Network Working Group P. Saint-Andre
Internet-Draft Cisco Systems, Inc.
Intended status: BCP J. Hodges
Expires: January 13, 2011 PayPal
July 12, 2010
Representation and Verification of Domain-Based Application Service
Identity in Certificates Used with Transport Layer Security
draft-saintandre-tls-server-id-check-08
Abstract
Many application technologies enable a secure connection between two
entities using certificates in the context of Transport Layer
Security (TLS). This document specifies best current practices for
representing and verifying the identity of application services in
such interactions.
Status of this Memo
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This Internet-Draft will expire on January 13, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. In Scope . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2. Out of Scope . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
1.4. Contributors . . . . . . . . . . . . . . . . . . . . . . . 10
1.5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 10
1.6. Discussion Venue . . . . . . . . . . . . . . . . . . . . . 10
2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1. Naming Application Services . . . . . . . . . . . . . . . 11
2.2. Subject Naming in PKIX Certificates . . . . . . . . . . . 12
3. Representation of Server Identity . . . . . . . . . . . . . . 14
4. Verification of Server Identity . . . . . . . . . . . . . . . 15
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Constructing an Ordered List of Reference Identifiers . . 16
4.3. Seeking a Match . . . . . . . . . . . . . . . . . . . . . 17
4.4. Verifying a Domain Name . . . . . . . . . . . . . . . . . 18
4.4.1. Checking of Traditional Domain Names . . . . . . . . . 18
4.4.2. Checking of Internationalized Domain Names . . . . . . 18
4.4.3. Checking of Wildcard Labels . . . . . . . . . . . . . 19
4.4.4. Checking of Common Names . . . . . . . . . . . . . . . 19
4.5. Verifying an Application Type . . . . . . . . . . . . . . 20
4.5.1. SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 20
4.5.2. URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 20
4.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.6.1. Case #1: Match Found . . . . . . . . . . . . . . . . . 20
4.6.2. Case #2: No Match Found, Cached Certificate . . . . . 20
4.6.3. Case #3: No Match Found, Uncached Certificate . . . . 21
4.6.3.1. Interactive User Agent . . . . . . . . . . . . . . 21
4.6.3.2. Automated Client . . . . . . . . . . . . . . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
5.1. Service Delegation . . . . . . . . . . . . . . . . . . . . 22
5.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 22
5.3. Internationalized Domain Names . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Normative References . . . . . . . . . . . . . . . . . . . 23
7.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Prior Art . . . . . . . . . . . . . . . . . . . . . . 28
A.1. IMAP, POP3, and ACAP (1999) . . . . . . . . . . . . . . . 28
A.2. HTTP (2000) . . . . . . . . . . . . . . . . . . . . . . . 29
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A.3. LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 30
A.4. SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 33
A.5. XMPP (2004) . . . . . . . . . . . . . . . . . . . . . . . 34
A.6. NNTP (2006) . . . . . . . . . . . . . . . . . . . . . . . 35
A.7. NETCONF (2006/2009) . . . . . . . . . . . . . . . . . . . 36
A.8. Syslog (2009) . . . . . . . . . . . . . . . . . . . . . . 38
A.9. SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 39
A.10. GIST (2010) . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
1.1. Motivation
The visible face of the Internet consists of services that employ a
client-server architecture in which an interactive or automated
client connects to an application services in order to retrieve or
upload information, communicate with other entities, or access a
broader network of services. When a client connects to an
application services using Transport Layer Security [TLS] (or, less
commonly, [DTLS]), it references some conception of the server's
identity while attempting to establish a secure connection (e.g.,
"the web site at example.com"). Likewise, during TLS negotiation the
server presents its conception of the server'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" (these rough ideas are defined more precisely
later in this document through the concept of particular
identifiers). In general, a client needs to verify that the server's
presented identity matches its reference identity so that it can be
sure that the certificate can legitimately be used to authenticate
the connection.
Many application technologies adhere to the pattern outlined here,
including but not limited to the following:
o The Internet Message Access Protocol [IMAP] and the Post Office
Protocol [POP3], for which see also [USINGTLS]
o The Hypertext Transfer Protocol [HTTP], for which see also
[HTTP-TLS]
o The Lightweight Directory Access Protocol [LDAP], for which see
also [LDAP-AUTH] and its predecessor [LDAP-TLS]
o The Simple Mail Transfer Protocol [SMTP], for which see also
[SMTP-AUTH] and [SMTP-TLS]
o The Extensible Messaging and Presence Protocol [XMPP], for which
see also [XMPPBIS]
o The Network News Transfer Protocol [NNTP], for which see also
[NNTP-TLS]
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o The NETCONF Configuration Protocol [NETCONF], for which see also
[NETCONF-SSH] and [NETCONF-TLS]
o The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]
o The Session Initiation Protocol [SIP], for which see also
[SIP-CERTS]
o The General Internet Signalling Transport [GIST]
Application protocols have traditionally specified their own rules
for representing and verifying server identities. Unfortunately,
this divergence of approaches has caused some confusion among
certification authorities, application developers, and protocol
designers.
Therefore, to codify best current practices regarding the
implementation and deployment of secure PKIX-based authentication,
this document specifies recommended procedures for representing and
verifying server identities in certificates intended for use in
applications employing TLS.
1.2. Scope
1.2.1. In Scope
This document applies only to server identities associated with
fully-qualified DNS domain names, only to TLS, and only to PKIX-based
systems. As a result, the scenarios described in the following
section are out of scope for this specification (although they might
be addressed by future specifications).
1.2.2. Out of Scope
The following topics are out of scope for this specification:
o Client or end-user identities.
Certificates representing client or end-user identities (e.g., the
rfc822Name identifier) can be used for mutual authentication
between a client and server or between two clients, thus enabling
stronger client-server security or end-to-end security. However,
certification authorities, application developers, and service
operators have less experience with client certificates than with
server certificates, thus gives us fewer models from which to
generalize and a less solid basis for defining best practices.
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o Identifiers other than fully-qualified DNS domain names.
Some certification authorities issue server certificates based on
IP addresses, but preliminary evidence indicates that such
certificates are a very small percentage of issued certificates
(e.g., less than 1%). Furthermore, IP addresses are not
necessarily reliable identifiers for application services because
of the existence of private internets [PRIVATE], host mobility,
multiple interfaces on a given host, Network Address Translators
(NATs) resulting in different addresses for a host from different
locations on the network, the practice of grouping many hosts
together behind a single IP address, etc. Most fundamentally,
most users find DNS domain names much easier to work with than IP
addresses, which is why the domain name system was designed in the
first place. We prefer to define best practices for the much more
common use case and not to complicate the rules in this
specification.
Furthermore, we do not discuss attributes unrelated to DNS domain
names, such as those defined in [X.520] and other such
specification (e.g., organizational attributes, geographical
attributes, company logos, and the like).
o Security protocols other than [TLS] or [DTLS].
