



Internet Engineering Task Force (IETF)                         H. Landau
Request for Comments: 8657                                 November 2019
Category: Standards Track                                               
ISSN: 2070-1721


   Certification Authority Authorization (CAA) Record Extensions for
  Account URI and Automatic Certificate Management Environment (ACME)
                             Method Binding

Abstract

   The Certification Authority Authorization (CAA) DNS record allows a
   domain to communicate an issuance policy to Certification Authorities
   (CAs) but only allows a domain to define a policy with CA-level
   granularity.  However, the CAA specification (RFC 8659) also provides
   facilities for an extension to admit a more granular, CA-specific
   policy.  This specification defines two such parameters: one allowing
   specific accounts of a CA to be identified by URIs and one allowing
   specific methods of domain control validation as defined by the
   Automatic Certificate Management Environment (ACME) protocol to be
   required.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8657.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Extensions to the CAA Record: The "accounturi" Parameter
     3.1.  Use with ACME
     3.2.  Use without ACME
   4.  Extensions to the CAA Record: The "validationmethods" Parameter
   5.  Security Considerations
     5.1.  Limited to CAs Processing CAA Records
     5.2.  Restrictions Ineffective without CA Recognition
     5.3.  Mandatory Consistency in CA Recognition
     5.4.  URI Ambiguity
     5.5.  Authorization Freshness
     5.6.  Use with and without DNSSEC
     5.7.  Restrictions Supersedable by DNS Delegation
     5.8.  Misconfiguration Hazards
     5.9.  Revelation of Account URIs
   6.  IANA Considerations
   7.  Normative References
   Appendix A.  Examples
   Author's Address

1.  Introduction

   This specification defines two parameters for the "issue" and
   "issuewild" Properties of the Certification Authority Authorization
   (CAA) DNS resource record [RFC8659].  The first, "accounturi", allows
   authorization conferred by a CAA policy to be restricted to specific
   accounts of a Certification Authority (CA), which are identified by
   URIs.  The second, "validationmethods", allows the set of validation
   methods supported by a CA to validate domain control to be limited to
   a subset of the full set of methods that it supports.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Extensions to the CAA Record: The "accounturi" Parameter

   This document defines the "accounturi" CAA parameter for the "issue"
   and "issuewild" Properties defined by [RFC8659].  The value of this
   parameter, if specified, MUST be a URI [RFC3986] identifying a
   specific CA account.

   "CA account" means an object that is maintained by a specific CA,
   that may request the issuance of certificates, and that represents a
   specific entity or group of related entities.

   The presence of this parameter constrains the Property to which it is
   attached.  Where a CAA Property has an "accounturi" parameter, a CA
   MUST only consider that Property to authorize issuance in the context
   of a given certificate issuance request if the CA recognizes the URI
   specified in the value portion of that parameter as identifying the
   account making that request.

   A Property without an "accounturi" parameter matches any account.  A
   Property with an invalid or unrecognized "accounturi" parameter is
   unsatisfiable.  A Property with multiple "accounturi" parameters is
   unsatisfiable.

   The presence of an "accounturi" parameter does not replace or
   supersede the need to validate the domain name specified in an
   "issue" or "issuewild" record in the manner described in the CAA
   specification [RFC8659].  CAs MUST still perform such validation.
   For example, a CAA "issue" Property that specifies a domain name
   belonging to CA A and an "accounturi" parameter identifying an
   account at CA B is unsatisfiable.

3.1.  Use with ACME

   An Automatic Certificate Management Environment (ACME) [RFC8555]
   account object MAY be identified by setting the "accounturi"
   parameter to the URI of the ACME account object.

   Implementations of this specification that also implement ACME MUST
   recognize such URIs.

3.2.  Use without ACME

   The "accounturi" specification provides a general mechanism to
   identify entities that may request certificate issuance via URIs.
   The use of specific kinds of URIs may be specified in future RFCs,
   and CAs not implementing ACME MAY assign and recognize their own URIs
   arbitrarily.

