wdiff rfc9525v1.txt rfc9525.txt

Internet Engineering Task Force (IETF) P. Saint-Andre
Request for Comments: 9525 Independent
Obsoletes: 6125 R. Salz
Category: Standards Track Akamai Technologies
ISSN: 2070-1721 October November 2023

Service Identity in TLS

Abstract

Many application technologies enable secure communication between two
entities by means of Transport Layer Security (TLS) with Internet
Public Key Infrastructure using X.509 (PKIX) certificates. This
document specifies procedures for representing and verifying the
identity of application services in such interactions.

This document obsoletes RFC 6125.

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/rfc9525.

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 and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Motivation
1.2. Applicability
1.3. Overview of Recommendations
1.4. Scope
1.4.1. In Scope
1.4.2. Out of Scope
1.5. Terminology
2. Identifying Application Services
3. Designing Application Protocols
4. Representing Server Identity
4.1. Rules
4.2. Examples
5. Requesting Server Certificates
6. Verifying Service Identity
6.1. Constructing a List of Reference Identifiers
6.1.1. Rules
6.1.2. Examples
6.2. Preparing to Seek a Match
6.3. Matching the DNS Domain Name Portion
6.4. Matching an IP Address Portion
6.5. Matching the Application Service Type Portion
6.6. Outcome
7. Security Considerations
7.1. Wildcard Certificates
7.2. Uniform Resource Identifiers
7.3. Internationalized Domain Names
7.4. IP Addresses
7.5. Multiple Presented Identifiers
7.6. Multiple Reference Identifiers
7.7. Certificate Trust
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Changes from RFC 6125
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

1.1. Motivation

The visible face of the Internet largely consists of services that
employ a client-server architecture in which a client communicates
with an application service. When a client communicates with an
application service using [TLS], [DTLS], or a protocol built on those
([QUIC] being a notable example), it has some notion of the server's
identity (e.g., "the website at bigcompany.example") while attempting
to establish secure communication. Likewise, during TLS negotiation,
the server presents its notion of the service's identity in the form
of a public key certificate that was issued by a certificate certification
authority (CA) in the context of the Internet Public Key
Infrastructure using X.509 [PKIX]. Informally, we can think of these
identities as the client's "reference identity" and the server's
"presented identity"; more formal definitions are given later. A
client needs to verify that the server's presented identity matches
its reference identity so it can deterministically and automatically
authenticate the communication.

This document defines procedures for how clients do perform this
verification. It therefore defines requirements on other parties,
such as the certificate certification authorities that issue certificates, the
service administrators requesting them, and the protocol designers
defining how things are named. interactions between clients and servers.

This document obsoletes RFC 6125 [VERIFY]. Changes from RFC 6125
[VERIFY] are described under Appendix A.

1.2. Applicability

This document does not supersede the rules for certificate issuance
or validation specified by [PKIX]. That document also governs any
certificate-related topic on which this document is silent. This
includes certificate syntax, extensions such as name constraints or
extended key usage, and handling of certification paths.

This document addresses only name forms in the leaf "end entity"
server certificate. It does not address the name forms in the chain
of certificates used to validate a certificate, nor does it create or
check the validity of such a chain. In order to ensure proper
authentication, applications need to verify the entire certification
path.

1.3. Overview of Recommendations

The previous version of this specification, [VERIFY], surveyed the
then-current practice from many IETF standards and tried to
generalize best practices (see Appendix A of [VERIFY] for details).

This document takes the lessons learned since then and codifies them.
The following is a summary of the rules, which are described at
greater length in the remainder of this document:

* Only check DNS domain names via the subjectAltName extension
designed for that purpose: dNSName.

* Allow use of even more specific subjectAltName extensions where
appropriate such as uniformResourceIdentifier, iPAddress, and the
otherName form SRVName.

* Wildcard support is now the default in certificates. Constrain
wildcard certificates so that the wildcard can only be the
complete left-most label of a domain name.

* Do not include or check strings that look like domain names in the
subject's Common Name.

1.4. Scope

1.4.1. In Scope

This document applies only to service identities that are used in TLS
or DTLS and that are included in PKIX certificates.

With regard to TLS and DTLS, these security protocols are used to
protect data exchanged over a wide variety of application protocols,
which use both the TLS or DTLS handshake protocol and the TLS or DTLS
record layer, either directly or through a profile as in Network Time
Security [NTS]. The TLS handshake protocol can also be used with
different record layers to define secure transport protocols; at
present, the most prominent example is QUIC [RFC9000]. The rules
specified here are intended to apply to all protocols in this
extended TLS "family".

With regard to PKIX certificates, the primary usage is in the context
of the public key infrastructure described in [PKIX]. In addition,
technologies such as DNS-Based Authentication of Named Entities
(DANE) [DANE] sometimes use certificates based on PKIX (more
precisely, certificates structured via [X.509] or specific encodings
thereof such as [X.690]), at least in certain modes. Alternatively,
a TLS peer could issue delegated credentials that are based on a CA-
issued certificate, as in [TLS-SUBCERTS]. In both cases, a TLS
client could learn of a service identity through its inclusion in the
relevant certificate. The rules specified here are intended to apply
whenever service identities are included in X.509 certificates or
credentials that are derived from such certificates.

1.4.2. Out of Scope

The following topics are out of scope for this specification:

* Security protocols other than those described above.

* Keys or certificates employed outside the context of PKIX-based
systems.

* Client or end-user identities. Other than as described above,
certificates representing client identities (e.g., rfc822Name) are
beyond the scope of this document.

* Identification of servers using other than a domain name, an IP
address, or an SRV service name. This document discusses Uniform
Resource Identifiers [URI] only to the extent that they are
expressed in certificates. Other aspects of a service such as a
specific resource (the URI "path" component) or parameters (the
URI "query" component) are the responsibility of specific
protocols or URI schemes.

* Certification authority policies. This includes items such as the
following:

- How to certify or validate fully qualified domain names (FQDNs)
and application service types (see [ACME]).