Although other secure, lower-layer protocols exist and even employ
PKIX certificates at times, e.g. [IPSEC], their use cases can
differ from those of TLS-based or DTLS-based application
technologies. Furthermore, application technologies have less
experience with IPsec than with TLS, thus making it more difficult
to gather feedback on proposed best practices.
o Keys or certificates employed outside the context of PKIX-based
systems.
Some deployed application technologies use a web of trust model
based on or similar to [OPENPGP], or use self-signed certificates,
or are deployed on networks are not directly connected to the
public Internet and therefore cannot depend on Certificate
Revocation Lists (CRLs) or the Online Certificate Status Protocol
[OCSP] to check CA-issued certificates. However, the syntax of
OpenPGP keys differs essentially from X.509 certificates, the data
in self-signed certificates has not been certified by a third
party in any way, and checking of CA-issued certificates via CRLs
or OSCP is critically important to maintaining the security of
PKIX-based systems. Attempting to define best practices for such
technologies would unduly complicate the rules defined in this
specification.
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Furthermore, this document also does not address various
certification authority policies, such as:
o What classes and types of certificates to issue and whether to
apply different policies for them (e.g., allow the wildcard
character in Class 2 certificates but not in Class 1 or Extended
Validation certificates).
o Whether to issue certificates based on IP addresses (or some other
form, such as relative domain names) in addition to fully-
qualified DNS domain names.
o Which identifiers to include (e.g., whether to include the SRVName
and uniformResourceIdentifier extensions).
o How to certify or validate fully-qualified domain names and
application service types.
o How to certify or validate other kinds of information that might
be included in a certificate (e.g., organization name).
Finally, this specification is mostly silent about user interface
issues, which in general are properly the responsibility of client
software developers and standards development organizations dedicated
to particular application technologies (see for example [WSC-UI]).
1.3. 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
interactive and automated 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 organization or individual that
hosts or deploys an application service.
attribute-type-and-value pair: A colloquial name for the ASN.1-based
construction comprising a Relative Distinguished Name (RDN), which
itself is a building-block component of Distinguished Names. See
Section 2 of [LDAP-DN].
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automated client: A software agent or device that is not directly
controlled by a natural person.
direct name: A name for an application service that is provided
directly to a client by a user, resulting in a source domain and
(optionally) a service type.
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 also used for matching
purposes; the types covered in this specification are:
* CN-ID = a Relative Distinguished Name (RDN) in the certificate
subject that contains one and only one attribute-type-and-value
pair of type Common Name (CN); see [PKIX] and also
[LDAP-SCHEMA]
* DNS-ID = a subjectAltName entry of type dNSName; see [PKIX]
* SRV-ID = a subjectAltName entry of type otherName whose name
form is SRVName; see [SRVNAME]
* URI-ID = a subjectAltName entry of type
uniformResourceIdentifier; see [PKIX]
indirect name: A name for an application service that is resolved by
a client based on a direct name provided by a user, resulting in a
target domain and (optionally) a service type.
interactive client: A software agent or device that is directly
controlled by a natural person. (Other specifications related to
security and application protocols often refer to this as a "user
agent", e.g., [WSC-UI].)
PKIX certificate: An X.509v3 certificate generated and employed in
the context of the Internet Public Key Infrastructure using X.509
[PKIX].
presented identifier: An identifier that is presented by a server to
a client within the server's PKIX certificate when the client
attempts to establish a secure connection with the server; the
certificate can include one or more presented identifiers of
different types.
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reference identifier: An identifier that is used by the client for
matching purposes when checking the presented identifiers; the
client can attempt to match multiple reference identifiers of
different types.
restricted name: A name that can be used only for one type of
service at an application service provider.
service type: A formal identifier for the application protocol used
to provide a particular kind of service at a domain; the service
type typically takes the form of a Uniform Resource Identifier
scheme [URI] or a DNS SRV Service [DNS-SRV].
source domain: The fully-qualified DNS domain name that a client
expects an application service to present in the certificate.
subjectAltName entry: A specific identifier placed in a
subjectAltName extension.
subjectAltName extension: A standard PKIX certificate extension as
described in [PKIX]. The subject alternative name extension
allows various identifiers of various types to be bound to the
certificate subject, in addition or in place of the subject name.
subject name: The name of a PKIX certificate's subject, encoded as
the X.501 type Name, and conveyed in a certificate's subject field
(see Section 4.1.2.6 of [PKIX]). Note that a subject's name(s)
can be represented in the subject field, the subjectAltName
extension, or both (see [PKIX] for details).
TLS client: An entity that assumes the role of a client in a
Transport Layer Security [TLS] negotiation; in this specfication
we generally assume that the TLS client is an (interactive or
automated) application client, however in application protocols
that enable server-to-server communication the TLS client could be
a peer application service.
TLS server: An entity that assumes the role of a server in a
Transport Layer Security [TLS] negotiation; in this specfication
we assume that the TLS server is an application service.
target 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) or that a natural person directly controlling
an interactive client has explicitly configured for connecting to
the source domain.
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unrestricted name: A name that can be used for any service type at
an application service provider.
Most security-related terms in this document are to be understood in
the sense defined in [SECTERMS]; such terms include, but are not
limited to, "attack", "authentication", "authorization",
"certification authority", "certification path", "certificate",
"credential", "identity", "self-signed certificate", "trust", "trust
anchor", "trust chain", "validate", and "verify".
The following capitalized keywords are to be interpreted as described
in [KEYWORDS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
"SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
"OPTIONAL".
1.4. Contributors
The following individuals made important contributions to the text of
this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.
1.5. Acknowledgements
The editors and contributors wish to thank the following individuals
for their feedback and suggestions: Nelson Bolyard, Kaspar Brand, Ben
Campbell, Scott Cantor, Dave Crocker, Cyrus Daboo, Charles Gardiner,
Philip Guenther, Bruno Harbulot, David Harrington, Paul Hoffman, Love
Hornquist Astrand, Harry Hotz, Geoff Keating, Scott Lawrence, Matt
McCutchen, Alexey Melnikov, Eddy Nigg, Ludwig Nussel, Joe Orton, Tom
Petch, Yngve Pettersen, Tim Polk, Eric Rescorla, Pete Resnick, Martin
Rex, Joe Salowey, Rob Stradling, Peter Sylvester, Dan Wing, and Dan
Winship.
1.6. Discussion Venue
[[ RFC Editor: please remove this section. ]]
The editors are actively seeking input from certification
authorities, application developers, and protocol designers regarding
the recommendations in this document. Please send feedback to the
editors directly or post to the <certid@ietf.org> mailing list, for
which archives and subscription information are available at
<https://www.ietf.org/mailman/listinfo/certid>.
2. Names
This section discusses naming of application services on the
Internet, followed by a brief tutorial about subject naming in PKIX.
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2.1. Naming Application Services
This specification assumes that the name of an application service is
based on a DNS domain name (e.g., "example.com") -- supplemented in
some circumstances by a service type (e.g., "the IMAP server at
example.com").
