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(The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (May 27, 2018) is 2158 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-18) exists of draft-ietf-acme-acme-12 ** Obsolete normative reference: RFC 6844 (Obsoleted by RFC 8659) Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACME Working Group H. Landau 3 Internet-Draft May 27, 2018 4 Intended status: Standards Track 5 Expires: November 28, 2018 7 CAA Record Extensions for Account URI and ACME Method Binding 8 draft-ietf-acme-caa-04 10 Abstract 12 The CAA DNS record allows a domain to communicate issuance policy to 13 CAs, but only allows a domain to define policy with CA-level 14 granularity. However, the CAA specification also provides facilities 15 for extension to admit more granular, CA-specific policy. This 16 specification defines two such parameters, one allowing specific 17 accounts of a CA to be identified by URI and one allowing specific 18 methods of domain control validation as defined by the ACME protocol 19 to be required. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on November 28, 2018. 38 Copyright Notice 40 Copyright (c) 2018 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (https://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 57 3. Extensions to the CAA Record: account-uri Parameter . . . . . 3 58 3.1. Use with ACME . . . . . . . . . . . . . . . . . . . . . . 3 59 3.2. Use without ACME . . . . . . . . . . . . . . . . . . . . 4 60 4. Extensions to the CAA Record: validation-methods Parameter . 4 61 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 62 5.1. Limited to CAs Processing CAA Records . . . . . . . . . . 5 63 5.2. Restrictions Ineffective without CA Recognition . . . . . 5 64 5.3. Mandatory Consistency in CA Recognition . . . . . . . . . 5 65 5.4. URI Ambiguity . . . . . . . . . . . . . . . . . . . . . . 6 66 5.5. Authorization Freshness . . . . . . . . . . . . . . . . . 6 67 5.6. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . . 7 68 5.7. Use without DNSSEC . . . . . . . . . . . . . . . . . . . 7 69 5.8. Restrictions Supercedable by DNS Delegation . . . . . . . 8 70 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 71 7. Normative References . . . . . . . . . . . . . . . . . . . . 8 72 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 9 73 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9 75 1. Introduction 77 This specification defines two parameters for the "issue" and 78 "issuewild" properties of the Certification Authority Authorization 79 (CAA) DNS resource record [RFC6844]. The first, "account-uri", 80 allows authorization conferred by a CAA policy to be restricted to 81 specific accounts of a CA, which are identified by URIs. The second, 82 "validation-methods", allows the set of validation methods supported 83 by a CA to validate domain control to be limited to a subset of the 84 full set of methods which it supports. 86 2. Terminology 88 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 89 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 90 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 91 [RFC2119] and indicate requirement levels for compliant ACME-CAA 92 implementations. 94 3. Extensions to the CAA Record: account-uri Parameter 96 A CAA parameter "account-uri" is defined for the "issue" and 97 "issuewild" properties defined by [RFC6844]. The value of this 98 parameter, if specified, MUST be a URI [RFC3986] identifying a 99 specific CA account. 101 "CA account" means an object maintained by a specific CA representing 102 a specific entity, or group of related entities, which may request 103 the issuance of certificates. 105 The presence of this parameter constrains the property to which it is 106 attached. Where a CAA property has an "account-uri" parameter, a CA 107 MUST NOT consider that property to authorize issuance in the context 108 of a given certificate issuance request unless the CA recognises the 109 URI specified as identifying the account making that request. 111 If a certificate issuance request is made to a CA such that no 112 account URI is available, because the request is made in the absence 113 of any account or the account has no URI assigned to it, a CA MUST 114 NOT consider any property having an "account-uri" parameter as 115 authorizing issuance. 