- What types or "classes" of certificates to issue and whether to
apply different policies for them.

- How to certify or validate other kinds of information that
might be included in a certificate (e.g., organization name).

* Resolution of DNS domain names. Although the process whereby a
client resolves the DNS domain name of an application service can
involve several steps, for the purposes of this specification, the
only relevant consideration is that the client needs to verify the
identity of the entity with which it will communicate once the
resolution process is complete. Thus, the resolution process
itself is out of scope for this specification.

* User interface issues. In general, such issues are properly the
responsibility of client software developers and standards
development organizations dedicated to particular application
technologies (for example, see [WSC-UI]).

1.5. Terminology

Because many concepts related to "identity" are often too vague to be
actionable in application protocols, we define a set of more concrete
terms for use in this specification.

application service: A service on the Internet that enables clients
to connect for the purpose of retrieving or uploading information,
communicating with other entities, or connecting to a broader
network of services.

application service provider: An entity that hosts or deploys an
application service.

application service type: A formal identifier for the application
protocol used to provide a particular kind of application service
at a domain. This often appears as a URI scheme [URI], a DNS SRV
Service [DNS-SRV], or an Application-Layer Protocol Negotiation
(ALPN) [ALPN] identifier.

identifier: A particular instance of an identifier type that is
either presented by a server in a certificate or referenced by a
client for matching purposes.

identifier type: A formally defined category of identifier that can
be included in a certificate and therefore be used for matching
purposes. For conciseness and convenience, we define the
following identifier types of interest:

DNS-ID: A subjectAltName entry of type dNSName as defined in
[PKIX].

IP-ID: A subjectAltName entry of type iPAddress as defined in
[PKIX].

SRV-ID: A subjectAltName entry of type otherName whose name form
is SRVName as defined in [SRVNAME].

URI-ID: A subjectAltName entry of type uniformResourceIdentifier
as defined in [PKIX]. See further discussion in Section 7.2.

PKIX: The short name for the Internet Public Key Infrastructure
using X.509 defined in [PKIX]. That document provides a profile
of the X.509v3 certificate specifications and X.509v2 certificate
revocation list (CRL) specifications for use on the Internet.

presented identifier: An identifier presented by a server to a
client within a PKIX certificate when the client attempts to
establish secure communication with the server. The certificate
can include one or more presented identifiers of different types,
and if the server hosts more than one domain, then the certificate
might present distinct identifiers for each domain.

reference identifier: An identifier used expected by the client when
examining presented identifiers. It is constructed from the
source domain and, optionally, an application service type.

Relative Distinguished Name (RDN): An ASN.1-based construction that
is itself a building-block component of Distinguished Names. See
[LDAP-DN], Section 2.

source domain: The FQDN that a client expects an application service
to present in the certificate. This is typically input by a human
user, configured into a client, or provided by reference such as a
URL. The combination of a source domain and, optionally, an
application service type enables a client to construct one or more
reference identifiers. This specification covers FQDNs. Use of
any names that are not fully qualified is out of scope and may
result in unexpected or undefined behavior.

subjectAltName entry: An identifier placed in a subjectAltName
extension.

subjectAltName extension: A standard PKIX extension enabling
identifiers of various types to be bound to the certificate
subject.

subjectName: The name of a PKIX certificate's subject, encoded in a
certificate's subject field (see [PKIX], Section 4.1.2.6).

TLS uses the words "client" and "server", where the client is the
entity that initiates the connection. In many cases, this is
consistent with common practice, such as a browser connecting to a
web origin. For the sake of clarity, and to follow the usage in
[TLS] and related specifications, we will continue to use the terms
client and server in this document. However, these are TLS-layer
roles, and the application protocol could support the TLS server
making requests to the TLS client after the TLS handshake; there is
no requirement that the roles at the application layer match the TLS
layer.

Security-related terms used in this document, but not defined here or
in [PKIX], should be understood in the sense defined in [SECTERMS].
Such terms include "attack", "authentication", "identity", "trust",
"validate", and "verify".

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.

2. Identifying Application Services

This document assumes that an application service is identified by a
DNS domain name (e.g., bigcompany.example), an IP address (IPv4 or
IPv6), or an identifier that contains additional supplementary
information. Supplementary information is limited to the application
service type as expressed in a DNS SRV record (e.g., "the IMAP server
at isp.example" for "_imap.isp.example") or a URI.

In a DNS-ID -- and in the DNS domain name portion of an SRV-ID or
URI-ID -- any characters outside the range described in [US-ASCII]
are prohibited, and internationalized domain labels are represented
as A-labels [IDNA-DEFS].

An IP address is either a 4-octet IPv4 address [IPv4] or a 16-octet
IPv6 address [IPv6]. The identifier might need to be converted from
a textual representation to obtain this value.

From the perspective of the application client or user, some
identifiers are _direct_ because they are provided directly by a
human user. This includes runtime input, prior configuration, or
explicit acceptance of a client communication attempt. Other names
are _indirect_ because they are automatically resolved by the
application based on user input, such as a target name resolved from
a source name using DNS SRV or the records described in [NAPTR]. The
distinction matters most for certificate consumption, specifically
verification as discussed in this document.

From the perspective of the application service, some identifiers are
_unrestricted_ because they can be used in any type of service, such
as a single certificate being used for both the HTTP and IMAP
services at the host "bigcompany.example". Other identifiers are
_restricted_ because they can only be used for one type of service,
such as a special-purpose certificate that can only be used for an
IMAP service. This distinction matters most for certificate
issuance.

The four identifier types can be categorized as follows:

* A DNS-ID is direct and unrestricted.

* An IP-ID is direct and unrestricted.

* An SRV-ID is typically indirect but can be direct, and it is
restricted.

* A URI-ID is direct and restricted.

It is important to keep these distinctions in mind because best
practices for the deployment and use of the identifiers differ. Note
that cross-protocol attacks such as those described in [ALPACA] are
possible when two different protocol services use the same
certificate. This can be addressed by using restricted identifiers
or deploying services so that they do not share certificates.
Protocol specifications MUST specify which identifiers are mandatory
to implement and SHOULD provide operational guidance when necessary.