From the perspective of the application client or user, some names
are direct because they are provided directly by the user (e.g., via
runtime input or prior configuration) whereas other names are
indirect because they are resolved by the client based on input
provided directly by the user (e.g., a target name resolved from a
source name using DNS SRV records). This dimension matters for
certificate verification.
From the perspective of the application service, some names are
unrestricted because they can be used in any type of service (e.g., a
certificate might be re-used for both the HTTP service and the IMAP
service at example.com) whereas other names are restricted because
they can be used in only one type of service (e.g., a special-purpose
certificate that can be used only for an IMAP service). This
dimension matters for certificate issuance.
Therefore:
o A CN-ID identifier is direct (provided by a user) and unrestricted
(can be used for any application).
o A DNS-ID identifier is direct (provided by a user) and
unrestricted (can be used for any application).
o An SRV-ID identifier is indirect (resolved by a client) and
restricted (can be used for only a single application).
o A URI-ID identifier is direct (provided by a user) and restricted
(can be used for only a single application).
We summarize this taxonomy in the following table.
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+-----------+-----------+---------------+
| | Direct | Restricted |
+-----------+-----------+---------------+
| CN-ID | Yes | No |
+-----------+-----------+---------------+
| DNS-ID | Yes | No |
+-----------+-----------+---------------+
| SRV-ID | No | Yes |
+-----------+-----------+---------------+
| URI-ID | Yes | Yes |
+-----------+-----------+---------------+
When implementing software, deploying services, and issuing
certificates for secure PKIX-based authentication, it is important to
keep these distinctions in mind. In particular, best practices
differ somewhat for application server implementations, application
client implementations, application service providers, and
certification authorities. Protocol specifications that reference
this document MUST specify which identifiers are mandatory-to-
implement by servers and clients, which identifiers are to be
preferred by application service providers, and which identifiers
ought to be supported by certificate issuers. Because these
requirements differ across applications, it is impossible to
categorically stipulate universal rules (e.g., that all software
implementations, service providers, and certification authorities for
all application protocols need to use or support DNS-IDs as a
baseline for the purpose of interoperability); however, it is
preferable that each application protocol will at least define a
baseline that applies to the community of software developers,
application service providers, and CAs actively using or supporting
that technology.
2.2. Subject Naming in PKIX Certificates
In theory, the Internet Public Key Infrastructure using X.509 [PKIX]
employs the global directory service model defined in [X.500] and
[X.501]. In this model, information is held in a directory
information base (DIB) and entries in the DIB are organized in a
hierarchy called the directory information tree (DIT). An entry in
that hierarchy consists of a set of attributes (each of which has a
defined type and one or more values) and is uniquely identified by a
Distinguished Name (DN). The DN of an entry is constructed by
combining one or more specially-nominated attributes of the entry
itself (which together comprise the Relative Distinguished Name (RDN)
of the entry) with the Distinguished Name of its superior entries in
the tree, up to the root of the DIT. An RDN is a set (i.e., an
unordered group) of attribute-type-and-value pairs (see also
[LDAP-DN]), each of which asserts some attribute about the entry.
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In practice, the certificates used in [X.509] and [PKIX] borrow key
concepts of X.500 and X.501 (e.g., DNs and RDNs) to identify
entities, but such certificates are not necessarily part of a global
directory information base. Specifically, the subject field of a
PKIX certificate is an X.501 type Name that "identifies the entity
associated with the public key stored in the subject public key
field" (see Section 4.1.2.6 of [PKIX]). However, it is perfectly
acceptable for the subject field to be empty, as long as the
certificate contains a subjectAltName extension that includes at
least one subjectAltName entry, because the subject alternative name
("subjectAltName") extension allows various identities to be bound to
the subject (see Section 4.2.1.6 of [PKIX]). The subjectAltName
extension itself is a sequence of typed entries, where each type is a
distinct kind of identifier.
For our purposes, an application service is identified by a name or
names carried in the subject field and/or in one of the following
subjectAltName entry types:
o dNSName -- a (fully-qualified) DNS domain name [PKIX]
o SRVName -- a DNS SRV service name [DNS-SRV] [SRVNAME]
o uniformResourceIdentifier -- a Uniform Resource Identifier [URI]
[PKIX]
We recognize that existing certificates often use a CN-ID in the
subject field to represent a fully-qualified DNS domain name; for
example, consider the following subjectName, where the attribute of
type Common Name contains a string whose form matches that of a
fully-qualified DNS domain name of "www.example.com":
cn=www.example.com,c=GB,ou=Web Services
However, in general, this specification recommends and prefers use of
subjectAltName entries over use of the subject field where possible,
as more completely described in the following sections.
Implementation Note: Confusion sometimes arises from different
renderings or encodings of the hierarchical information contained
in a certificate. Certificates are binary objects and are encoded
using the Distinguished Encoding Rules (DER) specified in [X.690].
However, some implementations generate displayable (a.k.a.
printable) renderings of certificate issuer, subject, and subject
alternative names, and these renderings convert the DER-encoded
sequences into a "string representation" before being displayed.
Because a Distinguished Name (DN) is an ordered sequence, order is
preserved in the string representation of a DN. However, because
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a Relative Distinguished Name (RDN) is an unordered group of
attribute-type-and-value pairs, the string representation of an
RDN can differ from the canonical DER encoding. Furthermore,
various specifications refer to the order of RDNs using
terminology that is not directly related to the information
hierarchy, such as "most specific" vs. "least specific", "left-
most" vs. "right-most", "first" vs. "last", or "most significant"
vs. "least significant" (see for example [LDAP-DN]). In this
specification we avoid such terms in an effort to reduce
confusion.
3. Representation of Server Identity
When a certification authority issues a certificate based on the
fully-qualified DNS domain name at which the application service
provider will provide the relevant application, the following rules
apply to the representation of application service identities.
1. The certificate SHOULD include a "DNS-ID" (i.e., a subjectAltName
entry of type dNSName) if possible as a baseline for
interoperability.
2. If the service using the certificate deploys a technology in
which a server is discovered by means of DNS SRV records
[DNS-SRV] (e.g., this is true of [XMPP]), then the certificate
SHOULD include an "SRV-ID" (i.e., a subjectAltName entry of type
otherName whose name form is SRVName as specified in [SRVNAME]).
3. If the service using the certificate deploys a technology in
which a server is typically associated with a URI (e.g., this is
true of [SIP]), then the certificate SHOULD include a URI-ID
(i.e., a subjectAltName entry of type uniformResourceIdentifier);
the scheme SHALL be that of the protocol associated with the
service type and the authority component SHALL be the fully-
qualified DNS domain name of the service.
4. The certificate MAY include other application-specific
identifiers for types that were defined before specification of
the SRVName extension (e.g., XmppAddr for [XMPP]) or for which
service names or URI schemes do not exist; however, such
application-specific identifiers are not generally applicable and
therefore are out of scope for this specification.