4.  Extensions to the CAA Record: The "validationmethods" Parameter

   This document also defines the "validationmethods" CAA parameter for
   the "issue" and "issuewild" Properties.  The value of this parameter,
   if specified, MUST be a comma-separated string of zero or more
   validation method labels.

   A validation method label identifies a validation method.  A
   validation method is a particular way in which a CA can validate
   control over a domain.

   The presence of this parameter constrains the Property to which it is
   attached.  A CA MUST only consider a Property with the
   "validationmethods" parameter to authorize issuance where the
   validation method being used is identified by one of the validation
   method labels listed in the comma-separated list.

   Each validation method label MUST be either the label of a method
   defined in the "ACME Validation Methods" IANA registry [RFC8555] or a
   CA-specific non-ACME validation method label as defined below.

   Where a CA supports both the "validationmethods" parameter and one or
   more non-ACME validation methods, it MUST assign labels to those
   methods.  If appropriate non-ACME labels are not present in the "ACME
   Validation Methods" IANA registry, the CA MUST use labels beginning
   with the string "ca-", which are defined to have CA-specific meaning.

   The value of the "validationmethods" parameter MUST comply with the
   following ABNF [RFC5234]:

      value = [*(label ",") label]
      label = 1*(ALPHA / DIGIT / "-")

5.  Security Considerations

   This specification describes an extension to the CAA record
   specification, increasing the granularity at which a CAA policy can
   be expressed.  This allows the set of entities capable of
   successfully requesting issuance of certificates for a given domain
   to be restricted beyond the set of entities would otherwise be
   possible, while still allowing issuance for specific accounts of a
   CA.  This improves the security of issuance for domains that choose
   to employ it, when combined with a CA that implements this
   specification.

5.1.  Limited to CAs Processing CAA Records

   All of the security considerations listed in [RFC8659] are inherited
   by this document.  This specification merely enables a domain with an
   existing relationship with a CA to further constrain that CA in its
   issuance practices, where that CA implements this specification.  In
   particular, it provides no additional security above that provided by
   using the unextended CAA specification alone as concerns matters
   relating to any other CA.  The capacity of any other CA to issue
   certificates for the given domain is completely unchanged.

   As such, a domain that, via CAA records, authorizes only CAs adopting
   this specification and that constrains its policy by means of this
   specification, remains vulnerable to unauthorized issuance by CAs
   that do not honor CAA records or that honor them only on an advisory
   basis.  Where a domain uses DNSSEC, it also remains vulnerable to CAs
   that honor CAA records but that do not validate CAA records by means
   of a trusted DNSSEC-validating resolver.

5.2.  Restrictions Ineffective without CA Recognition

   Because the parameters of "issue" or "issuewild" CAA Properties
   constitute a CA-specific namespace, the CA identified by an "issue"
   or "issuewild" Property decides what parameters to recognize and
   their semantics.  Accordingly, the CAA parameters defined in this
   specification rely on their being recognized by the CA named by an
   "issue" or "issuewild" CAA Property and are not an effective means of
   control over issuance unless a CA's support for the parameters is
   established beforehand.

   CAs that implement this specification SHOULD make available
   documentation indicating as such, including explicit statements as to
   which parameters are supported.  Domains configuring CAA records for
   a CA MUST NOT assume that the restrictions implied by the
   "accounturi" and "validationmethods" parameters are effective in the
   absence of explicit indication as such from that CA.

   CAs SHOULD also document whether they implement DNSSEC validation for
   DNS lookups done for validation purposes, as this affects the
   security of the "accounturi" and "validationmethods" parameters.

5.3.  Mandatory Consistency in CA Recognition

   A CA MUST ensure that its support for the "accounturi" and
   "validationmethods" parameters is fully consistent for a given domain
   name that a CA recognizes as identifying itself in a CAA "issue" or
   "issuewild" Property.  If a CA has multiple issuance systems (for
   example, an ACME-based issuance system and a non-ACME-based issuance
   system, or two different issuance systems resulting from a corporate
   merger), it MUST ensure that all issuance systems recognize the same
   parameters.

