Network Working Group P. Saint-Andre
Internet-Draft Mozilla
Obsoletes: 6125 (if approved) J. Hodges
Intended status: Standards Track Google
Expires: 22 May 2022 R. Salz
Akamai Technologies
18 November 2021
Service Names in TLS
draft-ietf-uta-rfc6125bis-04
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.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Using TLS in
Applications Working Group mailing list (uta@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/uta/.
Source for this draft and an issue tracker can be found at
https://github.com/richsalz/draft-ietf-uta-rfc6125bis.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 22 May 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Changes since RFC 6125 . . . . . . . . . . . . . . . . . 3
1.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Overview of Recommendations . . . . . . . . . . . . . . . 4
1.5. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5.1. In Scope . . . . . . . . . . . . . . . . . . . . . . 5
1.5.2. Out of Scope . . . . . . . . . . . . . . . . . . . . 5
1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Naming of Application Services . . . . . . . . . . . . . . . 8
3. Designing Application Protocols . . . . . . . . . . . . . . . 10
4. Representing Server Identity . . . . . . . . . . . . . . . . 10
4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Requesting Server Certificates . . . . . . . . . . . . . . . 12
6. Verifying Service Identity . . . . . . . . . . . . . . . . . 12
6.1. Constructing a List of Reference Identifiers . . . . . . 13
6.1.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1.2. Examples . . . . . . . . . . . . . . . . . . . . . . 14
6.2. Preparing to Seek a Match . . . . . . . . . . . . . . . . 15
6.3. Matching the DNS Domain Name Portion . . . . . . . . . . 16
6.4. Matching the Application Service Type Portion . . . . . . 17
6.5. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Wildcard Certificates . . . . . . . . . . . . . . . . . . 19
7.2. Internationalized Domain Names . . . . . . . . . . . . . 19
7.3. Multiple Presented Identifiers . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
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9.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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, it has some notion of the server's identity (e.g., "the
website at example.com") 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 CA in the context of the Internet
Public Key Infrastructure using X.509 [PKIX]. Informally, we can
think of these identities as the client's "reference identity" and
the server's "presented identity"; more formal definitions are given
later. A client needs to verify that the server's presented identity
matches its reference identity so it can deterministically and
automatically authenticate the communication.
This document defines procedures for how clients do this
verification. It therefore also defines requirements on other
parties, such as the CA's that issue certificates, the service
administrators requesting them, and the protocol designers defining
how things are named.
1.2. Changes since 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:
* All references have been updated to the current latest version.
* The TLS SNI extension is no longer new, it is commonplace.
* The only legal place for a certificate wildcard name is as the
left-most component 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].
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* References to the X.500 directory, the survey of prior art, and
the sample text in Appendix A have been removed.
* 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 6.5.
* The sections detailing different target audiences and which
sections to read (first) have been removed.
1.3. 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 cetrificate, let alone creating or
checking the validity of such a chain. In order to ensure proper
authentication, applications need to verify the entire certification
path as per [PKIX].
1.4. Overview of Recommendations
The previous version of this specification, [VERIFY], surveyed the
current practice from many IETF standards and tried to generalize
best practices. This document takes the lessons learned since then
and codifies them. The rules are brief:
* Only check DNS domain names via the subjectAlternativeName
extension designed for that purpose: dNSName.
* Allow use of even more specific subjectAlternativeName extensions
where appropriate such as uniformResourceIdentifier and the
otherName form SRVName.
* Constrain wildcard certificates so that the wildcard can only be
the left-most component of a domain name.
* Do not include or check strings that look like domain names in the
subject's Common Name.
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1.5. Scope
1.5.1. In Scope
This document applies only to service identities associated with
FQDNs only to TLS and DTLS, and only to PKIX-based systems.
TLS uses the words client and server, where the client is the entity
that initiates the connection. In many cases, this models 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 to use the terms client and
server in this document. Note that these are TLS-layer roles, and
that the application protocol could support the TLS server making
requests to the TLS client after the TLS handshake; these is no
requirement that the roles at the application layer match the TLS-
layer.
At the time of this writing, other protocols such as [QUIC] and
Network Time Security ([NTS]) use DTLS or TLS to do the initial
establishment of cryptographic key material. Such services MUST also
follow the rules specified here.
1.5.2. Out of Scope
The following topics are out of scope for this specification:
* Security protocols other than [TLS] or [DTLS] except as described
above.
