Internet DRAFT - draft-ietf-acme-subdomains
draft-ietf-acme-subdomains
ACME Working Group O. Friel
Internet-Draft R. Barnes
Intended status: Standards Track Cisco
Expires: 2 September 2023 T. Hollebeek
DigiCert
M. Richardson
Sandelman Software Works
1 March 2023
Automated Certificate Management Environment (ACME) for Subdomains
draft-ietf-acme-subdomains-07
Abstract
This document specifies how Automated Certificate Management
Environment (ACME) can be used by a client to obtain a certificate
for a subdomain identifier from a certification authority. This
document specifies how a client can fulfill a challenge against an
ancestor domain but may not need to fulfill a challenge against the
explicit subdomain if certification authority policy allows issuance
of the subdomain certificate without explicit subdomain ownership
proof.
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. ACME Workflow and Identifier Requirements . . . . . . . . . . 5
4. ACME Issuance of Subdomain Certificates . . . . . . . . . . . 6
4.1. Authorization Object . . . . . . . . . . . . . . . . . . 7
4.2. Pre-Authorization . . . . . . . . . . . . . . . . . . . . 8
4.3. New Orders . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Directory Object Metadata . . . . . . . . . . . . . . . . 11
5. Illustrative Call Flow . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6.1. Authorization Object Fields Registry . . . . . . . . . . 17
6.2. Directory Object Metadata Fields Registry . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Client Account Security . . . . . . . . . . . . . . . . . 19
7.2. Subdomain Determination . . . . . . . . . . . . . . . . . 19
7.3. ACME Server Policy Considerations . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
ACME [RFC8555] defines a protocol that a certification authority (CA)
and an applicant can use to automate the process of domain name
ownership validation and X.509v3 (PKIX) [RFC5280] certificate
issuance. The CA is the ACME server and the applicant is the ACME
client, and the client uses the ACME protocol to request certificate
issuance from the server. This document outlines how ACME can be
used to issue subdomain certificates, without requiring the ACME
client to explicitly fulfill an ownership challenge against the
subdomain identifiers - the ACME client need only fulfill an
ownership challenge against an ancestor domain identifier.
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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.
The following terms are defined in DNS Terminology [RFC8499] and are
reproduced here:
* Label: An ordered list of zero or more octets that makes up a
portion of a domain name. Using graph theory, a label identifies
one node in a portion of the graph of all possible domain names.
* Domain Name: An ordered list of one or more labels.
* Fully-Qualified Domain Name (FQDN): This is often just a clear way
of saying the same thing as "domain name of a node", as outlined
above. However, the term is ambiguous. Strictly speaking, a
fully-qualified domain name would include every label, including
the zero-length label of the root: such a name would be written
"www.example.net." (note the terminating dot). But, because every
name eventually shares the common root, names are often written
relative to the root (such as "www.example.net") and are still
called "fully qualified". This term first appeared in [RFC0819].
In this document, names are often written relative to the root.
The following definition for "subdomain" is taken from DNS
Terminology [RFC8499] and reproduced here, however the definition is
ambiguous and is further clarified below:
* Subdomain: "A domain is a subdomain of another domain if it is
contained within that domain. This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's
name." (Quoted from Section 3.1 of [RFC1034].) For example, in
the host name "nnn.mmm.example.com", both "mmm.example.com" and
"nnn.mmm.example.com" are subdomains of "example.com". Note that
the comparisons here are done on whole labels; that is,
"ooo.example.com" is not a subdomain of "oo.example.com".
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The definition is ambiguous as it appears to allow a subdomain to
include the given domain. That is, "mmm.example.com" ends with
"mmm.example.com" and thus is a subdomain of itself. This document
interprets the first sentence of the above definition as meaning "A
domain is a subdomain of a different domain if it is contained within
that different domain.". A domain cannot be a subdomain of itself.
For example, "mmm.example.com" is not a subdomain of
"mmm.example.com".