5. The certificate SHOULD NOT represent the server's fully-qualified
DNS domain name in a CN-ID, even though many deployed clients
still check for this legacy identifier configuration within
certificate subjectName. If a CN-ID is included, the
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certificate's subject field SHOULD NOT contain more than one
CN-ID, and MUST NOT contain RDNs which consist of multiple Common
Name attributes.
6. The fully-qualified DNS domain name portion of the DN-ID or CN-ID
MAY contain one instance of the wildcard character '*', but only
as the left-most label of the domain name component of the
identifier (following the definition of "label" from [DNS]).
Specifications that profile the rules defined in this document
MUST specify whether the wildcard character is or is not allowed
in certificates issued under that profile; by default wildcard
certificates SHOULD NOT be allowed.
4. Verification of Server Identity
4.1. Overview
At a high level, the client verifies the server's identity by
performing the following actions:
1. Before connecting to the server, the client constructs an ordered
list of reference identifiers against which to check the
presented identifiers.
2. The server provides its identifiers in the form of a PKIX
certificate.
3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.
4. When checking a reference identifier against a presented
identifier, the client (a) MUST match the source domain (or, in
some cases, target domain) of the identifiers and (b) MAY also
match the service type of the identifiers.
Implementation Note: Naturally, in addition to checking
identifiers, a client might complete further checks to ensure that
the server is authorized to provide the requested service.
However, such checking is not a matter of verifying the server
identity presented in a certificate, and therefore methods for
doing so (e.g., consulting local policy information) are out of
scope for this document.
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4.2. Constructing an Ordered List of Reference Identifiers
Before connecting to the server, the client MUST construct an ordered
list of acceptable reference identifiers.
The inputs here might be a URI that a user has typed into an
interface (e.g., an HTTP URL for a web site), configured account
information (e.g., the username of an IMAP or POP3 account for
retrieving email), or some other combination of information that can
yield a source domain and a service type.
The client might need to derive the source domain and service type
from the input(s) it has received. The derived data MUST include
only information that can be securely parsed out of the inputs (e.g.,
extracting the fully-qualified DNS domain name from the authority
component of a URI or extracting the service type from the scheme of
a URI) or information for which the derivation is performed in a
secure manner (e.g., using [DNSSEC]).
In some cases the inputs might include more than one fully-qualified
DNS domain name, because a user might have explicitly configured the
client to associate a target domain with the source domain. Such
delegation can occur by means of user-approved DNS SRV records (e.g.,
_xmpp-server._tcp.im.example.com might yield an address of
hosting.example.net) or a user-configured lookup table for host-to-
address or address-to-host translations (e.g., the Unix "hosts"
file). See under Section 5 for further discussion of service
delegation.
Using the combination of fully-qualified DNS domain name(s) and
service type, the client constructs a list of reference identifiers
in accordance with the following rules:
o The list MUST include a DNS-ID. A reference identifier of type
DNS-ID can be directly constructed from a fully-qualified DNS
domain name that is (a) contained in or securely derived from the
inputs (i.e., the source domain), or (b) explicitly associated
with the source domain by means of user configuration (i.e., a
target domain).
o If a server for the service type is typically discovered by means
of DNS SRV records , then the list SHOULD include an SRV-ID.
o If a server for the service type is typically associated with a
URI, then the list SHOULD include a URI-ID
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o The list SHOULD NOT include a CN-ID; however, the CN-ID (if
included) MUST be constructed only from the source domain and
never from a target domain.
The client does not need to actually construct the foregoing
identifiers in the formats found in a certificate (e.g., as ASN.1
object identifiers), only the functional equivalent of such
identifiers for matching purposes.
Security Note: A client MUST NOT construct a reference identifier
corresponding to Relative Distinguished Names (RDNs) other than
the Common Name and MUST NOT check for such RDNs in the presented
identifiers.
The client then orders the list in accordance with the following
rules:
o Reference identifiers that include the source domain MUST be
preferred over reference identifiers that include a target domain
(if any).
o Reference identifiers that include both a fully-qualified DNS
domain name and a service type MUST be preferred over reference
identifiers that include only a fully-qualified DNS domain name.
Therefore an SRV-ID or URI-ID is to be preferred over a DNS-ID.
o Notwithstanding any of the foregoing rules, reference identifiers
of type CN-ID (if included) MUST always be the lowest-priority
reference identifiers in the list.
To illustrate the ordering rules, consider the case where the inputs
are a source domain of "im.example.com" and a service type of "XMPP"
(for which application services are discovered via DNS SRV records)
and the user of the client has also explicitly configured a target
domain of "hosting.example.net". In this case, the ordered list
would be:
1. SRV-ID of "_xmpp.im.example.com"
2. DNS-ID of "im.example.com"
3. SRV-ID of "_xmpp.hosting.example.net"
4. DNS-ID of "hosting.example.net"
5. CN-ID of "im.example.com"
4.3. Seeking a Match
Once the client has constructed its order list of reference
identifiers and has received the server's presented identifiers in
the form of a PKIX certificate, the client checks its reference
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identifiers against the presented identifiers for the purpose of
finding a match. It does so by seeking a match in preference order
and aborting the search if any presented identifier matches one of
its reference identifiers. The search fails if the client exhausts
its list of reference identifiers without finding a match. Detailed
comparison rules for finding a match are provided in the following
sections.
Security Note: A client MUST NOT seek a match for a reference
identifier of CN-ID if the presented identifiers include an
SRV-ID, URI-ID, DNS-ID, or any application-specific subjectAltName
entry types supported by the client.
4.4. Verifying a Domain Name
This document assumes that each reference identifier contains a
fully-qualified DNS domain name that is a "traditional domain name"
or an "internationalized domain name". The client MUST match the
source domain of a reference identifier according to the following
rules.
4.4.1. Checking of Traditional Domain Names
The term "traditional domain name" is a contraction of this more
formal and accurate name: "traditional US-ASCII letter-digit-hyphen
DNS domain name". Traditional domain names are defined in
[DNS-CONCEPTS] and [DNS] in conjunction with [HOSTS] as further
explained in [IDNA2003]. In essence, a traditional domain name
consists of a set of one or more labels (e.g., "www", "example", and
"com"), with the labels usually shown separated by dots (e.g.,
"www.example.com"). Labels nominally consist of only [US-ASCII]
uppercase and lowercase letters, digits, and hyphen. There are
additional qualifications (please refer to the above-referenced
specifications for details) but they are not germane to this
specification.
If the source domain of a reference identifier is a "traditional
domain name", then matching of the reference identifier against the
presented identifier is performed by comparing the set of domain
components using a case-insensitive ASCII comparison, as clarified by
[DNS-CASE] (e.g., "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.
4.4.2. Checking of Internationalized Domain Names
The term "internationalized domain name" refers to a DNS domain name
that conforms to the overall form of a domain name (dot-separated
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labels) but that can include Unicode code points outside the
traditional US-ASCII range, as explained by and [IDNA2008].
