Service Identity in TLS
RFC 9525


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 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.
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
 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 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 perform this
 verification. It therefore defines requirements on other parties,
 such as the certification authorities that issue certificates, the
 service administrators requesting them, and the protocol designers
 defining 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 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 (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 indirect. Use of
 IP-IDs that are 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] and [DNS-OVER-TLS]
 do not change when the client uses the DNS resource records defined
 in [SVCB-FOR-HTTPS] or [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.
 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 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 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 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>, 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>.
 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> (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 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,
 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, 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
 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 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@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 one or more FQDNs or IP 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/> would have a single reference
 identifier: a DNS-ID of www.bigcompany.example.
 2. A web browser connecting to <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]>, 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 would be rejected because the DNS domain
 name portion does not match www.bigcompany.example.
 * With regard to the third example, a URI-ID of
 <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 exactly match the value of an
 iPAddress entry in subjectAltName, with no partial (e.g., 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> 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 not an
 internationalized domain 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 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 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. Consider the example of a
 messaging client that has two reference identifiers: (1) an SRV-ID of
 _xmpp-client.messenger.example and (2) a DNS-ID of app.example. The
 client MUST check (1) the combination of (a) an application service
 type of xmpp-client and (b) a DNS domain name of messenger.example 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 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 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 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 6.3). 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. Consider a system in which one component enforces a
 security rule that is conditional on the type of identifier but
 misclassifies an IP address as an FQDN, whereas a second component
 correctly classifies the identifier but incorrectly assumes that
 rules regarding IP addresses have been 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 the Server
 Name Indication (SNI), as discussed in [TLS-EXT], Section 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>.
 [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, November 2023,
 <https://www.rfc-editor.org/info/rfc9461>.
 [SVCB-FOR-HTTPS]
 Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
 and Parameter Specification via the DNS (SVCB and HTTPS
 Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
 November 2023, <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 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, 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 8825-1:2021 (E), ITU-T
 Recommendation X.690, 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 discussions within the 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