The Common Name RDN MUST NOT be used to identify a service because it
is not strongly typed (essentially (it is essentially free-form text) and
therefore suffers from ambiguities in interpretation.

For similar reasons, other RDNs within the subjectName MUST NOT be
used to identify a service.

An IP address that is the result of a DNS query is not direct. indirect. Use of
IP-IDs that are not direct indirect is out of scope for this document.

The IETF continues to define methods for looking up information
needed to make connections to network services. One recent example
is service binding via the "SVCB" and "HTTPS" DNS resource record
(RR) types. This document does not define any identity
representation or verification procedures that are specific to SVCB-
compatible records, because the use of such records during connection
establishment does not currently alter any of the PKIX validation
requirements specified herein or in any other relevant specification.
For example, the PKIX validation rules for [HTTP-OVER-TLS] [HTTP] and [DNS-OVER-TLS]
do not change when the client uses the DNS resource records defined
in [SVCB-FOR-HTTPS] or
[SVCB-FOR-DNS]. [SVCB-FOR-DNS] to look up connection
information. However, it is possible that future SVCB mapping
documents could specify altered PKIX rules for new use cases.

3. Designing Application Protocols

This section defines how protocol designers should reference this
document, which would typically be a normative reference in their
specification. Its

A specification MAY choose to allow only one of the identifier types
defined here.

If the technology does not use DNS SRV records to resolve the DNS
domain names of application services, then its the specification MUST
state that SRV-ID as defined in this document is not supported. Note
that many existing application technologies use DNS SRV records to
resolve the DNS domain names of application services, but they do not
rely on representations of those records in PKIX certificates by
means of SRV-IDs as defined in [SRVNAME].

If the technology does not use URIs to identify application services,
then its the specification MUST state that URI-ID as defined in this
document is not supported. Note that many existing application
technologies use URIs to identify application services, but they do
not rely on representation of those URIs in PKIX certificates by
means of URI-IDs.

A technology MAY disallow the use of the wildcard character in
presented identifiers. If it does so, then the specification MUST
state that wildcard certificates as defined in this document are not
supported.

A protocol can allow the use of an IP address in place of a DNS name.
This might use the same field without distinguishing the type of
identifier as, for example, in the "host" components of a URI. In
this case, applications need to be aware that the textual
representation of an IPv4 address is a valid DNS name. The two types
can be distinguished by first testing if the identifier is a valid
IPv4 address, as is done by the "first-match-wins" algorithm in
Section 3.2.2 of [URI].

4. Representing Server Identity

This section provides instructions for issuers of certificates.

4.1. Rules

When a certificate certification authority issues a certificate based on the FQDN
at which the application service provider will provide the relevant
application, the following rules apply to the representation of
application service identities. Note that some of these rules are
cumulative and can interact in important ways that are illustrated
later in this document.

1. The certificate MUST include at least one identifier.

2. The certificate SHOULD include a DNS-ID as a baseline for
interoperability. This is not mandatory because it is legitimate
for a certificate to include only an SRV-ID or URI-ID so as to
scope its use to a particular application type.

3. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
should include identifiers of type SRV-ID (e.g., this is true of
the Extensible Messaging and Presence Protocol (XMPP) as
described in [XMPP]), then the certificate SHOULD include an SRV-
ID. This identifier type could supplement the DNS-ID, unless the
certificate is meant to be scoped to only the protocol in
question.

4. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
should include identifiers of type URI-ID (e.g., this is true of
the Session Initiation Protocol [SIP] as specified by
[SIP-CERTS]), then the certificate SHOULD include a URI-ID. The
scheme MUST be that of the protocol associated with the
application service type, and the "host" component MUST be the
FQDN of the service. The application protocol specification MUST
specify which URI schemes are acceptable in URI-IDs contained in
PKIX certificates used for the application protocol (e.g., sip
but not sips or tel for SIP as described in [SIP-SIPS]).
Typically, this identifier type would supplement the DNS-ID,
unless the certificate is meant to be scoped to only the protocol
in question.

5. The certificate MAY contain more than one DNS-ID, SRV-ID, URI-ID,
or IP-ID as further explained in Section 7.5.

6. The certificate MAY include other application-specific
identifiers for compatibility with a deployed base, especially
identifiers for types that were defined before publication of
[SRVNAME] or for which SRV service names or URI schemes do not
exist. Such identifiers are out of scope for this specification.

4.2. Examples

Consider a simple website at www.bigcompany.example, <www.bigcompany.example>, which is not
discoverable via DNS SRV lookups. Because HTTP does not specify the
use of URIs in server certificates, a certificate for this service
might include only a DNS-ID of www.bigcompany.example. <www.bigcompany.example>.

Consider another website, which is reachable by a fixed IP address of
2001:db8::5c. If the two sites refer to the same web service, then
the certificate might also include this value in an IP-ID to allow
clients to use the fixed IP address as a reference identity.

Consider an IMAP-accessible email server at the host mail.isp.example
servicing email addresses of the form user@isp.example and
discoverable via DNS SRV lookups on the application service name of
isp.example. A certificate for this service might include SRV-IDs of
_imap.isp.example and _imaps.isp.example (see [EMAIL-SRV]) along with
DNS-IDs of isp.example and mail.isp.example.

Consider a SIP-accessible voice-over-IP (VoIP) server at the host
voice.college.example servicing SIP addresses of the form
user@voice.college.example and identified by a URI of
<sip:voice.college.example>. A certificate for this service would
include a URI-ID of sip:voice.college.example <sip:voice.college.example> (see [SIP-CERTS])
along with a DNS-ID of voice.college.example.

Consider an XMPP-compatible instant messaging (IM) server at the host
messenger.example servicing that services IM addresses of the form
user@messenger.example and that is discoverable via DNS SRV lookups
on the messenger.example domain. A certificate for this service
might include SRV-IDs of _xmpp-client.messenger.example and _xmpp-
server.messenger.example (see [XMPP]), as well as a DNS-ID of
messenger.example.