If the source domain of a reference identifier is an
internationalized domain name, then an implementation MUST convert
every label in the domain name to an A-label before checking the
domain anme.
4.4.3. Checking of Wildcard Labels
Unless forbidden by a specification that profiles the best practices
defined herein, a client employing this specification's rules MAY
match the reference identifier against a presented identifier
containing one instance of the wildcard character '*', but only as
the left-most label of the domain name, e.g. *.example.com (following
the definition of "label" from [DNS]).
If such a wildcard identifier is presented, the wildcard MUST be used
to match only the one position-wise corresponding label (thus
*.example.com matches foo.example.com but not bar.foo.example.com or
example.com). The client MUST fail to match a presented identifier
in which the wildcard character is contained within a label fragment
(e.g., baz*.example.net is not allowed and MUST NOT be taken to match
baz1.example.net and baz2.example.net), or in which the wildcard
character does not comprise the left-most label in the presented
identifier (e.g., neither bar.*.example.net nor bar.f*o.example.net
are allowed).
4.4.4. Checking of Common Names
As noted, a client MUST NOT seek a match for a reference identifier
of CN-ID if the presented identifiers include an SRV-ID, URI-ID,
DNS-ID, or any application-specific subjectAltName entry types
supported by the client.
Therefore, if and only if the identity set does not include a
subjectAltName entry of type dNSName, SRVName, or
uniformResourceIdentifier (or any application-specific subjectAltName
entry types supported by the client), the client MAY as a fallback
check for a string whose form matches that of a fully-qualified DNS
domain name in the CN-ID. If the client chooses to compare a
reference identifier of type CN-ID against that string, it MUST
follow the comparison rules for the source domain of an identifier of
type DNS-ID, SRV-ID, or URI-ID, as described under Section 4.4.
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4.5. Verifying an Application Type
As specified under the ordering rules for reference identifiers, a
client SHOULD check not only the domain name but also the service
type of the service to which it connects. This is a best practice
because typically a client is not designed to connect to all kinds of
services using all possible application protocols, but instead is
designed to connect to one kind of service, such as a web site, an
email service, or an instant messaging service.
The service type is verified by means of either an SRV-ID or URI-ID.
4.5.1. SRV-ID
The service name portion of an SRV-ID (e.g., "xmpp") 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.
4.5.2. URI-ID
The scheme name portion of a URI-ID (e.g., "sip") 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 therefore does not need to be included in any
comparison.
4.6. Outcome
The outcome of the checking procedure is one of the following cases.
4.6.1. Case #1: Match Found
If the client has found a presented identifier that matches a
reference identifier, matching has succeeded. In this case, the
client MUST use the matched reference identifier as the validated
identity of the server.
4.6.2. Case #2: No Match Found, Cached Certificate
If the client finds no presented identifier that matches any of the
reference identifiers but a natural person has permanently accepted
the certificate during a previous connection attempt or via
configured preferences, the certificate is cached. In this case, the
client MUST verify that the presented certificate matches the cached
certificate and (if it is an interactive client) MUST notify the user
if the certificate has changed since the last time a secure
connection was successfully negotiated (where causes of change
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include but are not limited to changes in the DNS domain name,
identifiers, issuer, certification path, and expiration date).
4.6.3. Case #3: No Match Found, Uncached Certificate
If the client finds no presented identifier that matches any of the
reference identifiers and a human user has not permanently accepted
the certificate for this application service during a previous
connection attempt, the client MUST NOT consider the certificate to
include a validated identity for the application service.
Instead, the client MUST proceed as follows.
4.6.3.1. Interactive User Agent
If the client is an interactive client that is directly controlled by
a natural person, then it MUST either do one of the following:
1. Automatically terminate the connection with a bad certificate
error; or
2. Actively warn the user that the certificate is untrusted and
encourage the user to terminate the connection, but give advanced
users the option to (a) view the entire certification path, (b)
accept the certificate for this application service either
temporarily (i.e., for this connection attempt only) or
permanently (i.e., for all future connection attempts) despite
the identity mismatch, and then (c) continue with the connection
attempt.
If a user permanently accepts a certificate for this application
service despite an identity mismatch (an action referred to in
[WSC-UI] as "pinning"), the client MUST cache the certificate (or
some non-forgeable representation such as a hash value) and in future
connection attempts MUST behave as in "Case #2: No Match Found,
Cached Certificate" Section 4.6.2. However, the client MUST provide
a method for revoking trust in cached certificates.
Security Note: It is the responsibility of the human user to
verify the hash value or fingerprint of the certificate with the
server over a trusted communication layer.
Informational Note: The guidelines provided here are roughly
consistent with those provided for web browsers and other HTTP-
aware interactive clients in [WSC-UI].
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4.6.3.2. Automated Client
If the client is an automated application that is not directly
controlled by a natural person, then it SHOULD terminate the
connection with a bad certificate error and log the error to an
appropriate audit log. An automated application MAY provide a
configuration setting that disables this check, but MUST enable the
check by default.
5. Security Considerations
5.1. Service Delegation
When the connecting application is an interactive client, the source
domain name and service type MUST be provided by a human user (e.g.
when specifying the server portion of the user's account name on the
server or when explicitly configuring the client to connect to a
particular host or URI as in [SIP-LOC]) and MUST NOT be derived from
the user inputs in an automated fashion (e.g., a host name or domain
name discovered through DNS resolution of the source domain). This
rule is important because only a match between the user inputs (in
the form of a reference identifier) and a presented identifier
enables the client to be sure that the certificate can legitimately
be used to secure the connection.
However, an interactive client MAY provide a configuration setting
that enables a human user to explicitly specify a particular host
name or domain name (called a "target domain") to be checked for
connection purposes.
5.2. Wildcard Certificates
Allowing the wildcard character in certificates has led to homograph
attacks involving non-ASCII characters that look similar to
characters commonly included in HTTP URLs, such as "/" and "?"; for
discussion, see for example [Defeating-SSL].
5.3. Internationalized Domain Names
In addition to the wildcard certificate attacks previously mentioned,
allowing internationalized domain names can lead to the inclusion of
visually similar (so-called "confusable") characters in certificates;
for discussion, see for example [IDNA-DEFS].
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6. IANA Considerations
This document specifies no actions for the IANA.
7. References
7.1. Normative References
[DNS] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[DNS-CONCEPTS]
Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[IDNA2003]
Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[IDNA2008]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol",
draft-ietf-idnabis-protocol-18 (work in progress),
January 2010.
[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LDAP-DN] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, June 2006.
[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, May 2008.
[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.
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[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
7.2. Informative References
[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", February 2009, <http://www.blackhat.com/
presentations/bh-dc-09/Marlinspike/
BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf>.
[DNS-CASE]
Eastlake, D., "Domain Name System (DNS) Case Insensitivity
Clarification", RFC 4343, January 2006.
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[GIST] Schulzrinne, H. and M. Stiemerling, "GIST: General
Internet Signalling Transport", draft-ietf-nsis-ntlp-20
(work in progress), June 2009.