* Keys or certificates employed outside the context of PKIX-based
systems.
* Client or end-user identities. Certificates representing client
identities other than that described above, such as rfc822Name,
are beyond the scope of this document.
* Identifiers other than FQDNs. Identifiers such as IP address are
not discussed. In addition, the focus of this document is on
application service identities, not specific resources located at
such services. Therefore this document discusses Uniform Resource
Identifiers [URI] only as a way to communicate a DNS domain name
(via the URI "host" component or its equivalent), not other
aspects of a service such as a specific resource (via the URI
"path" component) or parameters (via the URI "query" component).
* CA policies. This includes items such as the following:
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- How to certify or validate FQDNs and application service types
(see [ACME] for some definition of this).
- Issuing certificates with additional identifiers such as IP
address or other, in addition to FQDNs.
- 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 our purposes we care only about the
fact that the client needs to verify the identity of the entity
with which it communicates as a result of the resolution process.
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 (see, for example, [WSC-UI]).
1.6. 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], DNS SRV
Service [DNS-SRV], or an ALPN [ALPN] identifier.
delegated domain: A domain name or host name that is explicitly
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configured for communicating with the source domain, by either the
human user controlling the client or a trusted administrator. For
example, a server at mailhost.example.com for connecting to an
IMAP server hosting an email address of user@example.com.
derived domain: A domain name or host name that a client has derived
from the source domain in an automated fashion (e.g., by means of
a [DNS-SRV] lookup).
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 that can also 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].
* 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]. This entry
MUST include both a "scheme" and a "host" component (or its
equivalent) that matches the "reg-name" rule (where the quoted
terms represent the associated [ABNF] productions from [URI]).
If the entry does not have both, it is not a valid URI-ID and
MUST be ignored.
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 in the Internet.
presented identifier: An identifier presented by a server to a
client within a PKIX certificate when the client attempts to
establish secure communication with the server. The certificate
can include one or more presented identifiers of different types,
and if the server hosts more than one domain then the certificate
might present distinct identifiers for each domain.
reference identifier: An identifier used by the client when
examining presented identifiers. It is constructed from the
source domain, and optionally an application service type.
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Relative Distinguished Name (RDN): An ASN.1-based construction which
itself is 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.
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).
Security-related terms used in this document, but not defined here or
in [PKIX] should be understood in the 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. Naming of Application Services
This document assumes that the name of an application service is
based on a DNS domain name (e.g., example.com) -- supplemented in
some circumstances by an application service type (e.g., "the IMAP
server at example.com"). The DNS name conforms to one of the
following forms:
1. A "traditional domain name", a FQDN (see [DNS-CONCEPTS]) all of
whose labels are "LDH labels" as described in [IDNA-DEFS].
Informally, such labels are constrained to [US-ASCII] letters,
digits, and the hyphen, with the hyphen prohibited in the first
character position. Additional qualifications apply (refer to
the above-referenced specifications for details), but they are
not relevant here.
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2. An "internationalized domain name", a DNS domain name that
includes at least one label containing appropriately encoded
Unicode code points outside the traditional US-ASCII range. That
is, it contains at least one U-label or A-label, but otherwise
may contain any mixture of NR-LDH labels, A-labels, or U-labels,
as described in [IDNA-DEFS] and the associated documents.
From the perspective of the application client or user, some names
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 [NAPTR] records. The distinction matters most
for certificate consumption, specifically verification as discussed
in this document.
From the perspective of the application service, some names 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 example.com. Other names 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.
We can categorize the three identifier types as follows:
* A DNS-ID is direct and unrestricted.
* An SRV-ID is typically indirect but can be direct, and is
restricted.
* A URI-ID is direct and restricted.
It is important to keep these distinctions in mind, as best practices
for the deployment and use of the identifiers differ. Note that
cross-protocol attacks such as [ALPACA] are possibile when two
different protocol services use the same certificate. This can be
addressed by using restricted identifiers, or telling services to 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. Reasons
for this include:
* It is not strongly typed and therefore suffers from ambiguities in
interpretation.
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* It can appear more than once in the subjectName.
For similar reasons, other RDN's within the subjectName MUST NOT be
used to identify a service.
3. Designing Application Protocols
This section defines how protocol designers should reference this
document, which MUST be a normative reference in their specification.
