 596 on S), it now appears to the server S as if A is connecting to it, 597 even though it is really C. (If the server S does not simply use 598 Token Binding keys to identify clients, but also uses bound 599 authentication cookies, then A would also have to trick C into 600 sending one of A's cookies to S, which it can do through a variety of 601 means - inserting cookies through Javascript APIs, setting cookies 602 through related-domain attacks, etc.) In other words, A tricked C 603 into logging into A's account on S. This could lead to a loss of 604 privacy for C, since A presumably has some other way to also access 605 the account, and can thus indirectly observe A's behavior (for 606 example, if S has a feature that lets account holders see their 607 activity history on S). 609 Therefore, we need to protect the integrity of the Sec-Token-Binding 610 header field. One origin should not be able to set the Sec-Token- 611 Binding header field (through a DOM API or otherwise) that the User 612 Agent uses with another origin. Employing the "Sec-" header field 613 prefix helps to meet this requirement by denoting the header field 614 name to be a "forbidden header name", see [fetch-spec]. 616 5.4. Securing Federated Sign-On Protocols 618 As explained above, in a federated sign-in scenario a client will 619 prove possession of two different key pairs to a Token Provider: One 620 key pair is the "provided" Token Binding key pair (which the client 621 normally uses with the Token Provider), and the other is the 622 "referred" Token Binding key pair (which the client normally uses 623 with the Token Consumer). The Token Provider is expected to issue a 624 token that is bound to the referred Token Binding key. 626 Both proofs (that of the provided Token Binding key and that of the 627 referred Token Binding key) are necessary. To show this, consider 628 the following scenario: 630 o The client has an authentication token with the Token Provider 631 that is bound to the client's Token Binding key. 633 o The client wants to establish a secure (i.e., free of men-in-the- 634 middle) authenticated session with the Token Consumer, but has not 635 done so yet (in other words, we are about to run the federated 636 sign-on protocol). 638 o A man-in-the-middle is allowed to intercept the connection between 639 client and Token Consumer or between Client and Token Provider (or 640 both). 642 The goal is to detect the presence of the man-in-the-middle in these 643 scenarios. 645 First, consider a man-in-the-middle between the client and the Token 646 Provider. Recall that we assume that the client possesses a bound 647 authentication token (e.g., cookie) for the Token Provider. The man- 648 in-the-middle can intercept and modify any message sent by the client 649 to the Token Provider, and any message sent by the Token Provider to 650 the client. (This means, among other things, that the man-in-the- 651 middle controls the Javascript running at the client in the origin of 652 the Token Provider.) It is not, however, in possession of the 653 client's Token Binding key. Therefore, it can either choose to 654 replace the Token Binding key in requests from the client to the 655 Token Provider, and create a Sec-Token-Binding header field that 656 matches the TLS connection between the man-in-the-middle and the 657 Token Provider; or it can choose to leave the Sec-Token-Binding 658 header field unchanged. If it chooses the latter, the signature in 659 the Token Binding message (created by the original client on the 660 exported keying material (EKM) for the connection between client and 661 man-in-the-middle) will not match the EKM between man-in-the-middle 662 and the Token Provider. If it chooses the former (and creates its 663 own signature, with its own Token Binding key, over the EKM for the 664 connection between man-in-the-middle and Token Provider), then the 665 Token Binding message will match the connection between man-in-the- 666 middle and Token Provider, but the Token Binding key in the message 667 will not match the Token Binding key that the client's authentication 668 token is bound to. Either way, the man-in-the-middle is detected by 669 the Token Provider, but only if the proof of key possession of the 670 provided Token Binding key is required in the protocol (as we do 671 above). 673 Next, consider the presence of a man-in-the-middle between client and 674 Token Consumer. That man-in-the-middle can intercept and modify any 675 message sent by the client to the Token Consumer, and any message 676 sent by the Token Consumer to the client. The Token Consumer is the 677 party that redirects the client to the Token Provider. In this case, 678 the man-in-the-middle controls the redirect URL, and can tamper with 679 any redirect URL issued by the Token Consumer (as well as with any 680 Javascript running in the origin of the Token Consumer). The goal of 681 the man-in-the-middle is to trick the Token Issuer to issue a token 682 bound to _its_ Token Binding key, not to the Token Binding key of the 683 legitimate client. To thwart this goal of the man-in-the-middle, the 684 client's referred Token Binding key must be communicated to the Token 685 Producer in a manner that can not be affected by the man-in-the- 686 middle (who, as we recall, can modify redirect URLs and Javascript at 687 the client). Including the referred Token Binding message in the 688 Sec-Token-Binding header field (as opposed to, say, including the 689 referred Token Binding key in an application-level message as part of 690 the redirect URL) is one way to assure that the man-in-the-middle 691 between client and Token Consumer cannot affect the communication of 692 the referred Token Binding key to the Token Provider. 