[HOSTS] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[HTTP-TLS]
Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[IDNA-DEFS]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
draft-ietf-idnabis-defs-13 (work in progress),
January 2010.
[IP] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
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[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[IPSEC] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[LDAP-AUTH]
Harrison, R., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.
[LDAP-SCHEMA]
Sciberras, A., "Lightweight Directory Access Protocol
(LDAP): Schema for User Applications", RFC 4519,
June 2006.
[LDAP-TLS]
Hodges, J., Morgan, R., and M. Wahl, "Lightweight
Directory Access Protocol (v3): Extension for Transport
Layer Security", RFC 2830, May 2000.
[NETCONF] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[NETCONF-SSH]
Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure SHell (SSH)", RFC 4742,
December 2006.
[NETCONF-TLS]
Badra, M., "NETCONF over Transport Layer Security (TLS)",
RFC 5539, May 2009.
[NNTP] Feather, C., "Network News Transfer Protocol (NNTP)",
RFC 3977, October 2006.
[NNTP-TLS]
Murchison, K., Vinocur, J., and C. Newman, "Using
Transport Layer Security (TLS) with Network News Transfer
Protocol (NNTP)", RFC 4642, October 2006.
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
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[OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[PRIVATE] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[PKIX-OLD]
Housley, R., Ford, W., Polk, T., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, January 1999.
[SECTERMS]
Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[SIP-CERTS]
Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, June 2010.
[SIP-LOC] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[SMTP-AUTH]
Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[SMTP-TLS]
Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[SYSLOG] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[SYSLOG-TLS]
Miao, F., Ma, Y., and J. Salowey, "Transport Layer
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Security (TLS) Transport Mapping for Syslog", RFC 5425,
March 2009.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[US-ASCII]
American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[USINGTLS]
Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", World Wide Web Consortium
LastCall WD-wsc-ui-20100309, March 2010,
<http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.
[X.500] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The Directory:
Overview of concepts, models and services", ITU-
T Recommendation X.500, ISO Standard 9594-1, August 2005.
[X.501] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The Directory:
Models", ITU-T Recommendation X.501, ISO Standard 9594-2,
August 2005.
[X.509] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The Directory:
Public-key and attribute certificate frameworks", ITU-
T Recommendation X.509, ISO Standard 9594-8, August 2005.
[X.520] International Telecommunications Union, "Information
Technology - Open Systems Interconnection - The Directory:
Selected attribute types", ITU-T Recommendation X.509,
ISO Standard 9594-6, August 2005.
[X.690] International Telecommunications Union, "Information
Technology - ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, ISO Standard 8825-1, August 2008.
[XMPP] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
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[XMPPBIS] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-07 (work
in progress), April 2010.
Appendix A. Prior Art
(This section is non-normative.)
The recommendations in this document are an abstraction from
recommendations in specifications for a wide range of application
protocols. For the purpose of comparison and to delineate the
history of thinking about server identity verification within the
IETF, this informative section gathers together prior art by
including the exact text from various RFCs (the only modifications
are changes to the names of several references to maintain coherence
with the main body of this document, and the elision of irrelevant
text as marked by the characters "[...]").
A.1. IMAP, POP3, and ACAP (1999)
In 1999, [USINGTLS] specified the following text regarding server
identity verification in IMAP, POP3, and ACAP:
######
2.4. Server Identity Check
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use any
form of the server hostname derived from an insecure remote source
(e.g., insecure DNS lookup). CNAME canonicalization is not done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc. but would not match
example.com.
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o If the certificate contains multiple names (e.g. more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation, or terminate the connection and indicate the server's
identity is suspect.
######
A.2. HTTP (2000)
In 2000, [HTTP-TLS] specified the following text regarding server
identity verification in HTTP:
######
3.1. Server Identity
In general, HTTP/TLS requests are generated by dereferencing a URI.
As a consequence, the hostname for the server is known to the client.
If the hostname is available, the client MUST check it against the
server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted. (For instance, a
client may be connecting to a machine whose address and hostname are
dynamic but the client knows the certificate that the server will
present.) In such cases, it is important to narrow the scope of
acceptable certificates as much as possible in order to prevent man
in the middle attacks. In special cases, it may be appropriate for
the client to simply ignore the server's identity, but it must be
understood that this leaves the connection open to active attack.
If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by
[PKIX-OLD]. If more than one identity of a given type is present in
the certificate (e.g., more than one dNSName name, a match in any one
of the set is considered acceptable.) Names may contain the wildcard
character * which is considered to match any single domain name
component or component fragment. E.g., *.a.com matches foo.a.com but
not bar.foo.a.com. f*.com matches foo.com but not bar.com.
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In some cases, the URI is specified as an IP address rather than a
hostname. In this case, the iPAddress subjectAltName must be present
in the certificate and must exactly match the IP in the URI.
If the hostname does not match the identity in the certificate, user
oriented clients MUST either notify the user (clients MAY give the
user the opportunity to continue with the connection in any case) or
terminate the connection with a bad certificate error. Automated
clients MUST log the error to an appropriate audit log (if available)
and SHOULD terminate the connection (with a bad certificate error).
Automated clients MAY provide a configuration setting that disables
this check, but MUST provide a setting which enables it.
Note that in many cases the URI itself comes from an untrusted
source. The above-described check provides no protection against
attacks where this source is compromised. For example, if the URI
was obtained by clicking on an HTML page which was itself obtained
without using HTTP/TLS, a man in the middle could have replaced the
URI. In order to prevent this form of attack, users should carefully
examine the certificate presented by the server to determine if it
meets their expectations.
######
A.3. LDAP (2000/2006)
In 2000, [LDAP-TLS] specified the following text regarding server
identity verification in LDAP:
######
3.6. Server Identity Check
The client MUST check its understanding of the server's hostname
against the server's identity as presented in the server's
Certificate message, in order to prevent man-in-the-middle attacks.
Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the LDAP
connection as the value to compare against the server name as
expressed in the server's certificate. The client MUST NOT use
the server's canonical DNS name or any other derived form of name.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
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o Matching is case-insensitive.
o The "*" wildcard character is allowed. If present, it applies
only to the left-most name component.
E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
bar.com. If more than one identity of a given type is present in the
certificate (e.g. more than one dNSName name), a match in any one of
the set is considered acceptable.
If the hostname does not match the dNSName-based identity in the
certificate per the above check, user-oriented clients SHOULD either
notify the user (clients MAY give the user the opportunity to
continue with the connection in any case) or terminate the connection
and indicate that the server's identity is suspect. Automated
clients SHOULD close the connection, returning and/or logging an
error indicating that the server's identity is suspect.
Beyond the server identity checks described in this section, clients
SHOULD be prepared to do further checking to ensure that the server
is authorized to provide the service it is observed to provide. The
client MAY need to make use of local policy information.
