RFC 9525: Service Identity in TLS

RFC 9525	Service Identity in TLS	November 2023
Saint-Andre & Salz	Standards Track	[Page]

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.¶

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.¶

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.¶

The certificate MUST include at least one identifier.¶
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.¶
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.¶
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.¶
The certificate MAY contain more than one DNS-ID, SRV-ID, URI-ID, or IP-ID as further explained in Section 7.5.¶
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.¶

6. Verifying Service Identity

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

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

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.¶
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.¶
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.¶
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]).¶
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:¶

There is only one wildcard character.¶
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.¶

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/>.