117 If a CA finds multiple CAA records pertaining to it (i.e., having 118 property "issue" or "issuewild" as applicable and a domain that the 119 CA recognises as its own) with different "account-uri" parameters, 120 the CA MUST NOT consider the CAA record set to authorize issuance 121 unless at least one of the specified account URIs identifies the 122 account of the CA by which issuance is requested. A property without 123 an "account-uri" parameter matches any account. A property with an 124 invalid or unrecognised "account-uri" parameter is unsatisfiable. A 125 property with multiple "account-uri" parameters is unsatisfiable. 127 The presence of an "account-uri" parameter does not replace or 128 supercede the need to validate the domain name specified in an 129 "issue" or "issuewild" record in the manner described in the CAA 130 specification. CAs MUST still perform such validation. For example, 131 a CAA property which specifies a domain name belonging to CA A and an 132 account URI identifying an account at CA B is unsatisfiable. 134 3.1. Use with ACME 136 An ACME [I-D.ietf-acme-acme] account object MAY be identified by 137 setting the "account-uri" parameter to the URI of the ACME account 138 object. 140 Implementations of this specification which also implement ACME MUST 141 recognise such URIs. 143 3.2. Use without ACME 145 The "account-uri" specification provides a general mechanism to 146 identify entities which may request certificate issuance via URIs. 147 The use of specific kinds of URI may be specified in future RFCs, and 148 CAs not implementing ACME MAY assign and recognise their own URIs 149 arbitrarily. 151 4. Extensions to the CAA Record: validation-methods Parameter 153 A CAA parameter "validation-methods" is also defined for the "issue" 154 and "issuewild" properties. The value of this parameter, if 155 specified, MUST be a comma-separated string of challenge method 156 names. Each challenge method name MUST be either an ACME challenge 157 method name or a CA-assigned non-ACME challenge method name. 159 The presence of this parameter constrains the property to which it is 160 attached. A CA MUST only consider a property with the "validation- 161 methods" parameter to authorize issuance where the name of the 162 challenge method being used is one of the names listed in the comma- 163 separated list. 165 Where a CA supports both the "validation-methods" parameter and one 166 or more non-ACME challenge methods, it MUST assign identifiers to 167 those methods. These identifiers MUST be chosen to minimise the 168 likelihood of conflict with any ACME challenge method name; it is 169 RECOMMENDED that, at the very least, CAs avoid assigning identifiers 170 ending in a hyphen and two digits ("-00"). 172 A CA SHOULD assign individual identifiers to each of its non-ACME 173 challenge methods. However, if it is unable or unwilling to do so, 174 it MAY use the fallback identifier of "non-acme" to identify such 175 methods. 177 5. Security Considerations 179 This specification describes an extension to the CAA record 180 specification increasing the granularity at which CAA policy can be 181 expressed. This allows the set of entities capable of successfully 182 requesting issuance of certificates for a given domain to be 183 restricted beyond that which would otherwise be possible, while still 184 allowing issuance for specific accounts of a CA. This improves the 185 security of issuance for domains which choose to employ it, when 186 combined with a CA which implements this specification. 188 5.1. Limited to CAs Processing CAA Records 190 All of the security considerations of the CAA specification are 191 inherited by this document. This specification merely enables a 192 domain with an existing relationship with a CA to further constrain 193 that CA in its issuance practices, where that CA implements this 194 specification. In particular, it provides no additional security 195 above that provided by use of the unextended CAA specification alone 196 as concerns matters relating to any other CA. The capacity of any 197 other CA to issue certificates for the given domain is completely 198 unchanged. 200 As such, a domain which via CAA records authorizes only CAs adopting 201 this specification, and which constrains its policy by means of this 202 specification, remains vulnerable to unauthorized issuance by CAs 203 which do not honour CAA records, or which honour them only on an 204 advisory basis. 206 5.2. Restrictions Ineffective without CA Recognition 208 The CAA parameters specified in this specification rely on their 209 being recognised by the CA named by an "issue" or "issuewild" CAA 210 property. As such, the parameters are not an effective means of 211 control over issuance unless a CA's support for the parameters is 212 established beforehand. 