The technology MUST only use the identifiers defined in this
document. Its specification MAY choose to allow only one of them.
If the technology does not use DNS SRV records to resolve the DNS
domain names of application services then its 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 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 URI's to identify application
services, then its 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 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 DNS
names. If it does so, then the specification MUST state that
wildcard certificates as defined in this document are not supported.
4. Representing Server Identity
This section provides instructions for issuers of certificates.
4.1. Rules
When a CA 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 a "DNS-ID" as a baseline for
interoperability.
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2. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type SRV-ID (e.g., [XMPP]), then
the certificate SHOULD include an SRV-ID.
3. If the service using the certificate deploys a technology for
which the relevant specification stipulates that certificates
ought to include identifiers of type URI-ID (e.g., [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 (or its
equivalent) 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]).
4. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-
ID as further explained under Section 7.3.
5. The certificate MAY include other application-specific
identifiers for compatibility with a deployed base. Such
identifiers are out of scope for this specification.
4.2. Examples
Consider a simple website at www.example.com, 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.example.com.
Consider an IMAP-accessible email server at the host mail.example.net
servicing email addresses of the form user@example.net and
discoverable via DNS SRV lookups on the application service name of
example.net. A certificate for this service might include SRV-IDs of
_imap.example.net and _imaps.example.net (see [EMAIL-SRV]) along with
DNS-IDs of example.net and mail.example.net.
Consider a SIP-accessible voice-over-IP (VoIP) server at the host
voice.example.edu servicing SIP addresses of the form
user@voice.example.edu and identified by a URI of
<sip:voice.example.edu>. A certificate for this service would
include a URI-ID of sip:voice.example.edu (see [SIP-CERTS]) along
with a DNS-ID of voice.example.edu.
Consider an XMPP-compatible instant messaging (IM) server at the host
im.example.org servicing IM addresses of the form user@im.example.org
and discoverable via DNS SRV lookups on the im.example.org domain. A
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certificate for this service might include SRV-IDs of _xmpp-
client.im.example.org and _xmpp-server.im.example.org (see [XMPP]), a
DNS-ID of im.example.org. For backward compatibility, it may also
have an XMPP-specific XmppAddr of im.example.org (see [XMPP]).
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 of the identifier types that are required or recommended
for the application service type that will be secured using the
certificate to be issued.
If the certificate will be used for only a single type of application
service, the service provider SHOULD request a certificate that
includes a DNS-ID and, if appropriate for the application service
type, an SRV-ID or URI-ID that limits the deployment scope of the
certificate to only the defined application service type.
If the certificate might be used for any type of application service,
then the service provider SHOULD request a certificate that includes
only a DNS-ID. Again, because of multi-protocol attacks this
practice is discouraged; this can be mitigated by deploying only one
service on a host.
If a service provider offers multiple application service types and
wishes to limit the applicability of certificates using SRV-IDs or
URI-IDs, they SHOULD request multiple certificates, rather than a
single certificate containing multiple SRV-IDs or URI-IDs each
identifying ia 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 acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX
certificate.
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3. The client checks each of its reference identifiers against the
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. Such
checking is not a matter of verifying the application service
identity presented in a certificate, however, and methods for doing
so are therefore out of scope for this document.
6.1. Constructing a List of Reference Identifiers
6.1.1. Rules
The client MUST construct a list of acceptable reference identifiers,
and MUST do so independently of the identifiers presented by the
service.
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.
The client might need to extract the source domain and application
service type from the input(s) it has received. The extracted data
MUST include only information that can be securely parsed out of the
inputs, such as parsing the FQDN out of the "host" component or
deriving the application service type from the scheme of a URI.
Other possibilities include pulling the data from a delegated domain
that is explicitly established via client or system configuration,
resolving the data via [DNSSEC], or obtaining the data from a third-
party domain mapping service in which a human user has explicitly
placed trust and with which the client communicates over a connection
or association that provides both mutual authentication and integrity
checking. These considerations apply only to extraction of the
source domain from the inputs. Naturally, if the inputs themselves
are invalid or corrupt (e.g., a user has clicked a link provided by a
malicious entity in a phishing attack), then the client might end up
communicating with an unexpected application service.
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For example, given an input URI of <sips:alice@example.net>, a client
would derive the application service type sip from the scheme and
parse the domain name example.net from the host component.
