wdiff rfc8647.original rfc8647.txt

ACME Working Group
Internet Engineering Task Force (IETF) H. Landau

Intended status:
Request for Comments: 8647 August 2019
Category: Standards Track
Expires: December 22, 2019

CAA
ISSN: 2070-1721

Certification Authority Authorization (CAA) Record Extensions for
Account URI and ACME Automatic Certificate Management Environment (ACME)
Method Binding
draft-ietf-acme-caa-10

Abstract

The Certification Authority Authorization (CAA) DNS record allows a
domain to communicate an issuance policy to Certification Authorities
(CAs),
(CAs) but only allows a domain to define a policy with CA-level
granularity. However, the CAA specification (RFC 8644) also provides
facilities for an extension to admit a more granular, CA-specific
policy. This specification defines two such parameters, parameters: one allowing
specific accounts of a CA to be identified by URI URIs and one allowing
specific methods of domain control validation as defined by the ACME
Automatic Certificate Management Environment (ACME) protocol to be
required.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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Internet-Drafts are draft documents valid the IETF community. It has
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This Internet-Draft will expire on December 22, 2019.
https://www.rfc-editor.org/info/rfc8647.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Extensions to the CAA Record: accounturi The "accounturi" Parameter . . . . . 3
3.1. Use with ACME . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Use without ACME . . . . . . . . . . . . . . . . . . . . 3
4. Extensions to the CAA Record: validationmethods The "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 Supersedable 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 Properties of the Certification Authority Authorization
(CAA) DNS resource record [I-D.ietf-lamps-rfc6844bis]. [RFC8644]. The first, "accounturi", allows
authorization conferred by a CAA policy to be restricted to specific
accounts of a CA, 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 which that it supports.

2. Terminology

In this document, the

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: accounturi The "accounturi" Parameter

This document defines the "accounturi" CAA parameter "accounturi" is defined for the "issue"
and "issuewild" properties Properties defined by [I-D.ietf-lamps-rfc6844bis]. [RFC8644]. The value of this
parameter, if specified, MUST be a URI [RFC3986] identifying a
specific CA account.

"CA account" means an object, object that is maintained by a specific CA and which CA,
that may request the issuance of certificates, which and that represents a
specific entity or group of related entities.

The presence of this parameter constrains the property Property to which it is
attached. Where a CAA property Property has an "accounturi" parameter, a CA
MUST only consider that property Property to authorize issuance in the context
of a given certificate issuance request if the CA recognises recognizes the URI
specified in the value portion of that parameter as identifying the
account making that request.

A property Property without an "accounturi" parameter matches any account. A
property
Property with an invalid or unrecognised unrecognized "accounturi" parameter is
unsatisfiable. A property Property with multiple "accounturi" parameters is
unsatisfiable.

The presence of an "accounturi" parameter does not replace or
supercede
supersede the need to validate the domain name specified in an
"issue" or "issuewild" record in the manner described in the CAA
specification.
specification [RFC8644]. CAs MUST still perform such validation.
For example, a CAA "issue" property which 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 ACME 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 which that also implement ACME MUST
recognise
recognize such URIs.

3.2. Use without ACME

The "accounturi" specification provides a general mechanism to
identify entities which that may request certificate issuance via URIs.
The use of specific kinds of URI URIs may be specified in future RFCs,
and CAs not implementing ACME MAY assign and recognise recognize their own URIs
arbitrarily.

4. Extensions to the CAA Record: validationmethods The "validationmethods" Parameter

This document also defines the "validationmethods" CAA parameter "validationmethods" is also defined for
the "issue" and "issuewild" properties. 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 Property to which it is
attached. A CA MUST only consider a property 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 "ACME Validation Methods Methods" IANA registry, registry [RFC8555] or a CA-
specific
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 "ACME
Validation Methods 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
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 that which 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 which that choose
to employ it, when combined with a CA which that implements this
specification.

5.1. Limited to CAs Processing CAA Records

All of the security considerations of the CAA specification listed in [RFC8644] 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
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 which that, via CAA records records, authorizes only CAs adopting
this specification, specification and which that constrains its policy by means of this
specification, remains vulnerable to unauthorized issuance by CAs
which
that do not honour honor CAA records, records or which honour that honor them only on an advisory
basis. Where a domain uses DNSSEC, it also remains vulnerable to CAs which honour
that honor CAA records but which 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 Properties
constitute a CA-specific namespace, the CA identified by an "issue"
or "issuewild" property Property decides what parameters to recognise recognize and
their semantics. Accordingly, the CAA parameters defined in this
specification rely on their being recognised recognized by the CA named by an
"issue" or "issuewild" CAA property, Property and are not an effective means of
control over issuance unless a CA's support for the parameters is
established beforehand.

CAs which 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 which that a CA recognises recognizes as identifying itself in a CAA "issue" or
"issuewild" property. Property. If a CA has multiple issuance systems (for
example, an ACME-based issuance system and a non-ACME based non-ACME-based issuance
system, or two different issuance systems resulting from a corporate
merger), it MUST ensure that all issuance systems recognise recognize the same
parameters.

A CA which 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 recognise 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 which that is unable to ensure consistent processing of the
"accounturi" parameter or the "validationmethods" parameters parameter for a
given CA domain name as specifiable in CAA "issue" or "issuewild" properties
Properties MUST NOT implement support for these parameters. Failure
to do so would result in an implementation of these parameters which that
does not provide effective security.

5.4. URI Ambiguity

Suppose that CA A recognises recognizes "a.example.com" as identifying itself, itself
and CA B is a subsidiary of CA A which recognises 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 form:

"urn:example:account-id:1234"

If the CA domain name in a CAA record is specified as "a.example.com"
"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 recognize as pertaining to a
specific account of that CA are unique within the scope of all domain
names which that they recognise recognize as identifying that CA for the purpose of
CAA record validation.

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

"https://a.example.com/account/1234"

5.5. Authorization Freshness

The CAA specification [RFC8644] 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 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 which 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 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, 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 that
it controls are authorized to obtain certificates, or
2. Exclusively use validation methods which that rely solely on
information obtained via DNSSEC, DNSSEC and use the "validationmethods"
parameter to ensure that only such methods are used.

A CA supporting the "accounturi" parameter or the "validationmethods" parameters
parameter MUST perform CAA validation using a trusted, 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 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.

Use

The use of the "accounturi" parameter or the "validationmethods" parameters
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 which 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
DNSSEC-validating resolver.

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

None.

This document has no IANA actions. As per the CAA specification, [RFC8644], the parameter
namespace for the CAA "issue" and "issuewild" properties Properties has CA-defined semantics 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>.

[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>.

[RFC8644] Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
"DNS Certification Authority Authorization (CAA) Resource
Record", RFC 8644, DOI 10.17487/RFC8644, August 2019,
<https://www.rfc-editor.org/info/rfc8644>.

Appendix A. Examples

The following shows an example DNS zone file fragment which 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 which that restricts the ACME
methods which 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