The following additional terms are used in this document:
* Certification Authority (CA): An organization that is responsible
for the creation, issuance, revocation, and management of
Certificates. The term applies equally to both Root CAs and
Subordinate CAs. Refer to [RFC5280] for detailed information on
Certification Authorities.
* CSR: Certificate Signing Request as defined in [RFC2986]
* Ancestor Domain: a domain is an ancestor domain of a subdomain if
it contains that subdomain and has less labels than that
subdomain. A domain cannot be an ancestor domain of itself. For
example, for the host name "nnn.mmm.example.com", both
"mmm.example.com" and "example.com" are ancestor domains of
"nnn.mmm.example.com". However, "nnn.mmm.example.com" is not an
ancestor domain of "nnn.mmm.example.com". Note that the
comparisons here are done on whole labels; that is,
"oo.example.com" is not an ancestor domain of "ooo.example.com".
ACME [RFC8555] defines the following object types which are used in
this document:
* Order Object: An ACME order object represents a client's request
for a certificate and is used to track the progress of that order
through to issuance.
* Authorization Object: An ACME authorization object represents a
server's authorization for an account to represent an identifier.
* Challenge Object: An ACME challenge object represents a server's
offer to validate a client's possession of an identifier in a
specific way.
ACME [RFC8555] Section 6.3 introduces the following term which is
used in this document:
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* POST-as-GET Request: When a client wishes to fetch a resource from
the server, then it MUST send a POST request with a signed JWS
body, where the JWS body is specified in ACME [RFC8555]
Section 6.2. ACME refers to these as "POST-as-GET" requests.
3. ACME Workflow and Identifier Requirements
A typical ACME [RFC8555] workflow for issuance of certificates is as
follows:
1. client POSTs a newOrder request that contains a set of
"identifiers"
2. server replies with an order object that contains a set of links
to authorization object(s) and a "finalize" URI
3. client sends POST-as-GET requests to retrieve the authorization
object(s), with the downloaded authorization object(s) containing
the "identifier" that the client must prove that they control,
and a set of links to associated challenges objects, one of which
the client must fulfill
4. client proves control over the "identifier" in the authorization
object by completing one of the specified challenges, for
example, by publishing a DNS TXT record
5. client POSTs a CSR to the "finalize" API
6. server replies with an updated order object that includes a
"certificate" URI
7. client sends POST-as-GET request to the "certificate" URI to
download the certificate
ACME places the following restrictions on "identifiers":
* [RFC8555], Section 7.1.3: The authorizations required are dictated
by server policy; there may not be a 1:1 relationship between the
order identifiers and the authorizations required.
* [RFC8555], Section 7.1.4: the only type of "identifier" defined by
the ACME specification is an FQDN: "The only type of identifier
defined by this specification is a fully qualified domain name
(type: "dns"). The domain name MUST be encoded in the form in
which it would appear in a certificate."
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* [RFC8555], Section 7.4: the "identifier" in the CSR request must
match the "identifier" in the newOrder request: "The CSR MUST
indicate the exact same set of requested identifiers as the
initial newOrder request."
* [RFC8555], Section 8.3: the "identifier", or FQDN, in the
authorization object must be used when fulfilling challenges via
HTTP: "Construct a URL by populating the URL template ... where
the domain field is set to the domain name being verified"
* [RFC8555], Section 8.4: the "identifier", or FQDN, in the
authorization object must be used when fulfilling challenges via
DNS: "The client constructs the validation domain name by
prepending the label "_acme-challenge" to the domain name being
validated."
ACME does not mandate that the "identifier" in a newOrder request
matches the "identifier" in authorization objects.
The base ACME [RFC8555] document only specifies the "dns" identifier
type. Additional identifiers may be defined and registered in the
IANA [ACME-Identifier-Types] registry. For example, [RFC8738]
specifies the "ip" identifier type. This document is only relevant
for the "dns" identifier type.
