Internet Engineering Task Force (IETF) O. Friel

Internet Engineering Task Force (IETF) O. Friel
Request for Comments: 9444 R. Barnes
Category: Standards Track Cisco
ISSN: 2070-1721 T. Hollebeek
DigiCert
M. Richardson
Sandelman Software Works
August 2023

Automated Certificate Management Environment (ACME) for Subdomains

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.
Additionally, 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 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/rfc9444.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Terminology
3. ACME Workflow and Identifier Requirements
4. ACME Issuance of Subdomain Certificates
4.1. Authorization Object
4.2. Pre-authorization
4.3. New Orders
4.4. Directory Object Metadata
5. Illustrative Call Flow
6. IANA Considerations
6.1. Authorization Object Fields Registry
6.2. Directory Object Metadata Fields Registry
7. Security Considerations
7.1. Client Account Security
7.2. Subdomain Determination
7.3. ACME Server Policy Considerations
8. References
8.1. Normative References
8.2. Informative References
Authors' Addresses

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.

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.

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 fewer 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.

[RFC8555] defines the following object types that 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:

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 JSON Web Signature (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 workflow for issuance of certificates is as follows:

1. Client POSTs a newOrder request that contains a set of identifier
objects in the identifiers field of the ACME order object.

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 request(s) 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 challenge 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 a 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."

* [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 ACME base document [RFC8555] 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 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.
For example, this allows for a flow where a client proves ownership
of example.org and then successfully obtains a certificate for
sub.example.org.

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.

{
"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.

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 of a 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 a 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 a 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.

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 of a 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 of a 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.

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 |
| |---------->|
| 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 | |
|--------------------------->| |
| | |
| 200 OK | |
| PEM SAN "sub2.example.org" | |
|<---------------------------| |

* Step 1: Pre-authorization of ancestor domain.

The client sends a newAuthz request for the ancestor domain and
includes 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 that includes the subdomainAuthAllowed flag. The
object is initially in "pending" state.

{
"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 Section 7.5.1 of [RFC8555]
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.

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"
}

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.

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 has been added to the "ACME Authorization Object
Fields" registry defined in ACME [RFC8555].

+======================+============+==============+===========+
| Field Name | Field Type | Configurable | Reference |
+======================+============+==============+===========+
| subdomainAuthAllowed | boolean | false | RFC 9444 |
+----------------------+------------+--------------+-----------+

Table 1

6.2. Directory Object Metadata Fields Registry

The following field has been added to the "ACME Directory Metadata
Fields" registry defined in [RFC8555].

+======================+============+===========+
| Field Name | Field Type | Reference |
+======================+============+===========+
| subdomainAuthAllowed | boolean | RFC 9444 |
+----------------------+------------+-----------+

Table 2

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
document assumes that control over a domain may imply control over a
subdomain; 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
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 an 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 Baseline Requirements [CAB].

* 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 Internet of Things (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 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>.

[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",
<https://www.iana.org/assignments/acme/>.

[ACME-Validation-Methods]
IANA, "ACME Validation Methods",
<https://www.iana.org/assignments/acme/>.

[CAB] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates",
<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>.

[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: [email protected]

Richard Barnes
Cisco
Email: [email protected]

Tim Hollebeek
DigiCert
Email: [email protected]

Michael Richardson
Sandelman Software Works
Email: [email protected]