694 Therefore, the Sec-Token-Binding header field in the federated sign- 695 on use case contains both, a proof of possession of the provided 696 Token Binding key, as well as a proof of possession of the referred 697 Token Binding key. 699 6. Privacy Considerations 701 6.1. Scoping of Token Binding Keys 703 Clients use different Token Binding key pairs for different servers, 704 so as to not allow Token Binding to become a tracking tool across 705 different servers. However, the scoping of the Token Binding key 706 pairs to servers varies according to the scoping rules of the 707 application protocol ([I-D.ietf-tokbind-protocol] section 4.1). 709 In the case of HTTP cookies, servers may use Token Binding to secure 710 their cookies. These cookies can be attached to any sub-domain of 711 effective top-level domains, and clients therefore should use the 712 same Token Binding key across such subdomains. This will ensure that 713 any server capable of receiving the cookie will see the same Token 714 Binding ID from the client, and thus be able to verify the token 715 binding of the cookie. See Section 2.1, above. 717 6.2. Life Time of Token Binding Keys 719 Token Binding keys do not have an expiration time. This means that 720 they can potentially be used by a server to track a user across an 721 extended period of time (similar to a long-lived cookie). HTTPS 722 clients such as web user agents should therefore provide a user 723 interface for discarding Token Binding keys (similar to the 724 affordances provided to delete cookies). 726 If a user agent provides modes such as private browsing mode in which 727 the user is promised that browsing state such as cookies are 728 discarded after the session is over, the user agent should also 729 discard Token Binding keys from such modes after the session is over. 730 Generally speaking, users should be given the same level of control 731 over life time of Token Binding keys as they have over cookies or 732 other potential tracking mechanisms. 734 7. IANA Considerations 736 Below are the Internet Assigned Numbers Authority (IANA) Permanent 737 Message Header Field registration information per [RFC3864]. 739 Header field name: Sec-Token-Binding 740 Applicable protocol: HTTP 741 Status: standard 742 Author/Change controller: IETF 743 Specification document(s): this one 745 Header field name: Include-Referred-Token-Binding-ID 746 Applicable protocol: HTTP 747 Status: standard 748 Author/Change controller: IETF 749 Specification document(s): this one 751 [[TODO: possibly add further considerations wrt the behavior of the 752 above header fields, per ]] 755 8. Acknowledgements 757 This document incorporates comments and suggestions offered by Eric 758 Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony 759 Nadalin, Michael Jones, Bill Cox, Nick Harper, Brian Campbell and 760 others. 762 9. References 764 9.1. Normative References 766 [fetch-spec] 767 WhatWG, "Fetch", Living Standard , 768 . 770 [I-D.ietf-tokbind-negotiation] 771 Popov, A., Nystrom, M., Balfanz, D., and A. Langley, 772 "Transport Layer Security (TLS) Extension for Token 773 Binding Protocol Negotiation", draft-ietf-tokbind- 774 negotiation-03 (work in progress), July 2016. 776 [I-D.ietf-tokbind-protocol] 777 Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J. 778 Hodges, "The Token Binding Protocol Version 1.0", draft- 779 ietf-tokbind-protocol-07 (work in progress), July 2016. 781 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 782 Requirement Levels", BCP 14, RFC 2119, 783 DOI 10.17487/RFC2119, March 1997, 784 . 786 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 787 Procedures for Message Header Fields", BCP 90, RFC 3864, 788 DOI 10.17487/RFC3864, September 2004, 789 . 791 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 792 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, 793 . 795 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 796 (TLS) Protocol Version 1.2", RFC 5246, 797 DOI 10.17487/RFC5246, August 2008, 798 . 800 [RFC5705] Rescorla, E., "Keying Material Exporters for Transport 801 Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, 802 March 2010, . 804 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, 805 DOI 10.17487/RFC6265, April 2011, 806 . 808 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 809 Protocol (HTTP/1.1): Message Syntax and Routing", 810 RFC 7230, DOI 10.17487/RFC7230, June 2014, 811 . 813 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 814 Protocol (HTTP/1.1): Semantics and Content", RFC 7231, 815 DOI 10.17487/RFC7231, June 2014, 816 . 818 [RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for 819 HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015, 820 . 822 9.2. Informative References 824 [RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, 825 "Transport Layer Security (TLS) Renegotiation Indication 826 Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010, 827 . 829 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 830 RFC 6749, DOI 10.17487/RFC6749, October 2012, 831 . 833 [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization 834 Framework: Bearer Token Usage", RFC 6750, 835 DOI 10.17487/RFC6750, October 2012, 836 . 838 [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext 839 Transfer Protocol Version 2 (HTTP/2)", RFC 7540, 840 DOI 10.17487/RFC7540, May 2015, 841 . 843 [RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A., 844 Langley, A., and M. Ray, "Transport Layer Security (TLS) 845 Session Hash and Extended Master Secret Extension", 846 RFC 7627, DOI 10.17487/RFC7627, September 2015, 847 . 849 [TRIPLE-HS] 850 Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti, 851 A., and P. Strub, "Triple Handshakes and Cookie Cutters: 852 Breaking and Fixing Authentication over TLS. IEEE 853 Symposium on Security and Privacy", 2014. 855 Authors' Addresses 857 Andrei Popov 858 Microsoft Corp. 859 USA 861 Email: andreipo@microsoft.com 863 Magnus Nystroem 864 Microsoft Corp. 865 USA 867 Email: mnystrom@microsoft.com 869 Dirk Balfanz (editor) 870 Google Inc. 871 USA 873 Email: balfanz@google.com 875 Adam Langley 876 Google Inc. 877 USA 879 Email: agl@google.com 881 Jeff Hodges 882 Paypal 883 USA 885 Email: Jeff.Hodges@paypal.com