Note also that ACME supports multiple different validation methods
that can be used to fulfill challenges and prove ownership of
identifiers. Validation methods are registered in the IANA
[ACME-Validation-Methods] registry. This document does not mandate
use of any particular validation method or methods. ACME server
policy dictates which validation methods are supported. See
Section 7.3 for more information on ACME server policy.
4. ACME Issuance of Subdomain Certificates
As noted in the previous section, ACME [RFC8555] does not mandate
that the "identifier" in a newOrder request matches the "identifier"
in authorization objects. This means that the ACME specification
does not preclude an ACME server processing newOrder requests and
issuing certificates for a subdomain without requiring a challenge to
be fulfilled against that explicit subdomain.
ACME server policy could allow issuance of certificates for a
subdomain to a client where the client only has to fulfill an
authorization challenge for an ancestor domain of that subdomain.
This allows a flow where a client proves ownership of, for example,
"example.org" and then successfully obtains a certificate for
"sub.example.org".
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ACME server policy is out of scope of this document, however, some
commentary is provided in Section 7.3.
Clients need a mechanism to instruct the ACME server that they are
requesting authorization for all subdomains subordinate to the
specified domain, as opposed to just requesting authorization for an
explicit domain identifier. Clients need a mechanism to do this in
both newAuthz and newOrder requests. ACME servers need a mechanism
to indicate to clients that authorization objects are valid for all
subdomains under the specified domain. These are described in this
section.
4.1. Authorization Object
ACME ([RFC8555], Section 7.1.4) defines the authorization object.
This document defines a new "subdomainAuthAllowed" field for the
authorization object. When ACME server policy allows authorization
for subdomains subordinate to a domain, the server indicates this by
including the new "subdomainAuthAllowed" field in the authorization
object for that domain identifier:
subdomainAuthAllowed (optional, boolean): If present, this field
MUST be true for authorizations where ACME server policy
allows certificates to be issued for any subdomain subordinate
to the domain specified in the 'identifier' field of the
authorization object.
The following example shows an authorization object for the domain
example.org where the authorization covers the subdomains subordinate
to example.org.
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{
"status": "valid",
"expires": "2023-09-01T14:09:07.99Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"url": "https://example.com/acme/chall/prV_B7yEyA4",
"type": "http-01",
"status": "valid",
"token": "DGyRejmCefe7v4NfDGDKfA",
"validated": "2014-12-01T12:05:58.16Z"
}
],
"subdomainAuthAllowed": true
}
If the "subdomainAuthAllowed" field is not included, then the assumed
default value is false.
If ACME server policy allows issuance of certificates containing
wildcard identifiers under that authorization object, then the server
SHOULD include the "wildcard" field with a value of true, as per
[RFC8555], Section 7.1.4.
4.2. Pre-Authorization
The basic ACME workflow has authorization objects created reactively
in response to a certificate order. ACME also allows for pre-
authorization, where clients obtain authorization for an identifier
proactively, outside of the context of a specific issuance. With the
ACME pre-authorization flow, a client can pre-authorize for a domain
once, and then issue multiple newOrder requests for certificates with
identifiers in the subdomains subordinate to that domain.
ACME [RFC8555], Section 7.4.1 defines the "identifier" object for
newAuthz requests. This document defines a new
"subdomainAuthAllowed" field for the "identifier" object:
subdomainAuthAllowed (optional, boolean): An ACME client sets
this flag to indicate to the server that it is requesting an
authorization for the subdomains subordinate to the specified
domain identifier value
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Clients include the new "subdomainAuthAllowed" field in the
"identifier" object of newAuthz requests to indicate that they are
requesting a subdomain authorization. In the following example
newAuthz payload, the client is requesting pre-authorization for the
subdomains subordinate to example.org.
"payload": base64url({
"identifier": {
"type": "dns",
"value": "example.org",
"subdomainAuthAllowed": true
}
})
If the server is willing to allow a single authorization for the
subdomains, and there is not an existing authorization object for the
identifier, then it will create an authorization object and include
the "subdomainAuthAllowed" flag with value of true.