5. Requesting Server Certificates

This section provides instructions for service providers regarding
the information to include in certificate signing requests (CSRs).
In general, service providers SHOULD request certificates that
include all the identifier types that are required or recommended for
the application service type that will be secured using the
certificate to be issued.

A service provider SHOULD request certificates with as few
identifiers as necessary to identify a single service; see
Section 7.5.

If the certificate will be used for only a single type of application
service, the service provider SHOULD request a certificate that
includes DNS-ID or IP-ID values that identify that service or, if
appropriate for the application service type, SRV-ID or URI-ID values
that limit the deployment scope of the certificate to only the
defined application service type.

If the certificate might be used for any type of application service,
then
the service provider SHOULD request a certificate that includes only
DNS-IDs or IP-IDs. Again, because of multiprotocol attacks, this
practice is discouraged; it can be mitigated by deploying only one
service on a host.

If a service provider offers multiple application service types and
wishes to limit the applicability of certificates using SRV-IDs or
URI-IDs, they it SHOULD request that multiple certificates rather than a
single certificate containing multiple SRV-IDs or URI-IDs each
identify a different application service type. This rule does not
apply to application service type "bundles" that identify distinct
access methods to the same underlying application such as an email
application with access methods denoted by the application service
types of imap, imaps, pop3, pop3s, and submission as described in
[EMAIL-SRV].

6. Verifying Service Identity

At a high level, the client verifies the application service's
identity by performing the following actions:

1. The client constructs a list of reference identifiers it would
find acceptable based on the source domain and, if applicable,
the type of service to which the client is connecting.

2. The server provides its presented identifiers in the form of a
PKIX certificate.

3. The client checks each of its reference identifiers against the
server's presented identifiers for the purpose of finding a
match. When checking a reference identifier against a presented
identifier, the client matches the source domain of the
identifiers and, optionally, their application service type.

Naturally, in addition to checking identifiers, a client should
perform further checks, such as expiration and revocation, to ensure
that the server is authorized to provide the requested service.
Because such checking is not a matter of verifying the application
service identity presented in a certificate, methods for doing so are
out of scope for this document.

6.1. Constructing a List of Reference Identifiers

6.1.1. Rules

The client MUST construct a list of acceptable reference identifiers
and MUST do so independently of the identifiers presented by the
service.
server.

The inputs used by the client to construct its list of reference
identifiers might be a URI that a user has typed into an interface
(e.g., an HTTPS URL for a website), configured account information
(e.g., the domain name of a host for retrieving email, which might be
different from the DNS domain name portion of a username), a
hyperlink in a web page that triggers a browser to retrieve a media
object or script, or some other combination of information that can
yield a source domain and an application service type.

This document does not precisely define how reference identifiers are
generated. Defining reference identifiers is the responsibility of
applications or protocols that use this document. Because the
security of a system that uses this document will depend on how
reference identifiers are generated, great care should be taken in
this process. For example, a protocol or application could specify
that the application service type is obtained through a one-to-one
mapping of URI schemes to service types or that the protocol or
application supports only a restricted set of URI schemes.
Similarly, it could insist specify that a domain name or an IP address taken
as input to the reference identifier must be obtained in a secure
context such as a hyperlink embedded in a web page that was delivered
over an authenticated and encrypted channel (for instance, see
[SECURE-CONTEXTS] with regard to the web platform).

Naturally, if the inputs themselves are invalid or corrupt (e.g., a
user has clicked a hyperlink provided by a malicious entity in a
phishing attack), then the client might end up communicating with an
unexpected application service.

During the course of processing, a client might be exposed to
identifiers that look like, but are not, reference identifiers. For
example, DNS resolution that starts at a DNS-ID reference identifier
might produce intermediate domain names that need to be further
resolved. Unless an application defines a process for authenticating
intermediate identifiers in a way that then allows them to be used as
a reference identifier (for example, see [SMTP-TLS]), any
intermediate values are not reference identifiers and MUST NOT be
treated as such. In the DNS case, not treating intermediate domain
names as reference identifiers removes DNS and DNS resolution from
the attack surface.

As one example of the process of generating a reference identifier,
from the user input of the URI <sip:alice@college.example>, <sip:alice@voice.college.example>, a
client could derive the application service type sip from the URI
scheme and parse the domain name college.example from the "host"
component.

Using the combination of FQDN(s) one or more FQDNs or IP address(es), addresses, plus
optionally an application service type, the client MUST construct its
list of reference identifiers in accordance with the following rules:

* If a server for the application service type is typically
associated with a URI for security purposes (i.e., a formal
protocol document specifies the use of URIs in server
certificates), the reference identifier SHOULD be a URI-ID.

* If a server for the application service type is typically
discovered by means of DNS SRV records, the reference identifier
SHOULD be an SRV-ID.

* If the reference identifier is an IP address, the reference
identifier is an IP-ID.

* In the absence of more specific identifiers, the reference
identifier is a DNS-ID. A reference identifier of type DNS-ID can
be directly constructed from an FQDN that is (a) contained in or
securely derived from the inputs or (b) explicitly associated with
the source domain by means of user configuration.

Which identifier types a client includes in its list of reference
identifiers, and their priority, is a matter of local policy. For
example, a client that is built to connect only to a particular kind
of service might be configured to accept as valid only certificates
that include an SRV-ID for that application service type. By
contrast, a more lenient client, even if built to connect only to a
particular kind of service, might include SRV-IDs, DNS-IDs, and IP-
IDs in its list of reference identifiers.

6.1.2. Examples

The following examples are for illustrative purposes only and are not
intended to be comprehensive.

1. A web browser that is connecting via HTTPS to the website at
https://www.bigcompany.example/
<https://www.bigcompany.example/> would have a single reference
identifier: a DNS-ID of www.bigcompany.example.

