CAA Record Extensions for Account URI and ACME Method Bindinghlandau@devever.net
General
ACME Working GroupInternet-DraftThe 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.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”, “NOT RECOMMENDED”, “MAY”, and
“OPTIONAL” are to be interpreted as described in BCP 14
when, and only when, they appear in all capitals, as shown here.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 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 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 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 :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.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.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 would 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“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 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 ):“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.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 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.A CA supporting the “accounturi” or “validationmethods” parameters 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 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 validation.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).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).Because the “accounturi” and “validationmethods” parameters express restrictive
security policies, misconfiguration of said parameters may result in legitimate
issuance requests being refused.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.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.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]Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [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 name. 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. This document obsoletes RFC 6844.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Automatic Certificate Management Environment (ACME)Public Key Infrastructure using 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. As of this writing, 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.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” or
a CA-specific method “ca-foo” can be used.