If the server policy does not allow creation of subdomain
authorizations subordinate to that domain, the server can create an
authorization object for the indicated identifier, and MAY include
the "subdomainAuthAllowed" flag with value of false. If the server
creates an authorization object and does not include the
"subdomainAuthAllowed" flag, then the assumed value is false.
In both scenarios, handling of the pre-authorization follows the
process documented in ACME [RFC8555], Section 7.4.1.
4.3. New Orders
Clients need a mechanism to optionally indicate to servers whether or
not they are authorized to fulfill challenges against an ancestor
domain for a given identifier. For example, if a client places an
order for an identifier foo.bar.example.org, and is authorized to
fulfill a challenge against the ancestor domains bar.example.org or
example.org, then the client needs a mechanism to indicate control
over the ancestor domains to the ACME server.
In order to accomplish this, this document defines a new
"ancestorDomain" field for the identifier that is included in order
objects.
ancestorDomain (optional, string): This is an ancestor domain of
the requested identifier. The client MUST be able to fulfill
a challenge against the ancestor domain.
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This field specifies an ancestor domain of the identifier that the
client has DNS control over, and is capable of fulfilling challenges
against. Based on server policy, the server can choose to issue a
challenge against any ancestor domain of the identifier up to and
including the specified "ancestorDomain", and create a corresponding
authorization object against the chosen identifier.
In the following example newOrder payload, the client requests a
certificate for identifier foo.bar.example.org and indicates that it
can fulfill a challenge against the ancestor domain bar.example.org.
The server can then choose to issue a challenge against either
foo.bar.example.org or bar.example.org identifiers.
"payload": base64url({
"identifiers": [
{ "type": "dns",
"value": "foo.bar.example.org",
"ancestorDomain": "bar.example.org" }
],
"notBefore": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
})
In the following example newOrder payload, the client requests a
certificate for identifier foo.bar.example.org and indicates that it
can fulfill a challenge against the ancestor domain example.org. The
server can then choose to issue a challenge against any one of
foo.bar.example.org, bar.example.org or example.org identifiers.
"payload": base64url({
"identifiers": [
{ "type": "dns",
"value": "foo.bar.example.org",
"ancestorDomain": "example.org" }
],
"notBefore": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
})
If the client is unable to fulfill authorizations against an ancestor
domain, the client should not include the "ancestorDomain" field.
Server newOrder handling generally follows the process documented in
ACME, Section 7.4 of [RFC8555]. If the server is willing to allow
subdomain authorizations for the domain specified in
"ancestorDomain", then it creates an authorization object against
that ancestor domain and includes the "subdomainAuthAllowed" flag
with a value of true.
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If the server policy does not allow creation of subdomain
authorizations against that ancestor domain, then it can create an
authorization object for the indicated identifier value, and SHOULD
NOT include the "subdomainAuthAllowed" flag. As the client requested
a subdomain authorization for the ancestor domain, and not for the
indicated identifier, there is no need for the server to include the
"subdomainAuthAllowed" flag in the authorization object for the
indicated identifier.
4.4. Directory Object Metadata
This document defines a new "subdomainAuthAllowed" ACME directory
metadata field. An ACME server can advertise support for
authorization of subdomains by including the "subdomainAuthAllowed"
boolean flag in its "ACME Directory Metadata Fields" registry:
subdomainAuthAllowed (optional, bool): Indicates if an ACME
server supports authorization of subdomains.
If not specified, then the assumed default value is false. If an
ACME server supports authorization of subdomains, it can indicate
this by including this field with a value of "true".
5. Illustrative Call Flow
The call flow illustrated here uses the ACME pre-authorization flow
using DNS-based proof of ownership.