   A CA that is unable to do this MAY still implement the parameters by
   splitting the CA into two domain names for the purposes of CAA
   processing.  For example, a CA "example.com" with an ACME-based
   issuance system and a non-ACME-based issuance system could recognize
   only "acme.example.com" for the former and "example.com" for the
   latter, and then implement support for the "accounturi" and
   "validationmethods" parameters for "acme.example.com" only.

   A CA that is unable to ensure consistent processing of the
   "accounturi" parameter or the "validationmethods" parameter for a
   given CA domain name as specifiable in CAA "issue" or "issuewild"
   Properties MUST NOT implement support for these parameters.  Failure
   to do so would result in an implementation of these parameters that
   does not provide effective security.

5.4.  URI Ambiguity

   Suppose that CA A recognizes "a.example.com" as identifying itself
   and CA B is a subsidiary of CA A that recognizes both "a.example.com"
   and "b.example.com" as identifying itself.

   Suppose that both CA A and CA B issue account URIs of the form:

      "urn:example:account-id:1234"

   If the CA domain name in a CAA record is specified as
   "a.example.com", then this could be construed as identifying account
   number 1234 at CA A or at CA B.  These may be different accounts,
   creating ambiguity.

   Thus, CAs MUST ensure that the URIs they recognize as pertaining to a
   specific account of that CA are unique within the scope of all domain
   names that they recognize as identifying that CA for the purpose of
   CAA record validation.

   CAs SHOULD satisfy this requirement by using URIs that include an
   authority (see Section 3.2 of [RFC3986]):

      "https://a.example.com/account/1234"

5.5.  Authorization Freshness

   The CAA specification [RFC8659] governs the act of issuance by a CA.
   In some cases, a CA may establish authorization for an account to
   request certificate issuance for a specific domain separately from
   the act of issuance itself.  Such authorization may occur
   substantially prior to a certificate issuance request.  The CAA
   policy expressed by a domain may have changed in the meantime,
   creating the risk that a CA will issue certificates in a manner
   inconsistent with the presently published CAA policy.

   CAs SHOULD adopt practices to reduce the risk of such circumstances.
   Possible countermeasures include issuing authorizations with very
   limited validity periods, such as an hour, or revalidating the CAA
   policy for a domain at certificate issuance time.

5.6.  Use with and without DNSSEC

   The "domain validation" model of validation commonly used for
   certificate issuance cannot ordinarily protect against adversaries
   who can conduct global man-in-the-middle attacks against a particular
   domain.  A global man-in-the-middle attack is an attack that can
   intercept traffic to or from a given domain, regardless of the origin
   or destination of that traffic.  Such an adversary can intercept all
   validation traffic initiated by a CA and thus appear to have control
   of the given domain.

   Where a domain is signed using DNSSEC, the authenticity of its DNS
   data can be assured, providing that a given CA makes all DNS
   resolutions via a trusted DNSSEC-validating resolver.  A domain can
   use this Property to protect itself from the threat posed by an
   adversary capable of performing a global man-in-the-middle attack
   against that domain.

   In order to facilitate this, a CA validation process must either rely
   solely on information obtained via DNSSEC or meaningfully bind the
   other parts of the validation transaction using material obtained via
   DNSSEC.

   The CAA parameters described in this specification can be used to
   ensure that only validation methods meeting these criteria are used.
   In particular, a domain secured via DNSSEC SHOULD either:

   1.  Use the "accounturi" parameter to ensure that only accounts that
       it controls are authorized to obtain certificates, or

   2.  Exclusively use validation methods that rely solely on
       information obtained via DNSSEC and use the "validationmethods"
       parameter to ensure that only such methods are used.

   A CA supporting the "accounturi" parameter or the "validationmethods"
   parameter MUST perform CAA validation using a trusted
   DNSSEC-validating resolver.

   "Trusted" in this context means that the CA both trusts the resolver
   itself and ensures that the communications path between the resolver
   and the system performing CAA validation is secure.  It is
   RECOMMENDED that a CA ensure this by using a DNSSEC-validating
   resolver running on the same machine as the system performing CAA
   validation.