Each reference identifier in the list MUST be based on the source
domain and MUST NOT be based on a derived domain such as a domain
name discovered through DNS resolution of the source domain. This
rule is important because only a match between the user inputs and a
presented identifier enables the client to be sure that the
certificate can legitimately be used to secure the client's
communication with the server. This removes DNS and DNS resolution
from the attack surface.
Using the combination of FQDN(s) and application service type, the
client MUST construct its list of reference identifiers in accordance
with the following rules:
* The list SHOULD include a DNS-ID. A reference identifier of type
DNS-ID can be directly constructed from a 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.
* If a server for the application service type is typically
discovered by means of DNS SRV records, then the list SHOULD
include an SRV-ID.
* 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), then the list SHOULD include a URI-ID.
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 both SRV-IDs and DNS-IDs in
its list of reference identifiers.
6.1.2. Examples
A web browser that is connecting via HTTPS to the website at
www.example.com would have a single reference identifier: a DNS-ID of
www.example.com.
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A mail user agent that is connecting via IMAPS to the email service
at example.net (resolved as mail.example.net) might have three
reference identifiers: an SRV-ID of _imaps.example.net (see
[EMAIL-SRV]), and DNS-IDs of example.net and mail.example.net. An
email user agentthat does not support [EMAIL-SRV] would probably be
explicitly configured to connect to mail.example.net, whereas an SRV-
aware user agent would derive example.net from an email address of
the form user@example.net but might also accept mail.example.net as
the DNS domain name portion of reference identifiers for the service.
A voice-over-IP (VoIP) user agent that is connecting via SIP to the
voice service at voice.example.edu might have only one reference
identifier: a URI-ID of sip:voice.example.edu (see [SIP-CERTS]).
An instant messaging (IM) client that is connecting via XMPP to the
IM service at im.example.org might have three reference identifiers:
an SRV-ID of _xmpp-client.im.example.org (see [XMPP]), a DNS-ID of
im.example.org, and an XMPP-specific XmppAddr of im.example.org (see
[XMPP]).
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 its DNS domain name portion and its application service type
portion, as follows:
* A DNS-ID reference identifier MUST be used directly as the DNS
domain name and 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.example.net has a DNS domain name
portion of example.net and an application service type portion of
imaps, which maps to the IMAP application protocol as explained in
[EMAIL-SRV].
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* 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 defind above.
Matching only the "reg-name" rule from [URI] limits verification
to DNS domain names, thereby differentiating a URI-ID from a
uniformResourceIdentifier entry that contains an IP address or a
mere host name, or that does not contain a "host" component at
all. Furthermore, note that extraction of the "reg-name" might
necessitate normalization of the URI (as explained in [URI]). For
example, a URI-ID of sip:voice.example.edu would be split into a
DNS domain name portion of voice.example.edu and an application
service type of sip (associated with an application protocol of
SIP as explained in [SIP-CERTS]).
A client MUST match the DNS name, and if an application service type
is present it MUST also match the service type as well. These are
described below.
6.3. Matching the DNS Domain Name Portion
This section describes how the client must determine if the the
presented DNS name matches the reference DNS name. The rules differ
depending on whether the domain to be checked is a traditional domain
name or an internationalized domain name, as defined in Section 2.
For clients that support names containing the wildcard character "*",
this section also specifies a supplemental rule for such "wildcard
certificates". This section uses the description of labels and
domain names in [DNS-CONCEPTS].
If the DNS domain name portion of a reference identifier is a
traditional domain name, then 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.Example.Com would be
lower-cased to www.example.com for comparison purposes. Each label
MUST match in order for the names to be considered to match, except
as supplemented by the rule about checking of wildcard labels 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. 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 of wildcard labels given
below.
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If the technology specification supports wildcards, 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 content of the left-
most label.
If the requirements are not met, the presented identifier is invalid
and MUST be ignored.
A wildcard in a presented identifier can only match exactly one label
in a reference identifier. Note that this is not the same as 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].
For information regarding the security characteristics of wildcard
certificates, see Section 7.1.
6.4. Matching the Application Service Type Portion
The rules for matching the application service type depend on whether
the identifier is an SRV-ID or a URI-ID.