+--------+ +------+ +-----+
| Client | | ACME | | DNS |
+--------+ +------+ +-----+
| | |
STEP 1: Pre-Authorization of ancestor domain
| | |
| POST /newAuthz | |
| "example.org" | |
|--------------------------->| |
| | |
| 201 authorizations | |
|<---------------------------| |
| | |
| Publish DNS TXT | |
| "example.org" | |
|--------------------------------------->|
| | |
| POST /challenge | |
|--------------------------->| |
| | Verify |
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| |---------->|
| 200 status=valid | |
|<---------------------------| |
| | |
| Delete DNS TXT | |
| "example.org" | |
|--------------------------------------->|
| | |
STEP 2: Place order for sub1.example.org
| | |
| POST /newOrder | |
| "sub1.example.org" | |
|--------------------------->| |
| | |
| 201 status=ready | |
|<---------------------------| |
| | |
| POST /finalize | |
| CSR SAN "sub1.example.org" | |
|--------------------------->| |
| | |
| 200 OK status=valid | |
|<---------------------------| |
| | |
| POST /certificate | |
|--------------------------->| |
| | |
| 200 OK | |
| PEM SAN "sub1.example.org" | |
|<---------------------------| |
| | |
STEP 3: Place order for sub2.example.org
| | |
| POST /newOrder | |
| "sub2.example.org" | |
|--------------------------->| |
| | |
| 201 status=ready | |
|<---------------------------| |
| | |
| POST /finalize | |
| CSR SAN "sub2.example.org" | |
|--------------------------->| |
| | |
| 200 OK status=valid | |
|<---------------------------| |
| | |
| POST /certificate | |
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|--------------------------->| |
| | |
| 200 OK | |
| PEM SAN "sub2.example.org" | |
|<---------------------------| |
* STEP 1: Pre-authorization of ancestor domain
The client sends a newAuthz request for the ancestor domain
including the "subdomainAuthAllowed" flag in the identifier
object.
POST /acme/new-authz HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "uQpSjlRb4vQVCjVYAyyUWg",
"url": "https://example.com/acme/new-authz"
}),
"payload": base64url({
"identifier": {
"type": "dns",
"value": "example.org",
"subdomainAuthAllowed": true
}
}),
"signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
}
The server creates and returns an authorization object for the
identifier including the "subdomainAuthAllowed" flag. The object is
initially in "pending" state.
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{
"status": "pending",
"expires": "2023-09-01T14:09:07.99Z",
"identifier": {
"type": "dns",
"value": "example.org"
},
"challenges": [
{
"url": "https://example.com/acme/chall/prV_B7yEyA4",
"type": "dns-01",
"status": "pending",
"token": "DGyRejmCefe7v4NfDGDKfA",
"validated": "2023-08-01T12:05:58.16Z"
}
],
"subdomainAuthAllowed": true
}
The example illustrates the client completing a DNS challenge by
publishing a DNS TXT record. The client then posts to the challenge
resource to inform the server that it can validate the challenge.
Once the server validates the challenge by checking the DNS TXT
record, the server will transition the authorization object and
associated challenge object status to "valid".
The call flow above illustrates the ACME server replying to the
client's challenge with status of "valid" after the ACME server has
validated the DNS challenge. However, the validation flow may take
some time. If this is the case, the ACME server may reply to the
client's challenge immediately with a status of "processing", and the
client will then need to poll the authorization resource to see when
it is finalized. Refer to ACME [RFC8555], Section 7.5.1 for more
details.
* STEP 2: The client places a newOrder for sub1.example.org
The client sends a newOrder request to the server and includes the
subdomain identifier. Note that the identifier is a subdomain of
the ancestor domain that has been pre-authorized in step 1. The
client does not need to include the "subdomainAuthAllowed" field
in the "identifier" object as it has already pre-authorized the
ancestor domain.
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POST /acme/new-order HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{ "type": "dns", "value": "sub1.example.org" }
],
"notBefore": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
As an authorization object already exists for the ancestor domain,
the server replies with an order object with a status of "ready" that
includes a link to the existing "valid" authorization object.