214 CAs which implement this specification SHOULD make available 215 documentation indicating as such, including explicit statements as to 216 which parameters are supported. Domains configuring CAA records for 217 a CA MUST NOT assume that the restrictions implied by the "account- 218 uri" and "validation-methods" parameters are effective in the absence 219 of explicit indication as such from that CA. 221 CAs SHOULD also document whether they implement DNSSEC validation for 222 DNS lookups done for validation purposes, as this affects the 223 security of the "account-uri" and "validation-methods" parameters. 225 5.3. Mandatory Consistency in CA Recognition 227 A CA MUST ensure that its support for the "account-uri" and 228 "validation-methods" parameters is fully consistent for a given 229 domain name which a CA recognises as identifying itself in a CAA 230 "issue" or "issuewild" property. If a CA has multiple issuance 231 systems (for example, an ACME-based issuance system and a non-ACME 232 based issuance system, or two different issuance systems resulting 233 from a corporate merger), it MUST ensure that all issuance systems 234 recognise the same parameters. 236 A CA which is unable to do this MAY still implement the parameters by 237 splitting the CA into two domain names for the purposes of CAA 238 processing. For example, a CA "example.com" with an ACME-based 239 issuance system and a non-ACME-based issuance system could recognise 240 only "acme.example.com" for the former and "example.com" for the 241 latter, and then implement support for the "account-uri" and 242 "validation-methods" parameters for "acme.example.com" only. 244 A CA which is unable to ensure consistent processing of the "account- 245 uri" or "validation-methods" parameters for a given CA domain name as 246 specifiable in CAA "issue" or "issuewild" properties MUST NOT 247 implement support for these parameters. Failure to do so will result 248 in an implementation of these parameters which does not provide 249 effective security. 251 5.4. URI Ambiguity 253 Suppose that CA A recognises "a.example.com" as identifying itself, 254 CA B is a subsidiary of CA A which recognises both "a.example.com" 255 and "b.example.com" as identifying itself. 257 Suppose that both CA A and CA B issue account URIs of the form 259 "account-id:1234" 261 If the CA domain name in a CAA record is specified as "a.example.com" 262 then this could be construed as identifying account number 1234 at CA 263 A or at CA B. These may be different accounts, creating ambiguity. 265 Thus, CAs MUST ensure that the URIs they recognise as pertaining to a 266 specific account of that CA are unique within the scope of all domain 267 names which they recognise as identifying that CA for the purpose of 268 CAA record validation. 270 It is RECOMMENDED that CAs satisfy this requirement by using URIs 271 which include an authority: 273 "https://a.example.com/account/1234" 275 5.5. Authorization Freshness 277 The CAA specification governs the act of issuance by a CA. In some 278 cases, a CA may establish authorization for an account to request 279 certificate issuance for a specific domain separately to the act of 280 issuance itself. Such authorization may occur substantially prior to 281 a certificate issuance request. The CAA policy expressed by a domain 282 may have changed in the meantime, creating the risk that a CA will 283 issue certificates in a manner inconsistent with the presently 284 published CAA policy. 286 CAs SHOULD consider adopting practices to reduce the risk of such 287 circumstances. Possible countermeasures include issuing 288 authorizations with very limited validity periods, such as an hour, 289 or revalidating the CAA policy for a domain at certificate issuance 290 time. 292 5.6. DNSSEC 294 Where a domain chooses to secure its nameservers using DNSSEC, the 295 authenticity of its DNS data can be assured, providing that a CA 296 makes all DNS resolutions via an appropriate, trusted DNSSEC- 297 validating resolver. A domain can use this property to protect 298 itself from the threat posed by a global adversary capable of 299 performing man-in-the-middle attacks, which is not ordinarily 300 mitigated by the "domain validation" model. 302 In order to facilitate this, a CA validation process must either rely 303 solely on information obtained via DNSSEC, or meaningfully bind the 304 other parts of the validation transaction using material obtained via 305 DNSSEC. 