ACME Working Group                                             H. Landau
Internet-Draft                                           August 30, 2017
Intended status: Standards Track
Expires: March 3, 2018

     CAA Record Extensions for Account URI and ACME Method Binding


   The CAA DNS record allows a domain to communicate issuance policy to
   CAs, but only allows a domain to define policy with CA-level
   granularity.  However, the CAA specification also provides facilities
   for extension to admit more granular, CA-specific policy.  This
   specification defines two such parameters, one allowing specific
   accounts of a CA to be identified by URI and one allowing specific
   methods of domain control validation as defined by the ACME protocol
   to be required.

Status of This Memo

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   This Internet-Draft will expire on March 3, 2018.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Extensions to the CAA Record: account-uri Parameter . . . . .   3
     3.1.  Use with ACME . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Use without ACME  . . . . . . . . . . . . . . . . . . . .   4
   4.  Extensions to the CAA Record: validation-methods Parameter  .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
     5.1.  Limited to CAs Processing CAA Records . . . . . . . . . .   5
     5.2.  Restrictions Ineffective without CA Recognition . . . . .   5
     5.3.  Mandatory Consistency in CA Recognition . . . . . . . . .   5
     5.4.  URI Ambiguity . . . . . . . . . . . . . . . . . . . . . .   6
     5.5.  Authorization Freshness . . . . . . . . . . . . . . . . .   6
     5.6.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.7.  Use without DNSSEC  . . . . . . . . . . . . . . . . . . .   7
     5.8.  Restrictions Supercedable by DNS Delegation . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This specification defines two parameters for the "issue" and
   "issuewild" properties of the Certification Authority Authorization
   (CAA) DNS resource record [RFC6844].  The first, "account-uri",
   allows authorization conferred by a CAA policy to be restricted to
   specific accounts of a CA, which are identified by URIs.  The second,
   "validation-methods", 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 which it supports.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant ACME-CAA

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3.  Extensions to the CAA Record: account-uri Parameter

   A CAA parameter "account-uri" is defined for the "issue" and
   "issuewild" properties defined by [RFC6844].  The value of this
   parameter, if specified, MUST be a URI [RFC3986] identifying a
   specific CA account.

   "CA account" means an object maintained by a specific CA representing
   a specific entity, or group of related entities, which may request
   the issuance of certificates.

   The presence of this parameter constrains the property to which it is
   attached.  Where a CAA property has an "account-uri" parameter, a CA
   MUST NOT consider that property to authorize issuance in the context
   of a given certificate issuance request unless the CA recognises the
   URI specified as identifying the account making that request.

   If a certificate issuance request is made to a CA such that no
   account URI is available, because the request is made in the absence
   of any account or the account has no URI assigned to it, a CA MUST
   NOT consider any property having an "account-uri" parameter as
   authorizing issuance.

   If a CA finds multiple CAA records pertaining to it (i.e., having
   property "issue" or "issuewild" as applicable and a domain that the
   CA recognises as its own) with different "account-uri" parameters,
   the CA MUST NOT consider the CAA record set to authorize issuance
   unless at least one of the specified account URIs identifies the
   account of the CA by which issuance is requested.  A property without
   an "account-uri" parameter matches any account.  A property with an
   invalid or unrecognised "account-uri" parameter is unsatisfiable.  A
   property with multiple "account-uri" parameters is unsatisfiable.

   The presence of an "account-uri" parameter does not replace or
   supercede the need to validate the domain name specified in an
   "issue" or "issuewild" record in the manner described in the CAA
   specification.  CAs MUST still perform such validation.  For example,
   a CAA property which specifies a domain name belonging to CA A and an
   account URI identifying an account at CA B is unsatisfiable.

3.1.  Use with ACME

   An ACME [I-D.ietf-acme-acme] account object MAY be identified by
   setting the "account-uri" parameter to the URI of the ACME account

   Implementations of this specification which also implement ACME MUST
   recognise such URIs.

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3.2.  Use without ACME

   The "account-uri" specification provides a general mechanism to
   identify entities which may request certificate issuance via URIs.
   The use of specific kinds of URI may be specified in future RFCs, and
   CAs not implementing ACME MAY assign and recognise their own URIs

4.  Extensions to the CAA Record: validation-methods Parameter

   A CAA parameter "validation-methods" is also defined for the "issue"
   and "issuewild" properties.  The value of this parameter, if
   specified, MUST be a comma-separated string of challenge method
   names.  Each challenge method name MUST be either an ACME challenge
   method name or a CA-assigned non-ACME challenge method name.

   The presence of this parameter constrains the property to which it is
   attached.  A CA MUST only consider a property with the "validation-
   methods" parameter to authorize issuance where the name of the
   challenge method being used is one of the names listed in the comma-
   separated list.

   Where a CA supports both the "validation-methods" parameter and one
   or more non-ACME challenge methods, it MUST assign identifiers to
   those methods.  These identifiers MUST be chosen to minimise the
   likelihood of conflict with any ACME challenge method name; it is
   RECOMMENDED that, at the very least, CAs avoid assigning identifiers
   ending in a hyphen and two digits ("-00").