   The use of the "accounturi" parameter or the "validationmethods"
   parameter does not confer additional security against an attacker
   capable of performing a man-in-the-middle attack against all
   validation attempts made by a given CA that is authorized by CAA
   where:

   1.  A domain does not secure its nameservers using DNSSEC, or

   2.  That CA does not perform CAA validation using a trusted
       DNSSEC-validating resolver.

   Moreover, the use of the "accounturi" parameter or the
   "validationmethods" parameter does not mitigate man-in-the-middle
   attacks against CAs that do not validate CAA records or that do not
   do so using a trusted DNSSEC-validating resolver, regardless of
   whether or not those CAs are authorized by CAA; see Section 5.1.

   In these cases, the "accounturi" and "validationmethods" parameters
   still provide an effective means of administrative control over
   issuance, except where control over DNS is subdelegated (see below).

5.7.  Restrictions Supersedable by DNS Delegation

   CAA records are located during validation by walking up the DNS
   hierarchy until one or more records are found.  CAA records are
   therefore not an effective way of restricting or controlling issuance
   for subdomains of a domain, where control over those subdomains is
   delegated to another party (such as via DNS delegation or by
   providing limited access to manage subdomain DNS records).

5.8.  Misconfiguration Hazards

   Because the "accounturi" and "validationmethods" parameters express
   restrictive security policies, misconfiguration of said parameters
   may result in legitimate issuance requests being refused.

5.9.  Revelation of Account URIs

   Because CAA records are publicly accessible, the use of the
   "accounturi" parameter enables third parties to observe the
   authorized account URIs for a domain.  This may allow third parties
   to identify a correlation between domains if those domains use the
   same account URIs.

   CAs are encouraged to select and process account URIs under the
   assumption that untrusted third parties may learn of them.

6.  IANA Considerations

   This document has no IANA actions.  As per [RFC8659], the parameter
   namespace for the CAA "issue" and "issuewild" Properties has CA-
   defined semantics, and the identifiers within that namespace may be
   freely and arbitrarily assigned by a CA.  This document merely
   specifies recommended semantics for parameters of the names
   "accounturi" and "validationmethods", which CAs may choose to adopt.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC8659]  Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
              "DNS Certification Authority Authorization (CAA) Resource
              Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
              <https://www.rfc-editor.org/info/rfc8659>.

Appendix A.  Examples

   The following shows an example DNS zone file fragment that nominates
   two account URIs as authorized to issue certificates for the domain
   "example.com".  Issuance is restricted to the CA "example.net".

   example.com. IN CAA 0 issue "example.net; \
     accounturi=https://example.net/account/1234"
   example.com. IN CAA 0 issue "example.net; \
     accounturi=https://example.net/account/2345"

   The following shows a zone file fragment that restricts the ACME
   methods that can be used; only ACME methods "dns-01" and "xyz-01" can
   be used.

   example.com. IN CAA 0 issue "example.net; \
     validationmethods=dns-01,xyz-01"

   The following shows an equivalent way of expressing the same
   restriction:

   example.com. IN CAA 0 issue "example.net; validationmethods=dns-01"
   example.com. IN CAA 0 issue "example.net; validationmethods=xyz-01"

   The following shows a zone file fragment in which one account can be
   used to issue with the "dns-01" method and one account can be used to
   issue with the "http-01" method.

   example.com. IN CAA 0 issue "example.net; \
     accounturi=https://example.net/account/1234; \
     validationmethods=dns-01"
   example.com. IN CAA 0 issue "example.net; \
     accounturi=https://example.net/account/2345; \
     validationmethods=http-01"

   The following shows a zone file fragment in which only ACME method
   "dns-01" or a CA-specific method "ca-foo" can be used.

   example.com. IN CAA 0 issue "example.net; \
     validationmethods=dns-01,ca-foo"

Author's Address

   Hugo Landau

   Email: hlandau@devever.net