######
In 2006, [LDAP-AUTH] specified the following text regarding server
identity verification in LDAP:
######
3.1.3. Server Identity Check
In order to prevent man-in-the-middle attacks, the client MUST verify
the server's identity (as presented in the server's Certificate
message). In this section, the client's understanding of the
server's identity (typically the identity used to establish the
transport connection) is called the "reference identity".
The client determines the type (e.g., DNS name or IP address) of the
reference identity and performs a comparison between the reference
identity and each subjectAltName value of the corresponding type
until a match is produced. Once a match is produced, the server's
identity has been verified, and the server identity check is
complete. Different subjectAltName types are matched in different
ways. Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
various subjectAltName types.
The client may map the reference identity to a different type prior
to performing a comparison. Mappings may be performed for all
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available subjectAltName types to which the reference identity can be
mapped; however, the reference identity should only be mapped to
types for which the mapping is either inherently secure (e.g.,
extracting the DNS name from a URI to compare with a subjectAltName
of type dNSName) or for which the mapping is performed in a secure
manner (e.g., using [DNSSEC], or using user- or admin-configured
host-to-address/address-to-host lookup tables).
The server's identity may also be verified by comparing the reference
identity to the Common Name (CN) [LDAP-SCHEMA] value in the last
Relative Distinguished Name (RDN) of the subjectName field of the
server's certificate (where "last" refers to the DER-encoded order,
not the order of presentation in a string representation of DER-
encoded data). This comparison is performed using the rules for
comparison of DNS names in Section 3.1.3.1, below, with the exception
that no wildcard matching is allowed. Although the use of the Common
Name value is existing practice, it is deprecated, and Certification
Authorities are encouraged to provide subjectAltName values instead.
Note that the TLS implementation may represent DNs in certificates
according to X.500 or other conventions. For example, some X.500
implementations order the RDNs in a DN using a left-to-right (most
significant to least significant) convention instead of LDAP's right-
to-left convention.
If the server identity check fails, user-oriented clients SHOULD
either notify the user (clients may give the user the opportunity to
continue with the LDAP session in this case) or close the transport
connection and indicate that the server's identity is suspect.
Automated clients SHOULD close the transport connection and then
return or log an error indicating that the server's identity is
suspect or both.
Beyond the server identity check described in this section, clients
should be prepared to do further checking to ensure that the server
is authorized to provide the service it is requested to provide. The
client may need to make use of local policy information in making
this determination.
3.1.3.1. Comparison of DNS Names
If the reference identity is an internationalized domain name,
conforming implementations MUST convert it to the ASCII Compatible
Encoding (ACE) format as specified in Section 4 of RFC 3490
[IDNA2003] before comparison with subjectAltName values of type
dNSName. Specifically, conforming implementations MUST perform the
conversion operation specified in Section 4 of RFC 3490 as follows:
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o in step 1, the domain name SHALL be considered a "stored string";
o in step 3, set the flag called "UseSTD3ASCIIRules";
o in step 4, process each label with the "ToASCII" operation; and
o in step 5, change all label separators to U+002E (full stop).
After performing the "to-ASCII" conversion, the DNS labels and names
MUST be compared for equality according to the rules specified in
Section 3 of RFC3490.
The '*' (ASCII 42) wildcard character is allowed in subjectAltName
values of type dNSName, and then only as the left-most (least
significant) DNS label in that value. This wildcard matches any
left-most DNS label in the server name. That is, the subject
*.example.com matches the server names a.example.com and
b.example.com, but does not match example.com or a.b.example.com.
3.1.3.2. Comparison of IP Addresses
When the reference identity is an IP address, the identity MUST be
converted to the "network byte order" octet string representation
[IP] [IPv6]. For IP Version 4, as specified in RFC 791, the octet
string will contain exactly four octets. For IP Version 6, as
specified in RFC 2460, the octet string will contain exactly sixteen
octets. This octet string is then compared against subjectAltName
values of type iPAddress. A match occurs if the reference identity
octet string and value octet strings are identical.
3.1.3.3. Comparison of Other subjectName Types
Client implementations MAY support matching against subjectAltName
values of other types as described in other documents.
######
A.4. SMTP (2002/2007)
In 2002, [SMTP-TLS] specified the following text regarding server
identity verification in SMTP:
######
4.1 Processing After the STARTTLS Command
[...]
The decision of whether or not to believe the authenticity of the
other party in a TLS negotiation is a local matter. However, some
general rules for the decisions are:
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o A SMTP client would probably only want to authenticate an SMTP
server whose server certificate has a domain name that is the
domain name that the client thought it was connecting to.
[...]
######
In 2006, [SMTP-AUTH] specified the following text regarding server
identity verification in SMTP:
######
14. Additional Requirements When Using SASL PLAIN over TLS
[...]
After a successful [TLS] negotiation, the client MUST check its
understanding of the server hostname against the server's identity as
presented in the server Certificate message, in order to prevent man-
in-the-middle attacks. If the match fails, the client MUST NOT
attempt to authenticate using the SASL PLAIN mechanism. Matching is
performed according to the following rules:
The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use any
form of the server hostname derived from an insecure remote source
(e.g., insecure DNS lookup). CNAME canonicalization is not done.
If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
Matching is case-insensitive.
A "*" wildcard character MAY be used as the leftmost name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
######
A.5. XMPP (2004)
In 2004, [XMPP] specified the following text regarding server
identity verification in XMPP:
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######
14.2. Certificate Validation
When an XMPP peer communicates with another peer securely, it MUST
validate the peer's certificate. There are three possible cases:
Case #1: The peer contains an End Entity certificate which appears
to be certified by a certification path terminating in a trust
anchor (as described in Section 6.1 of [PKIX]).
Case #2: The peer certificate is certified by a Certificate
Authority not known to the validating peer.
Case #3: The peer certificate is self-signed.
In Case #1, the validating peer MUST do one of two things:
1. Verify the peer certificate according to the rules of [PKIX].
The certificate SHOULD then be checked against the expected
identity of the peer following the rules described in [HTTP-TLS],
except that a subjectAltName extension of type "xmpp" MUST be
used as the identity if present. If one of these checks fails,
user-oriented clients MUST either notify the user (clients MAY
give the user the opportunity to continue with the connection in
any case) or terminate the connection with a bad certificate
error. Automated clients SHOULD terminate the connection (with a
bad certificate error) and log the error to an appropriate audit
log. Automated clients MAY provide a configuration setting that
disables this check, but MUST provide a setting that enables it.
2. The peer SHOULD show the certificate to a user for approval,
including the entire certification path. The peer MUST cache the
certificate (or some non-forgeable representation such as a
hash). In future connections, the peer MUST verify that the same
certificate was presented and MUST notify the user if it has
changed.
In Case #2 and Case #3, implementations SHOULD act as in (2) above.
######
At the time of this writing, [XMPPBIS] refers to this document for
rules regarding server identity verification in XMPP.
A.6. NNTP (2006)
In 2006, [NNTP-TLS] specified the following text regarding server
identity verification in NNTP:
######
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5. Security Considerations
[...]
