CAA Record Extensions for Account URI and ACME Method Bindinghlandau@devever.net
General
ACME Working GroupInternet-DraftThe 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.This specification defines two parameters for the “issue” and “issuewild”
properties of the Certification Authority Authorization (CAA) DNS resource
record . 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.In this document, the key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL
NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” are to be
interpreted as described in BCP 14, RFC 2119 and indicate
requirement levels for compliant ACME-CAA implementations.A CAA parameter “accounturi” is defined for the “issue” and “issuewild”
properties defined by . The value of this parameter, if specified,
MUST be a URI 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 “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 as identifying the
account making that request.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 “accounturi” parameters, the CA MUST consider the CAA
record set to authorize issuance if and only if at least one of the specified
account URIs identifies the account of the CA by which issuance is requested.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 unsatisfiable.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 unsatisfiable.An ACME 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.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 arbitrarily.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 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 “validationmethods” 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 “validationmethods” parameter and one or more
non-ACME challenge methods, it MUST assign identifiers to those methods. If
appropriate non-ACME identifiers are not present in the ACME Validation Methods
IANA registry, the CA MUST use identifiers beginning with the string
“ca-“, which are defined to have CA-specific meaning.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.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
unchanged.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.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 “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.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 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.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 “example.com” with an ACME-based issuance system and a non-ACME-based
issuance system could recognise 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 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 will result in an implementation of these
parameters which does not provide effective security.Suppose that CA A recognises “a.example.com” as identifying itself, CA B is a
subsidiary of CA A which recognises 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“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 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 MUST satisfy this requirement by using URIs which include an authority:“https://a.example.com/account/1234”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.Where a domain chooses to secure its nameservers using DNSSEC, the authenticity
of its DNS data can be assured, providing that a given 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 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:Use the “accounturi” parameter to ensure that only accounts which it
controls are authorized to obtain certificates, orExclusively use validation methods which rely solely on information
obtained via DNSSEC, and use the “validationmethods” parameter to ensure
that only such methods are used.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:A domain does not secure its nameservers using DNSSEC, orThat 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
.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).Because CAA records are located during validation by walking up the DNS
hierarchy until one or more records are found, the use of the “accounturi” and
“validationmethods” 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).Because they express a restrictive security policy, misconfiguration of the
“accounturi” or “validationmethods” parameters may result in legitimate
issuance requests being refused.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 “accounturi” and
“validationmethods”.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Uniform Resource Identifier (URI): Generic SyntaxA Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK]DNS Certification Authority Authorization (CAA) Resource RecordThe Certification Authority Authorization (CAA) DNS Resource Record allows a DNS domain name holder to specify one or more Certification Authorities (CAs) authorized to issue certificates for that domain. CAA Resource Records allow a public Certification Authority to implement additional controls to reduce the risk of unintended certificate mis-issue. This document defines the syntax of the CAA record and rules for processing CAA records by certificate issuers. [STANDARDS-TRACK]Automatic Certificate Management Environment (ACME)Public Key Infrastructure X.509 (PKIX) certificates are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. Today, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a CA and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation. RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH: The source for this draft is maintained in GitHub. Suggested changes should be submitted as pull requests at https://github.com/ietf-wg-acme/acme [1]. Instructions are on that page as well. Editorial changes can be managed in GitHub, but any substantive change should be discussed on the ACME mailing list (acme@ietf.org).The following shows an example DNS zone file fragment which nominates two
account URIs as authorized to issue certificates for the domain “example.com”.
Issuance is restricted to the CA “example.net”.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.The following shows an equivalent way of expressing the same restriction: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.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.