2. A web browser connecting to https://192.0.2.107/ <https://192.0.2.107/> would have a
single IP-ID reference identifier of 192.0.2.107. Likewise, if
connecting to https://[2001:db8::abcd], <https://[2001:db8::abcd]>, it would have a single
IP-ID reference identifier of 2001:db8::abcd.

3. A mail user agent that is connecting via IMAPS to the email
service at isp.example (resolved as mail.isp.example) might have
three reference identifiers: an SRV-ID of _imaps.isp.example (see
[EMAIL-SRV]) and DNS-IDs of isp.example and mail.isp.example. An
email user agent that does not support [EMAIL-SRV] would probably
be explicitly configured to connect to mail.isp.example, whereas
an SRV-aware user agent would derive isp.example from an email
address of the form user@isp.example but might also accept
mail.isp.example as the DNS domain name portion of reference
identifiers for the service.

4. A VoIP user agent that is connecting via SIP to the voice service
at voice.college.example might have only one reference
identifier: a URI-ID of sip:voice.college.example (see
[SIP-CERTS]).

5. An IM client that is connecting via XMPP to the IM service at
messenger.example might have three reference identifiers: an SRV-
ID of _xmpp-client.messenger.example (see [XMPP]), a DNS-ID of
messenger.example, and an XMPP-specific XmppAddr of
messenger.example (see [XMPP]).

In all these cases, presented identifiers that do not match the
reference identifier(s) would be rejected; for instance:

* With regard to the first example, a DNS-ID of
"web.bigcompany.example"
web.bigcompany.example would be rejected because the DNS domain
name portion does not match "www.bigcompany.example". www.bigcompany.example.

* With regard to the third example, a URI-ID of
"sip:www.college.example"
<sip:www.college.example> would be rejected because the DNS domain
name portion does not match "voice.college.example", and a DNS-ID
of "voice.college.example" would be rejected because it lacks the
appropriate application service type portion (i.e., it does not
specify a "sip:" URI).

6.2. Preparing to Seek a Match

Once the client has constructed its list of reference identifiers and
has received the server's presented identifiers, the client checks
its reference identifiers against the presented identifiers for the
purpose of finding a match. The search fails if the client exhausts
its list of reference identifiers without finding a match. The
search succeeds if any presented identifier matches one of the
reference identifiers, at which point the client SHOULD stop the
search.

Before applying the comparison rules provided in the following
sections, the client might need to split the reference identifier
into components. Each reference identifier produces either a domain
name or an IP address and optionally an application service type as
follows:

* A DNS-ID reference identifier MUST be used directly as the DNS
domain name, and there is no application service type.

* An IP-ID reference identifier MUST be exactly equal to match the value of an
iPAddress entry in subjectAltName, with no partial (e.g.,
network-level) network-
level) matching. There is no application service type.

* For an SRV-ID reference identifier, the DNS domain name portion is
the Name and the application service type portion is the Service.
For example, an SRV-ID of _imaps.isp.example has a DNS domain name
portion of isp.example and an application service type portion of
imaps, which maps to the IMAP application protocol as explained in
[EMAIL-SRV].

* For a reference identifier of type URI-ID, the DNS domain name
portion is the "reg-name" part of the "host" component and the
application service type portion is the scheme, as defined above.
Matching only the "reg-name" rule from [URI] limits the additional
domain name validation (Section 6.3) to DNS domain names or non-IP
hostnames. A URI that contains an IP address might be matched
against an IP-ID in place of a URI-ID by some lenient clients.
This document does not describe how a URI that contains no "host"
component can be matched. Note that extraction of the "reg-name"
might necessitate normalization of the URI (as explained in
Section 6 of [URI]). For example, a URI-ID of
sip:voice.college.example
<sip:voice.college.example> would be split into a DNS domain name
portion of voice.college.example and an application service type
of sip (associated with an application protocol of SIP as
explained in [SIP-CERTS]).

If the reference identifier produces a domain name, the client MUST
match the DNS name; see Section 6.3. If the reference identifier
produces an IP address, the client MUST match the IP address; see
Section 6.4. If an application service type is present, it MUST also
match the service type; see Section 6.5.

6.3. Matching the DNS Domain Name Portion

This section describes how the client must determine if the presented
DNS name matches the reference DNS name. The rules differ depending
on whether the domain to be checked is an internationalized domain
name, as defined in Section 2, or not. For clients that support
presented identifiers containing the wildcard character "*", this
section also specifies a supplemental rule for such "wildcard
certificates". This section uses the description of labels and
domain names in [DNS-CONCEPTS].

If the DNS domain name portion of a reference identifier is a
"traditional not an
internationalized domain name" name (i.e., an FQDN that conforms to
"preferred name syntax" as described in Section 3.5 of
[DNS-CONCEPTS]), then the matching of the reference identifier
against the presented identifier MUST be performed by comparing the
set of domain name labels using a case-insensitive ASCII comparison,
as clarified by [DNS-CASE]. For example, WWW.BigCompany.Example
would be lower-cased to www.bigcompany.example for comparison
purposes. Each label MUST match in order for the names to be
considered a match, except as supplemented by the rule about checking
wildcard labels in presented identifiers given below.

If the DNS domain name portion of a reference identifier is an
internationalized domain name, then the client MUST convert any
U-labels [IDNA-DEFS] in the domain name to A-labels before checking
the domain name or comparing it with others. In accordance with
[IDNA-PROTO], A-labels MUST be compared as case-insensitive ASCII.
Each label MUST match in order for the domain names to be considered
to match, except as supplemented by the rule about checking wildcard
labels in presented identifiers given below.

If the technology specification supports wildcards in presented
identifiers, then the client MUST match the reference identifier
against a presented identifier whose DNS domain name portion contains
the wildcard character "*" in a label, provided these requirements
are met:

1. There is only one wildcard character.

2. The wildcard character appears only as the complete content of
the left-most label.

If the requirements are not met, the presented identifier is invalid
and MUST be ignored.