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the
connection (or the hostname specified in TLS "server_name"
extension [TLS]) as the value to compare against the server name
as expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is not
done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
o If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation or terminate the connection with a QUIT command and
indicate the server's identity is suspect.
Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [PKIX] for general certificate validation, but MAY supplement that
algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
######
A.7. NETCONF (2006/2009)
In 2006, [NETCONF-SSH] specified the following text regarding server
identity verification in NETCONF:
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######
6. Security Considerations
The identity of the server MUST be verified and authenticated by the
client according to local policy before password-based authentication
data or any configuration or state data is sent to or received from
the server. The identity of the client MUST also be verified and
authenticated by the server according to local policy to ensure that
the incoming client request is legitimate before any configuration or
state data is sent to or received from the client. Neither side
should establish a NETCONF over SSH connection with an unknown,
unexpected, or incorrect identity on the opposite side.
######
In 2009, [NETCONF-TLS] specified the following text regarding server
identity verification in NETCONF:
######
3.1. Server Identity
During the TLS negotiation, the client MUST carefully examine the
certificate presented by the server to determine if it meets the
client's expectations. Particularly, the client MUST check its
understanding of the server hostname against the server's identity as
presented in the server Certificate message, in order to prevent man-
in-the-middle attacks.
Matching is performed according to the rules below (following the
example of [NNTP-TLS]):
o The client MUST use the server hostname it used to open the
connection (or the hostname specified in the TLS "server_name"
extension [TLS]) as the value to compare against the server name
as expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is not
done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it MUST be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the leftmost name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
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o If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client MUST either ask for explicit user
confirmation or terminate the connection and indicate the server's
identity is suspect.
Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [PKIX] for general certificate validation, but MAY supplement that
algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted.
######
A.8. Syslog (2009)
In 2009, [SYSLOG-TLS] specified the following text regarding server
identity verification in Syslog:
######
5.2. Subject Name Authorization
Implementations MUST support certification path validation [PKIX].
In addition, they MUST support specifying the authorized peers using
locally configured host names and matching the name against the
certificate as follows.
o Implementations MUST support matching the locally configured host
name against a dNSName in the subjectAltName extension field and
SHOULD support checking the name against the common name portion
of the subject distinguished name.
o The '*' (ASCII 42) wildcard character is allowed in the dNSName of
the subjectAltName extension (and in common name, if used to store
the host name), but only as the left-most (least significant) DNS
label in that value. This wildcard matches any left-most DNS
label in the server name. That is, the subject *.example.com
matches the server names a.example.com and b.example.com, but does
not match example.com or a.b.example.com. Implementations MUST
support wildcards in certificates as specified above, but MAY
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provide a configuration option to disable them.
o Locally configured names MAY contain the wildcard character to
match a range of values. The types of wildcards supported MAY be
more flexible than those allowed in subject names, making it
possible to support various policies for different environments.
For example, a policy could allow for a trust-root-based
authorization where all credentials issued by a particular CA
trust root are authorized.
o If the locally configured name is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons, as
specified in Section 7 of [PKIX].
o Implementations MAY support matching a locally configured IP
address against an iPAddress stored in the subjectAltName
extension. In this case, the locally configured IP address is
converted to an octet string as specified in [PKIX], Section
4.2.1.6. A match occurs if this octet string is equal to the
value of iPAddress in the subjectAltName extension.
######
A.9. SIP (2010)
At the time of this writing, [SIP-CERTS] specified text regarding
server identity verification in the Session Initiation Protocol
(SIP). However, that specification has not yet been approved by the
IESG and text cannot be considered final.
The relevant text follows.
######
7.2. Comparing SIP Identities
When an implementation (either client or server) compares two values
as SIP domain identities:
Implementations MUST compare only the DNS name component of each
SIP domain identifier; an implementation MUST NOT use any scheme
or parameters in the comparison.
Implementations MUST compare the values as DNS names, which means
that the comparison is case insensitive as specified by
[DNS-CASE]. Implementations MUST handle Internationalized Domain
Names (IDNs) in accordance with Section 7.2 of [PKIX].
Implementations MUST match the values in their entirety:
Implementations MUST NOT match suffixes. For example,
"foo.example.com" does not match "example.com".
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Implemenations MUST NOT match any form of wildcard, such as a
leading "." or "*." with any other DNS label or sequence of
labels. For example, "*.example.com" matches only
"*.example.com" but not "foo.example.com". Similarly,
".example.com" matches only ".example.com", and does not match
"foo.example.com."
[HTTP-TLS] allows the dNSName component to contain a
wildcard; e.g., "DNS:*.example.com". [PKIX], while not
disallowing this explicitly, leaves the interpretation of
wildcards to the individual specification. [SIP] does not
provide any guidelines on the presence of wildcards in
certificates. Through the rule above, this document
prohibits such wildcards in certificates for SIP domains.
######
A.10. GIST (2010)
In 2010, [GIST] specified the following text regarding server
identity verification in the General Internet Signalling Transport:
######
5.7.3.1. Identity Checking in TLS
After TLS authentication, a node MUST check the identity presented by
the peer in order to avoid man-in-the-middle attacks, and verify that
the peer is authorised to take part in signalling at the GIST layer.
The authorisation check is carried out by comparing the presented
identity with each Authorised Peer Database (APD) entry in turn, as
discussed in Section 4.4.2. This section defines the identity
comparison algorithm for a single APD entry.
For TLS authentication with X.509 certificates, an identity from the
DNS namespace MUST be checked against each subjectAltName extension
of type dNSName present in the certificate. If no such extension is
present, then the identity MUST be compared to the (most specific)
Common Name in the Subject field of the certificate. When matching
DNS names against dNSName or Common Name fields, matching is case-
insensitive. Also, a "*" wildcard character MAY be used as the left-
most name component in the certificate or identity in the APD. For
example, *.example.com in the APD would match certificates for
a.example.com, foo.example.com, *.example.com, etc., but would not
match example.com. Similarly, a certificate for *.example.com would
be valid for APD identities of a.example.com, foo.example.com,
*.example.com, etc., but not example.com.
Additionally, a node MUST verify the binding between the identity of
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the peer to which it connects and the public key presented by that
peer. Nodes SHOULD implement the algorithm in Section 6 of [PKIX]
for general certificate validation, but MAY supplement that algorithm
with other validation methods that achieve equivalent levels of
verification (such as comparing the server certificate against a
local store of already-verified certificates and identity bindings).
For TLS authentication with pre-shared keys, the identity in the
psk_identity_hint (for the server identity, i.e. the Responding node)
or psk_identity (for the client identity, i.e. the Querying node)
MUST be compared to the identities in the APD.
######
Authors' Addresses
Peter Saint-Andre
Cisco Systems, Inc.
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA
Phone: +1-303-308-3282
Email: psaintan@cisco.com
Jeff Hodges
PayPal
2211 North First Street
San Jose, California 95131
US
Email: Jeff.Hodges@PayPal.com
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