307 The CAA parameters described in this specification can be used to 308 ensure that only validation methods meeting these criteria are used. 309 In particular, a domain secured via DNSSEC SHOULD either: 311 1. Use the "account-uri" parameter to ensure that only accounts 312 which it controls are authorized to obtain certificates, or 314 2. Exclusively use validation methods which rely solely on 315 information obtained via DNSSEC, and use the "validation-methods" 316 parameter to ensure that only such methods are used. 318 5.7. Use without DNSSEC 320 Where a domain does not secure its nameservers using DNSSEC, or one 321 or more of the CAs it authorizes do not perform CAA validation 322 lookups using a trusted DNSSEC-validating resolver, use of the 323 "account-uri" or "validation-methods" parameters does not confer 324 additional security against an attacker capable of performing a man- 325 in-the-middle attack against all validation attempts made by a CA, as 326 such an attacker could simply fabricate the responses to DNS lookups 327 for CAA records. 329 In this case, the "account-uri" and "validation-methods" parameters 330 still provide an effective means of administrative control over 331 issuance, except where control over DNS is subdelegated (see below). 333 5.8. Restrictions Supercedable by DNS Delegation 335 Because CAA records are located during validation by walking up the 336 DNS hierarchy until one or more records are found, the use of the 337 "account-uri" and "validation-methods" parameters, or any CAA policy, 338 is not an effective way to restrict or control issuance for 339 subdomains of a domain, where control over those subdomains is 340 delegated to another party (such as via DNS delegation or by 341 providing limited access to manage subdomain DNS records). 343 6. IANA Considerations 345 None. As per the CAA specification, the parameter namespace for the 346 CAA "issue" and "issuewild" properties has CA-defined semantics. 347 This document merely specifies a RECOMMENDED semantic for parameters 348 of the names "account-uri" and "validation-methods". 350 7. Normative References 352 [I-D.ietf-acme-acme] 353 Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. 354 Kasten, "Automatic Certificate Management Environment 355 (ACME)", draft-ietf-acme-acme-12 (work in progress), April 356 2018. 358 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 359 Requirement Levels", BCP 14, RFC 2119, 360 DOI 10.17487/RFC2119, March 1997, 361 . 363 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 364 Resource Identifier (URI): Generic Syntax", STD 66, 365 RFC 3986, DOI 10.17487/RFC3986, January 2005, 366 . 368 [RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification 369 Authority Authorization (CAA) Resource Record", RFC 6844, 370 DOI 10.17487/RFC6844, January 2013, 371 . 373 Appendix A. Examples 375 The following shows an example DNS zone file fragment which nominates 376 two account URIs as authorized to issue certificates for the domain 377 "example.com". Issuance is restricted to the CA "example.net". 379 example.com. IN CAA 0 issue "example.net; \ 380 account-uri=https://example.net/account/1234" 381 example.com. IN CAA 0 issue "example.net; \ 382 account-uri=https://example.net/account/2345" 384 The following shows a zone file fragment which restricts the ACME 385 methods which can be used; only ACME methods "dns-01" and "xyz-01" 386 can be used. 388 example.com. IN CAA 0 issue "example.net; \ 389 validation-methods=dns-01,xyz-01" 391 The following shows an equivalent way of expressing the same 392 restriction: 394 example.com. IN CAA 0 issue "example.net; validation-methods=dns-01" 395 example.com. IN CAA 0 issue "example.net; validation-methods=xyz-01" 397 The following shows a zone file fragment in which one account can be 398 used to issue with the "dns-01" method and one account can be used to 399 issue with the "http-01" method. 401 example.com. IN CAA 0 issue "example.net; \ 402 account-uri=https://example.net/account/1234; \ 403 validation-methods=dns-01" 404 example.com. IN CAA 0 issue "example.net; \ 405 account-uri=https://example.net/account/2345; \ 406 validation-methods=http-01" 408 The following shows a zone file fragment in which only ACME method 409 "dns-01" can be used, but non-ACME methods of issuance are also 410 allowed. 412 example.com. IN CAA 0 issue "example.net; \ 413 validation-methods=dns-01,non-acme" 415 Author's Address 417 Hugo Landau 419 Email: hlandau@devever.net