A wildcard in a presented identifier can only match exactly one label in a
reference identifier. This specification covers only wildcard
characters in presented identifiers, not wildcard characters in
reference identifiers or in DNS domain names more generally.
Therefore, the use of wildcard characters as described herein is not
to be confused with DNS wildcard matching, where the "*" label always
matches at least one whole label and sometimes more; see
[DNS-CONCEPTS], Section 4.3.3 and [DNS-WILDCARDS]. In particular, it
also deviates from [DNS-WILDCARDS], Section 2.1.3.

For information regarding the security characteristics of wildcard
certificates, see Section 7.1.

6.4. Matching an IP Address Portion

Matching of an IP-ID matches is based on an octet-for-octet comparison of the
bytes of the reference identity with the bytes contained in the
iPAddress subjectAltName.

For an IP address that appears in a URI-ID, the "host" component of
both the reference identity and the presented identifier must match.
These are parsed as either an "IPv6address" (following [URI],
Section 3.2.2) or an "IPv4address" (following [IPv4]). If the
resulting octets are equal, the IP address matches.

This document does not specify how an SRV-ID reference identity can
include an IP address, as [SRVNAME] only defines string names, not
octet identifiers such as an IP address.

6.5. Matching the Application Service Type Portion

The rules for matching the application service type depend on whether
the identifier is an SRV-ID or a URI-ID.

These identifiers provide an application service type portion to be
checked, but that portion is combined only with the DNS domain name
portion of the SRV-ID or URI-ID itself. For example, if a client's
list Consider the example of a
messaging client that has two reference identifiers includes identifiers: (1) an SRV-ID of _xmpp-
client.messenger.example
_xmpp-client.messenger.example and (2) a DNS-ID of app.example, the app.example. The
client MUST check both (1) the combination of (a) an application service
type of xmpp-
client xmpp-client and (b) a DNS domain name of messenger.example and, separately, as
well as (2) a DNS domain name of app.example. However, the client
MUST NOT check the combination of an application service type of
xmpp-client and a DNS domain name of app.example because it does not
have an SRV-ID of _xmpp-client.app.example in its list of reference
identifiers.

If the identifier is an SRV-ID, then the application service name
MUST be matched in a case-insensitive manner, in accordance with
[DNS-SRV]. Note that per [SRVNAME], the _ character underscore "_" is part of
the service name in DNS SRV records and in SRV-IDs.

If the identifier is a URI-ID, then the scheme name portion MUST be
matched in a case-insensitive manner, in accordance with [URI]. Note
that the : character colon ":" is a separator between the scheme name and the
rest of the URI and thus does not need to be included in any
comparison.

6.6. Outcome

If the client has found a presented identifier that matches a
reference identifier, then the service identity check has succeeded.
In this case, the client MUST use the matched reference identifier as
the validated identity of the application service.

If the client does not find a presented identifier matching any of
the reference identifiers, then the client MUST proceed as described
as follows.

If the client is an automated application, then it SHOULD terminate
the communication attempt with a bad certificate error and log the
error appropriately. The application MAY provide a configuration
setting to disable this behavior, but it MUST NOT disable this
security control by default.

If the client is one that is directly controlled by a human user,
then it SHOULD inform the user of the identity mismatch and
automatically terminate the communication attempt with a bad
certificate error in order to prevent users from inadvertently
bypassing security protections in hostile situations. Such clients
MAY give advanced users the option of proceeding with acceptance
despite the identity mismatch. Although this behavior can be
appropriate in certain specialized circumstances, it needs to be
handled with extreme caution, for example by first encouraging even
an advanced user to terminate the communication attempt and, if they
choose to proceed anyway, by forcing the user to view the entire
certification path before proceeding.

The application MAY also present the user with the ability to accept
the presented certificate as valid for subsequent connections. Such
ad hoc "pinning" SHOULD NOT restrict future connections to just the
pinned certificate. Local policy that statically enforces a given
certificate for a given peer SHOULD be made available only as prior
configuration rather than a just-in-time override for a failed
connection.

7. Security Considerations

7.1. Wildcard Certificates

Wildcard certificates automatically vouch for any single-label
hostnames within their domain, but not multiple levels of domains.
This can be convenient for administrators but also poses the risk of
vouching for rogue or buggy hosts. For example, see [Defeating-SSL]
(beginning at slide 91) and [HTTPSbytes] (slides 38-40).

As specified in Section 6.3, restricting the presented identifiers in
certificates to only one wildcard character (e.g.,
"*.bigcompany.example" but not "*.*.bigcompany.example") and
restricting the use of wildcards to only the left-most domain label
can help to mitigate certain aspects of the attack described in
[Defeating-SSL].

That same attack also relies on the initial use of a cleartext HTTP
connection, which is hijacked by an active on-path attacker and
subsequently upgraded to HTTPS. In order to mitigate such an attack,
administrators and software developers are advised to follow the
strict TLS guidelines provided in [TLS-REC], Section 3.2.

Because the attack described in [HTTPSbytes] relies on an underlying
cross-site scripting (XSS) attack, web browsers and applications are
advised to follow best practices to prevent XSS attacks; for example,
see [XSS], which was published by the Open Web Application Security
Project (OWASP).

Protection against a wildcard that identifies a public suffix
[Public-Suffix], such as *.co.uk or *.com, is beyond the scope of
this document.

As noted in Section 3, application protocols can disallow the use of
wildcard certificates entirely as a more foolproof mitigation.

7.2. Uniform Resource Identifiers

The URI-ID type is a subjectAltName entry of type
uniformResourceIdentifier as defined in [PKIX]. For the purposes of
this specification, the URI-ID MUST include both a "scheme" and a
"host" component that matches the "reg-name" rule; if the entry does
not include both, it is not a valid URI-ID and MUST be ignored. Any
other components are ignored because only the "scheme" and "host"
components are used for certificate matching as specified under
Section 6.