   A CA SHOULD assign individual identifiers to each of its non-ACME
   challenge methods.  However, if it is unable or unwilling to do so,
   it MAY use the fallback identifier of "non-acme" to identify such

5.  Security Considerations

   This specification describes an extension to the CAA record
   specification increasing the granularity at which 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 that which would otherwise be possible, while still
   allowing issuance for specific accounts of a CA.  This improves the
   security of issuance for domains which choose to employ it, when
   combined with a CA which implements this specification.

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5.1.  Limited to CAs Processing CAA Records

   All of the security considerations of the CAA specification 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 use of 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

   As such, a domain which via CAA records authorizes only CAs adopting
   this specification, and which constrains its policy by means of this
   specification, remains vulnerable to unauthorized issuance by CAs
   which do not honour CAA records, or which honour them only on an
   advisory basis.

5.2.  Restrictions Ineffective without CA Recognition

   The CAA parameters specified in this specification rely on their
   being recognised by the CA named by an "issue" or "issuewild" CAA
   property.  As such, the parameters are not an effective means of
   control over issuance unless a CA's support for the parameters is
   established beforehand.

   CAs which 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 "account-
   uri" and "validation-methods" 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 "account-uri" and "validation-methods" parameters.

5.3.  Mandatory Consistency in CA Recognition

   A CA MUST ensure that its support for the "account-uri" and
   "validation-methods" parameters is fully consistent for a given
   domain name which a CA recognises 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
   recognise the same parameters.

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   A CA which 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 "" with an ACME-based
   issuance system and a non-ACME-based issuance system could recognise
   only "" for the former and "" for the
   latter, and then implement support for the "account-uri" and
   "validation-methods" parameters for "" only.

   A CA which is unable to ensure consistent processing of the "account-
   uri" or "validation-methods" parameters 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 will result
   in an implementation of these parameters which does not provide
   effective security.

5.4.  URI Ambiguity

   Suppose that CA A recognises "" as identifying itself,
   CA B is a subsidiary of CA A which recognises both ""
   and "" as identifying itself.

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


   If the CA domain name in a CAA record is specified as ""
   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 recognise as pertaining to a
   specific account of that CA are unique within the scope of all domain
   names which they recognise as identifying that CA for the purpose of
   CAA record validation.

   It is RECOMMENDED that CAs satisfy this requirement by using URIs
   which include an authority:


5.5.  Authorization Freshness

   The CAA specification 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 to 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

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   issue certificates in a manner inconsistent with the presently
   published CAA policy.

   CAs SHOULD consider adopting 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

5.6.  DNSSEC

   Where a domain chooses to secure its nameservers using DNSSEC, the
   authenticity of its DNS data can be assured, providing that a CA
   makes all DNS resolutions via an appropriate, trusted DNSSEC-
   validating resolver.  A domain can use this property to protect
   itself from the threat posed by a global adversary capable of
   performing man-in-the-middle attacks, which is not ordinarily
   mitigated by the "domain validation" model.

   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

   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 "account-uri" parameter to ensure that only accounts
       which it controls are authorized to obtain certificates, or

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

5.7.  Use without DNSSEC

   Where a domain does not secure its nameservers using DNSSEC, or one
   or more of the CAs it authorizes do not perform CAA validation
   lookups using a trusted DNSSEC-validating resolver, use of the
   "account-uri" or "validation-methods" parameters does not confer
   additional security against an attacker capable of performing a man-
   in-the-middle attack against all validation attempts made by a CA, as
   such an attacker could simply fabricate the responses to DNS lookups
   for CAA records.

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   In this case, the "account-uri" and "validation-methods" parameters
   still provide an effective means of administrative control over
   issuance, except where control over DNS is subdelegated (see below).

5.8.  Restrictions Supercedable by DNS Delegation

   Because CAA records are located during validation by walking up the
   DNS hierarchy until one or more records are found, the use of the
   "account-uri" and "validation-methods" parameters, or any CAA policy,
   is not an effective way to restrict or control 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).

6.  IANA Considerations

   None.  As per the CAA specification, the parameter namespace for the
   CAA "issue" and "issuewild" properties has CA-defined semantics.
   This document merely specifies a RECOMMENDED semantic for parameters
   of the names "account-uri" and "validation-methods".

7.  Normative References

              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-07 (work in progress), June 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997, <

   [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,

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013, <https://www.rfc-

Appendix A.  Examples

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

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     account-uri=" IN CAA 0 issue "; \

   The following shows a zone file fragment which restricts the ACME
   methods which can be used; only ACME methods "dns-01" and "xyz-01"
   can be used. IN CAA 0 issue "; \

   The following shows an equivalent way of expressing the same
   restriction: IN CAA 0 issue "; validation-methods=dns-01" IN CAA 0 issue "; validation-methods=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. IN CAA 0 issue "; \
     account-uri=; \
     validation-methods=dns-01" IN CAA 0 issue "; \
     account-uri=; \

   The following shows a zone file fragment in which only ACME method
   "dns-01" can be used, but non-ACME methods of issuance are also
   allowed. IN CAA 0 issue "; \

Author's Address

   Hugo Landau


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