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.
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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