The quoted component names in the previous paragraph represent the
associated [ABNF] productions from the IETF Proposed Standard for
Uniform Resource Identifiers [URI]. Although the reader should be
aware that some applications (e.g., web browsers) might instead
conform to the Uniform Resource Locator (URL) specification
maintained by the WHATWG [URL], it is not expected that differences
between the URI and URL specifications would manifest themselves in
certificate matching.

7.3. Internationalized Domain Names

This document specifies only matching between reference identifiers
and presented identifiers, not the visual presentation of domain
names. Specifically, the matching of internationalized domain names
is performed on A-labels only (Section 6). 6.3). The limited scope of
this specification likely mitigates potential confusion caused by the
use of visually similar characters in domain names (for example, as
described in Section 4.4 of [IDNA-DEFS], [UTS-36], and [UTS-39]); in
any case, such concerns are a matter for application-level protocols
and user interfaces, not the matching of certificates.

7.4. IP Addresses

The TLS Server Name Indication (SNI) extension only conveys domain
names. Therefore, a client with an IP-ID reference identity cannot
present any information about its reference identity when connecting
to a server. Servers that wish to present an IP-ID therefore need to
present this identity when a connection is made without SNI.

The textual representation of an IPv4 address might be misinterpreted
as a valid FQDN in some contexts. This can result in different
security treatment that might cause different components of a system
to classify the value differently, which might lead to
vulnerabilities. For example, one Consider a system in which one component enforces a
security rule that is conditional on the type of identifier. This
component identifier but
misclassifies an IP address as an FQDN. A different FQDN, whereas a second component
correctly classifies the identifier but might incorrectly
assume assumes that
rules regarding IP addresses have been enforced. enforced by the first
component. As a result, the system as a whole might behave in an
insecure manner. Consistent classification of identifiers avoids
this problem.

See also Section 3, particularly the last paragraph.

7.5. Multiple Presented Identifiers

A given application service might be addressed by multiple DNS domain
names for a variety of reasons, and a given deployment might service
multiple domains or protocols. TLS extensions such as SNI, the Server
Name Indication (SNI), as discussed in [TLS], [TLS-EXT], Section 4.4.2.2, 3, and
ALPN, as discussed in [ALPN], provide a way for the application to
indicate the desired identifier and protocol to the server, which it
can then use to select the most appropriate certificate.

This specification allows multiple DNS-IDs, IP-IDs, SRV-IDs, or URI-
IDs in a certificate. As a result, an application service can use
the same certificate for multiple hostnames, such as when a client
does not support the TLS SNI extension, or for multiple protocols,
such as SMTP and HTTP, on a single hostname. Note that the set of
names in a certificate is the set of names that could be affected by
a compromise of any other server named in the set: the strength of
any server in the set of names is determined by the weakest of those
servers that offer the names.

The way to mitigate this risk is to limit the number of names that
any server can speak for and to ensure that all servers in the set
have a strong minimum configuration as described in [TLS-REC],
Section 3.9.

7.6. Multiple Reference Identifiers

This specification describes how a client may construct multiple
acceptable reference identifiers and may match any of those reference
identifiers with the set of presented identifiers. [PKIX],
Section 4.2.1.10 describes a mechanism to allow CA certificates to be
constrained in the set of presented identifiers that they may include
within server certificates. However, these constraints only apply to
the explicitly enumerated name forms. For example, a CA that is only
name-constrained for DNS-IDs is not constrained for SRV-IDs and URI-
IDs, unless those name forms are also explicitly included within the
name constraints extension.

A client that constructs multiple reference identifiers of different
types, such as both DNS-IDs and SRV-IDs as described in
Section 6.1.1, SHOULD take care to ensure that CAs issuing such
certificates are appropriately constrained. This MAY take the form
of local policy through agreement with the issuing CA or MAY be
enforced by the client requiring that if one form of presented
identifier is constrained, such as a dNSName name constraint for DNS-
IDs, then all other forms of acceptable reference identities are also
constrained, such as requiring a uniformResourceIndicator name
constraint for URI-IDs.

7.7. Certificate Trust

This document assumes that if a client trusts a given CA, it trusts
all certificates issued by that CA. The certificate checking process
does not include additional checks for bad behavior by the hosts
identified with such certificates, for instance, rogue servers or
buggy applications. Any additional checks (e.g., checking the server
name against trusted block lists) are the responsibility of the
application protocol or the client itself.

8. IANA Considerations

This document has no IANA actions.

9. References

9.1. Normative References

[DNS-CONCEPTS]
Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.

[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.

[DNS-WILDCARDS]
Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<https://www.rfc-editor.org/info/rfc4592>.

[IDNA-DEFS]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.

[IDNA-PROTO]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<https://www.rfc-editor.org/info/rfc5891>.

[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[IPv6] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[LDAP-DN] Zeilenga, K., Ed., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, DOI 10.17487/RFC4514, June 2006,
<https://www.rfc-editor.org/info/rfc4514>.

[PKIX] 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>.

[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>.

[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<https://www.rfc-editor.org/info/rfc4985>.

[TLS-REC] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/info/rfc9325>.

[URI] 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>.

9.2. Informative References

[ABNF] 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>.

[ACME] 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>.

[ALPACA] Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D.,
Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel,
"ALPACA: Application Layer Protocol Confusion - Analyzing
and Mitigating Cracks in TLS Authentication", 30th USENIX
Security Symposium (USENIX Security 21), September 2021,
<https://alpaca-attack.com/ALPACA.pdf>.

[ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[DANE] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.

[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", Black Hat DC, February 2009,
<https://www.blackhat.com/presentations/bh-dc-
09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-
SSL.pdf>.

[DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343,
DOI 10.17487/RFC4343, January 2006,
<https://www.rfc-editor.org/info/rfc4343>.

[DNS-OVER-TLS]
Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.

[DTLS] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.

[EMAIL-SRV]
Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<https://www.rfc-editor.org/info/rfc6186>.

[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[HTTP-OVER-TLS]
Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.