HTTP/1.1 201 Created
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Link: <https://example.com/acme/directory>;rel="index"
Location: https://example.com/acme/order/TOlocE8rfgo
{
"status": "ready",
"expires": "2023-09-01T14:09:07.99Z",
"notBefore": "2023-09-01T00:00:00Z",
"notAfter": "2023-09-08T00:00:00Z",
"identifiers": [
{ "type": "dns", "value": "sub1.example.org" }
],
"authorizations": [
"https://example.com/acme/authz/PAniVnsZcis"
],
"finalize": "https://example.com/acme/order/TOlocrfgo/finalize"
}
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The client can proceed to finalize the order by posting a CSR to the
"finalize" resource. The client can then download the certificate
for sub1.example.org.
* STEP 3: The client places a newOrder for sub2.example.org
The client sends a newOrder request to the server and includes the
subdomain identifier. Note that the identifier is a subdomain of
the ancestor domain that has been pre-authorized in step 1. The
client does not need to include the "subdomainAuthAllowed" field
in the "identifier" object as it has already pre-authorized the
ancestor domain.
POST /acme/new-order HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{ "type": "dns", "value": "sub2.example.org" }
],
"notBefore": "2023-09-01T00:04:00+04:00",
"notAfter": "2023-09-08T00:04:00+04:00"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
As an authorization object already exists for the ancestor domain,
the server replies with an order object with a status of "ready" that
includes a link to the existing "valid" authorization object.
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HTTP/1.1 201 Created
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Link: <https://example.com/acme/directory>;rel="index"
Location: https://example.com/acme/order/TOlocE8rfgo
{
"status": "ready",
"expires": "2023-09-01T14:09:07.99Z",
"notBefore": "2023-09-01T00:00:00Z",
"notAfter": "2023-09-08T00:00:00Z",
"identifiers": [
{ "type": "dns", "value": "sub2.example.org" }
],
"authorizations": [
"https://example.com/acme/authz/PAniVnsZcis"
],
"finalize": "https://example.com/acme/order/ROni7rdde/finalize"
}
The client can proceed to finalize the order by posting a CSR to the
"finalize" resource. The client can then download the certificate
for sub2.example.org.
6. IANA Considerations
6.1. Authorization Object Fields Registry
The following field is added to the "ACME Authorization Object
Fields" registry defined in ACME [RFC8555].
+----------------------+------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+----------------------+------------+--------------+-----------+
| subdomainAuthAllowed | boolean | false | RFC XXXX |
+----------------------+------------+--------------+-----------+
6.2. Directory Object Metadata Fields Registry
The following field is added to the "ACME Directory Metadata Fields"
registry defined in ACME [RFC8555].
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+----------------------+------------+-----------+
| Field Name | Field Type | Reference |
+----------------------+------------+-----------+
| subdomainAuthAllowed | boolean | RFC XXXX |
+----------------------+------------+-----------+
7. Security Considerations
This document specifies enhancements to ACME [RFC8555] that optimize
the protocol flows for issuance of certificates for subdomains. The
underlying goal of ACME for Subdomains remains the same as that of
ACME: managing certificates that attest to identifier/key bindings
for these subdomains. Thus, ACME for Subdomains has the same two
security goals as ACME:
1. Only an entity that controls an identifier can get an
authorization for that identifier
2. Once authorized, an account key's authorizations cannot be
improperly used by another account
ACME for Subdomains makes no changes to:
* account or account key management
* ACME channel establishment, security mechanisms or threat model
* Validation channel establishment, security mechanisms or threat
model
Therefore, all Security Considerations in ACME in the following areas
are equally applicable to ACME for Subdomains:
* Threat Model
* Integrity of Authorizations
* Denial-of-Service Considerations
* Server-Side Request Forgery
* CA Policy Considerations
The only exception is that in order to satisfy goal (1) above, this
draft assumes that control over a domain may imply control over a
subdomain, and therefore authorization for certificate issuance for
the former may imply authorization for certificate issuance for the
latter. In many ecosystems, this is a safe assumption, especially
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because control over the domain can often be leveraged to
successfully demonstrate control over subdomains anyway, for example
by temporarily modifying DNS for the subdomain to point to a server
the ancestor domain owner controls, rendering the distinction moot.
