ACME Working Group                                             H. Landau
Internet-Draft                                             June 20, 2019
Intended status: Standards Track
Expires: December 22, 2019

     CAA Record Extensions for Account URI and ACME Method Binding


   The Certification Authority Authorization (CAA) DNS record allows a
   domain to communicate issuance policy to Certification Authorities
   (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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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Extensions to the CAA Record: accounturi Parameter  . . . . .   3
     3.1.  Use with ACME . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Use without ACME  . . . . . . . . . . . . . . . . . . . .   3
   4.  Extensions to the CAA Record: validationmethods 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 . . . . . . . . . . . . . . . . .   7
     5.6.  Use with and without DNSSEC . . . . . . . . . . . . . . .   7
     5.7.  Restrictions Supercedable by DNS Delegation . . . . . . .   8
     5.8.  Misconfiguration Hazards  . . . . . . . . . . . . . . . .   9
     5.9.  Revelation of Account URIs  . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This specification defines two parameters for the "issue" and
   "issuewild" properties of the Certification Authority Authorization
   (CAA) DNS resource record [I-D.ietf-lamps-rfc6844bis].  The first,
   "accounturi", allows authorization conferred by a CAA policy to be
   restricted to specific accounts of a 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 which it supports.

2.  Terminology

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

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3.  Extensions to the CAA Record: accounturi Parameter

   A CAA parameter "accounturi" is defined for the "issue" and
   "issuewild" properties defined by [I-D.ietf-lamps-rfc6844bis].  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 and which
   may request the issuance of certificates, which 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 recognises 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 unrecognised "accounturi" parameter is
   unsatisfiable.  A property with multiple "accounturi" parameters is

   The presence of an "accounturi" 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 "issue" property which specifies a domain name belonging to CA
   A and an "accounturi" parameter identifying an account at CA B is

3.1.  Use with ACME

   An ACME [RFC8555] account object MAY be identified by setting the
   "accounturi" parameter to the URI of the ACME account object.

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

3.2.  Use without ACME

   The "accounturi" 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

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4.  Extensions to the CAA Record: validationmethods Parameter

   A CAA parameter "validationmethods" is also defined 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, 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 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.  Where a domain uses DNSSEC, it also remains
   vulnerable to CAs which honour CAA records but which 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 recognise and
   their semantics.  Accordingly, the CAA parameters defined in this
   specification rely on their being recognised 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 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
   "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 which a CA recognises as identifying itself in a CAA "issue" or
   "issuewild" property.  If a CA has multiple issuance systems (for

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   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

   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 "accounturi" and
   "validationmethods" parameters for "" only.

   A CA which is unable to ensure consistent processing of the
   "accounturi" or "validationmethods" 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 would
   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.

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


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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
   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 which 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

   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 which
       it controls are authorized to obtain certificates, or

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   2.  Exclusively use validation methods which 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" or "validationmethods" parameters
   MUST perform CAA validation using a trusted, DNSSEC-validating

   "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 are 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

   Use of the "accounturi" or "validationmethods" 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
   given CA which 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, use of the "accounturi" or "validationmethods" parameters
   does not mitigate against man-in-the-middle attacks against CAs which
   do not validate CAA records, or which do not do so using a trusted
   DNSSEC-validating resolver, regardless of whether those CAs are
   authorized by CAA or not; 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 Supercedable 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).

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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 publically accessible, 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

   None.  As per the CAA specification, 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

              Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
              "DNS Certification Authority Authorization (CAA) Resource
              Record", draft-ietf-lamps-rfc6844bis-07 (work in
              progress), May 2019.

   [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,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

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 "". IN CAA 0 issue "; \
     accounturi=" 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 "; validationmethods=dns-01" IN CAA 0 issue "; 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. IN CAA 0 issue "; \
     accounturi=; \
     validationmethods=dns-01" IN CAA 0 issue "; \
     accounturi=; \

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

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Author's Address

   Hugo Landau


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