[HTTPSbytes]
Sokol, J. and R. Hansen, "HTTPS Can Byte Me", Black Hat
Briefings, November 2010, <https://media.blackhat.com/bh-
ad-10/Hansen/Blackhat-AD-2010-Hansen-Sokol-HTTPS-Can-Byte-
Me-slides.pdf>.

[NAPTR] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, DOI 10.17487/RFC3403, October 2002,
<https://www.rfc-editor.org/info/rfc3403>.

[NTS] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time
Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
<https://www.rfc-editor.org/info/rfc8915>.

[Public-Suffix]
Mozilla Foundation, "Public Suffix List",
<https://publicsuffix.org>.

[QUIC] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.

[SECURE-CONTEXTS]
West, M., "Secure Contexts", W3C Candidate Recommendation
Draft, September 2021,
<https://www.w3.org/TR/secure-contexts/>.

[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[SIP-CERTS]
Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, DOI 10.17487/RFC5922, June 2010,
<https://www.rfc-editor.org/info/rfc5922>.

[SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630,
DOI 10.17487/RFC5630, October 2009,
<https://www.rfc-editor.org/info/rfc5630>.

[SMTP-TLS] Fenton, J., "SMTP Require TLS Option", RFC 8689,
DOI 10.17487/RFC8689, November 2019,
<https://www.rfc-editor.org/info/rfc8689>.

[SVCB-FOR-DNS]
Schwartz, B., "Service Binding Mapping for DNS Servers",
RFC 9461, DOI 10.17487/RFC9461, October November 2023,
<https://www.rfc-editor.org/info/rfc9461>.

[SVCB-FOR-HTTPS]
Schwartz, B., Bishop, M., and E. Nygren, "Service binding Binding
and parameter specification Parameter Specification via the DNS (DNS SVCB (SVCB and HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-
dnsop-svcb-https-12, 11 March
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-12>. <https://www.rfc-editor.org/info/rfc9460>.

[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[TLS-EXT] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.

[TLS-SUBCERTS]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS and DTLS", RFC 9345,
DOI 10.17487/RFC9345, July 2023,
<https://www.rfc-editor.org/info/rfc9345>.

[URL] van Kesteren, A., "URL", WHATWG Living Standard, September
2023, <https://url.spec.whatwg.org/>.

[US-ASCII] American National Standards Institute, "Coded Character
Sets - 7-bit American Standard Code for Information
Interchange (7-Bit ASCII)", ANSI X3.4-1986, 1986. INCITS 4-1986 (R2007),
June 2007.

[UTS-36] Davis, M. and M. Suignard, "Unicode Security
Considerations", Revision 15, Unicode Technical
Report #36, September 2014,
<https://unicode.org/reports/tr36/>.

[UTS-39] Davis, M. and M. Suignard, "Unicode Security Mechanisms",
Version 15.1.0, Revision 28, Unicode Technical
Standard #39, September 2023,
<https://unicode.org/reports/tr39/>.

[VERIFY] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", W3C Recommendation REC-wsc-ui-
20100812, August 2010,
<https://www.w3.org/TR/2010/REC-wsc-ui-20100812/>.

[X.509] ITU-T, "Information Technology - Open Systems
Interconnection - The Directory: Public-key and attribute
certificate frameworks", ISO/IEC 9594-8, ITU-T
Recommendation X.509, August 2005. October 2019.

[X.690] ITU-T, "Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO/IEC 8825-1, 8825-1:2021 (E), ITU-T
Recommendation X.690,
November 2008. February 2021.

[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.

[XSS] Kirsten, S., et al., "Cross Site Scripting (XSS)", OWASP
Foundation, 2020,
<https://owasp.org/www-community/attacks/xss/>.

Appendix A. Changes from RFC 6125

This document revises and obsoletes [VERIFY] based on the decade of
experience and changes since it was published. The major changes, in
no particular order, include:

* The only legal place for a certificate wildcard is as the complete
left-most label in a domain name.

* The server identity can only be expressed in the subjectAltNames
extension; it is no longer valid to use the commonName RDN, known
as CN-ID in [VERIFY].

* Detailed discussion of pinning (configuring use of a certificate
that doesn't match the criteria in this document) has been removed
and replaced with two paragraphs in Section 6.6.

* The sections detailing different target audiences and which
sections to read (first) have been removed.

* References to the X.500 directory, the survey of prior art, and
the sample text in Appendix A have been removed.

* All references have been updated to the latest versions.

* The TLS SNI extension is no longer new; it is commonplace.

* Additional text on multiple identifiers, and their security
considerations, has been added.

* IP-ID reference identifiers have been added. This builds on the
definition in [HTTP], Section 4.3.5.

* The document title has been shortened because the previous title
was difficult to cite.

Acknowledgements

We gratefully acknowledge everyone who contributed to the previous
version of this specification [VERIFY]. Thanks also to Carsten
Bormann for converting the previous version of this specification to
Markdown so that we could more easily use Martin Thomson's
i-d-template software.

In addition to discussion on discussions within the mailing list, UTA Working Group, the
following people provided official reviews or especially significant
feedback: Corey Bonnell, Roman Danyliw, Viktor Dukhovni, Lars Eggert,
Patrik Fältström, Jim Fenton, Olle Johansson, John Klensin, Murray
Kucherawy, Warren Kumari, John Mattson, Alexey Melnikov, Derrell
Piper, Maria Ines Robles, Rob Sayre, Yaron Sheffer, Ryan Sleevi,
Brian Smith, Petr Špaček, Orie Steele, Martin Thomson, Joe Touch,
Éric Vyncke, Paul Wouters, and Qin Wu.

A few descriptive sentences were borrowed from [TLS-REC].

Contributors

Jeff Hodges coauthored the previous version of this specification
[VERIFY]. The authors gratefully acknowledge his essential
contributions to this work.

Martin Thomson contributed the text on the handling of IP-IDs.

Authors' Addresses

Peter Saint-Andre
Independent
United States of America
Email: stpeter@stpeter.im

Rich Salz
Akamai Technologies
United States of America
Email: rsalz@akamai.com