Internet DRAFT - draft-ietf-acme-caa
draft-ietf-acme-caa
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
draft-ietf-acme-caa-10
Abstract
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 22, 2019.
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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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
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
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.
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
arbitrarily.
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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
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.
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
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.
5.4. URI Ambiguity
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 [RFC3986]):
"https://a.example.com/account/1234"
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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
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 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
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:
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
[I-D.ietf-lamps-rfc6844bis]
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,
<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>.
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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, <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>.
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
"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 which restricts the ACME
methods which 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.
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example.com. IN CAA 0 issue "example.net; \
validationmethods=dns-01,ca-foo"
Author's Address
Hugo Landau
Email: hlandau@devever.net
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