These identifiers provide an application service type portion to be
checked, but that portion is combined only with the DNS domain name
portion of the SRV-ID or URI-ID itself. For example, if a client's
list of reference identifiers includes an SRV-ID of _xmpp-
client.im.example.org and a DNS-ID of apps.example.net, the client
MUST check both the combination of an application service type of
xmpp-client and a DNS domain name of im.example.org and a DNS domain
name of apps.example.net. However, the client MUST NOT check the
combination of an application service type of xmpp-client and a DNS
domain name of apps.example.net because it does not have an SRV-ID of
_xmpp-client.apps.example.net 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 the _ character is prepended to the service
identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and
thus does not need to be included in any comparison.
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If the identifier is a URI-ID, then the scheme name portion MUST be
matched in a case-insensitive manner, in accordance with [URI]. Note
that the : character is a separator between the scheme name and the
rest of the URI, and thus does not need to be included in any
comparison.
6.5. Outcome
If the client has found a presented identifier that matches a
reference identifier, then the service identity check has succeeded.
In this case, the client MUST use the matched reference identifier as
the validated identity of the application service.
If the client does not find a presented identifier matching any of
the reference identifiers then the client MUST proceed as described
as follows.
If the client is an automated application, then it SHOULD terminate
the communication attempt with a bad certificate error and log the
error appropriately. The application MAY provide a configuration
setting to disable this behavior, but it MUST enable it 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 is SHOULD made available only as prior
configuration, rather than a just-in-time override for a failed
connection.
7. Security Considerations
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7.1. Wildcard Certificates
Wildcard certificates automatically vouch for any single-label host
names 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. See for example [Defeating-SSL]
(beginning at slide 91) and [HTTPSbytes] (slides 38-40).
Protection against a wildcard that identifies a public suffix
[Public-Suffix], such as *.co.uk or *.com, is beyond the scope of
this document.
7.2. Internationalized Domain Names
Allowing internationalized domain names can lead to visually similar
characters, also referred to as "confusables", being included within
certificates. For discussion, see for example [IDNA-DEFS],
Section 4.4 and [UTS-39].
7.3. 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 TLS Server
Name Identification (SNI), discussed in [TLS], Section 4.4.2.2, and
Application Layer Protocol Negotiation (ALPN), discussed in [ALPN],
provide a way for the client to indicate the desired identifier and
protocol to the server, which can then select the most appropriate
certificate.
To accommodate the workaround that was needed before the development
of the SNI extension, this specification allows multiple DNS-IDs,
SRV-IDs, or URI-IDs in a certificate.
8. IANA Considerations
This document has no actions for IANA.
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/rfc/rfc1034>.
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[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/rfc/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/rfc/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/rfc/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/rfc/rfc5891>.
[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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/rfc4985>.
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[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/rfc/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/rfc/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/rfc/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", 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/rfc/rfc7301>.
[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", BlackHat DC, February 2009,
<http://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/rfc/rfc4343>.
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/rfc/rfc4033>.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.
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[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/rfc/rfc6186>.
[HTTPSbytes]
Sokol, J. and R. Hansen, "HTTPS Can Byte Me", BlackHat Abu
Dhabi, 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/rfc/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/rfc/rfc8915>.
[Public-Suffix]
"Public Suffix List", 2020, <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/rfc/rfc9001>.
[SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/rfc/rfc4949>.
[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/rfc/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/rfc/rfc5922>.
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[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/rfc/rfc5630>.
[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/rfc/rfc8446>.
[US-ASCII] American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[UTS-39] Davis, M. and M. Suignard, "Unicode Security Mechanisms",
n.d., <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/rfc/rfc6125>.
[WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", August 2010,
<https://www.w3.org/TR/2010/REC-wsc-ui-20100812/>.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/rfc/rfc6120>.
Acknowledgements
We gratefully acknowledge everyone who contributed to the previous
version of this document, [VERIFY]. Thanks also to Carsten Bormann
for converting the previous document to Markdown so that we could
more easily use Martin Thomson's i-d-template software.
In addition to discussion on the mailing list, the following people
contributed significant changes: Viktor Dukhovni, Jim Fenton, Olle
Johansson, and Ryan Sleevi.
Authors' Addresses
Peter Saint-Andre
Mozilla
United States of America
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Email: stpeter@mozilla.com
Jeff Hodges
Google
United States of America
Email: jdhodges@google.com
Rich Salz
Akamai Technologies
United States of America
Email: rsalz@akamai.com
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