For example, the CA/Browser Forum Baseline Requirements may consider
control of an ancestor domain sufficient for issuance of certificates
for subdomains, but only if specific processes and procedures are
used for validating ownership of the ancestor domain.
In ecosystems where control of an ancestor domain may not imply
control over subdomains or authorization for issuance of certificates
for subdomains, a more complicated threat analysis and server policy
might be needed.
Some additional comments on ACME server policy are given later in
this section.
7.1. Client Account Security
There may be scenarios were a client wishes to deactivate an
authorization object for an ancestor domain, or deactivate its
account completely. For example, a client may want to do this if an
account key is compromised, or if a authorization object covering
domains subordinate to an ancestor domain is no longer needed. The
client can deactivate an authorization using the mechanism specified
in [RFC8555], Section 7.5.2 and can deactivate an account using the
mechanism specified in [RFC8555], Section 7.3.6.
7.2. Subdomain Determination
The [RFC8499] definition of a subdomain is reproduced in Section 2.
When comparing domains to determine if one is a subdomain of the
other, it is important to compare entire labels, and not rely on a
string prefix match. Relying on string prefix matches may yield
incorrect results.
7.3. ACME Server Policy Considerations
The ACME for Subdomains and the ACME specifications do not mandate
any specific ACME server or CA policies, or any specific use cases
for issuance of certificates. For example, an ACME server could be
used:
* to issue Web PKI certificates where the ACME server must comply
with CA/Browser Forum [CAB] Baseline Requirements.
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* as a Private CA for issuance of certificates within an
organization. The organization could enforce whatever policies
they desire on the ACME server.
* for issuance of IoT device certificates. There are currently no
IoT device certificate policies that are generally enforced across
the industry. Organizations issuing IoT device certificates can
enforce whatever policies they desire on the ACME server.
ACME server policy could specify whether:
* issuance of subdomain certificates is allowed based on proof of
ownership of an ancestor domain
* issuance of subdomain certificates is allowed, but only for a
specific set of ancestor domains
* DNS based proof of ownership, or HTTP based proof of ownership, or
both, are allowed
The CA policy considerations listed in [RFC8555], Section 10.5 are
equally applicable here. These include, but are not limited to:
* Is the claimed identifier syntactically valid?
* For domain names:
* Is the name on the Public Suffix List?
* Is the name a high-value name?
* Is the key in the CSR sufficiently strong?
Refer to [RFC8555], Section 10.5 for more CA policy considerations.
ACME server policy specification is explicitly out of scope of this
document.
8. References
8.1. 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>.
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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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[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>.
8.2. Informative References
[ACME-Identifier-Types]
IANA, "ACME Identifier Types", n.d.,
<https://www.iana.org/assignments/acme/acme.xhtml#acme-
identifier-types>.
[ACME-Validation-Methods]
IANA, "ACME Validation Methods", n.d.,
<https://www.iana.org/assignments/acme/acme.xhtml#acme-
validation-methods>.
[CAB] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates", n.d.,
<https://cabforum.org/baseline-requirements-documents/>.
[RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for
Internet User Applications", RFC 819,
DOI 10.17487/RFC0819, August 1982,
<https://www.rfc-editor.org/info/rfc819>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
<https://www.rfc-editor.org/info/rfc2986>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
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[RFC8738] Shoemaker, R.B., "Automated Certificate Management
Environment (ACME) IP Identifier Validation Extension",
RFC 8738, DOI 10.17487/RFC8738, February 2020,
<https://www.rfc-editor.org/info/rfc8738>.
Authors' Addresses
Owen Friel
Cisco
Email: ofriel@cisco.com
Richard Barnes
Cisco
Email: rlb@ipv.sx
Tim Hollebeek
DigiCert
Email: tim.hollebeek@digicert.com
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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