Network Working Group P. Saint-Andre
Internet-Draft Cisco
Intended status: BCP J. Hodges
Expires: April 23, 2011 PayPal
October 20, 2010
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)
draft-saintandre-tls-server-id-check-10
Abstract
Many application technologies enable a secure connection between two
entities by means of Internet Public Key Infrastructure Using X.509
(PKIX) 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Applicability and Audience . . . . . . . . . . . . . . . . 5
1.3. Overview of Recommendations . . . . . . . . . . . . . . . 6
1.4. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4.1. In Scope . . . . . . . . . . . . . . . . . . . . . . . 7
1.4.2. Out of Scope . . . . . . . . . . . . . . . . . . . . . 7
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
1.6. Contributors . . . . . . . . . . . . . . . . . . . . . . . 12
1.7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 12
2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1. Naming Application Services . . . . . . . . . . . . . . . 13
2.2. DNS Domain Names . . . . . . . . . . . . . . . . . . . . . 14
2.3. Subject Naming in PKIX Certificates . . . . . . . . . . . 15
3. Representation of Server Identity . . . . . . . . . . . . . . 17
3.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18
4. Verification of Service Identity . . . . . . . . . . . . . . . 18
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2. Constructing a List of Reference Identifiers . . . . . . . 19
4.2.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 21
4.3. Seeking a Match . . . . . . . . . . . . . . . . . . . . . 21
4.4. Verifying a Domain Name . . . . . . . . . . . . . . . . . 21
4.4.1. Checking of Traditional Domain Names . . . . . . . . . 22
4.4.2. Checking of Internationalized Domain Names . . . . . . 22
4.4.3. Checking of Wildcard Labels . . . . . . . . . . . . . 22
4.4.4. Checking of Common Names . . . . . . . . . . . . . . . 23
4.5. Verifying an Application Type . . . . . . . . . . . . . . 23
4.5.1. SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 23
4.5.2. URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 24
4.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.6.1. Case #1: Match Found . . . . . . . . . . . . . . . . . 24
4.6.2. Case #2: No Match Found, Pinned Certificate . . . . . 24
4.6.3. Case #3: No Match Found, No Pinned Certificate . . . . 24
4.6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . . 24
5. Security Considerations . . . . . . . . . . . . . . . . . . . 25
5.1. Pinned Certificates . . . . . . . . . . . . . . . . . . . 25
5.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 25
5.3. Internationalized Domain Names . . . . . . . . . . . . . . 26
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
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7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Normative References . . . . . . . . . . . . . . . . . . . 26
7.2. Informative References . . . . . . . . . . . . . . . . . . 27
Appendix A. Prior Art . . . . . . . . . . . . . . . . . . . . . . 32
A.1. IMAP, POP3, and ACAP (1999) . . . . . . . . . . . . . . . 32
A.2. HTTP (2000) . . . . . . . . . . . . . . . . . . . . . . . 33
A.3. LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 34
A.4. SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 37
A.5. XMPP (2004) . . . . . . . . . . . . . . . . . . . . . . . 39
A.6. NNTP (2006) . . . . . . . . . . . . . . . . . . . . . . . 40
A.7. NETCONF (2006/2009) . . . . . . . . . . . . . . . . . . . 41
A.8. Syslog (2009) . . . . . . . . . . . . . . . . . . . . . . 42
A.9. SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 43
A.10. SNMP (2010) . . . . . . . . . . . . . . . . . . . . . . . 44
A.11. GIST (2010) . . . . . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46
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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 service 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 service using Transport Layer Security [TLS] or Datagram
Transport Layer Security [DTLS], it references some conception of the
server's identity while attempting to establish a secure connection
(e.g., "the website at example.com"). Likewise, during TLS
negotiation the server presents its conception of the service's
identity in the form of a public-key certificate that was issued by a
certification authority (CA) in the context of the Internet Public
Key Infrastructure using X.509 [PKIX]. Informally, we can think of
these identities as the client's "reference identity" and the
server's "presented identity" (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 it can be sure
that the certificate can legitimately be used to authenticate the
connection.
Many application technologies adhere to the pattern just outlined,
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 [XMPP-OLD]
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] and
[SYSLOG-DTLS]
o The Session Initiation Protocol [SIP], for which see also
[SIP-CERTS]
o The Simple Network Management Protocol [SNMP], for which see also
[SNMP-TLS]
o The General Internet Signalling Transport [GIST]
Application protocols have traditionally specified their own rules
for representing and verifying application service identity.
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 application service identity in certificates intended for
use in applications employing TLS.
1.2. Applicability and Audience
This document does not supersede the rules for certificate issuance
or validation provided in [PKIX], which governs any certificate-
related topic on which this document is silent (e.g., certificate
syntax, certificate extensions such as name constraints and extended
key usage, and handling of certification paths). Specifically, in
order to ensure proper authentication, application clients need to
verify the entire certification path, because this document addresses
only name forms in the leaf server certificate, not any name forms in
the chain of certificates used to validate the server certificate.
This document also does not supersede the rules for verifying service
identity provided in specifications for existing application
protocols, such as those mentioned under Appendix A. However, the
best current practices described here can be referenced by future
specifications, including updates to specifications for existing
application protocols if the relevant technology communities agree to
do so. It is also expected that this document will be updated or
obsoleted in the future as best practices for issuance and
verification of PKIX certificates continue to evolve through more
widespread implementation and deployment of TLS-protected application
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services over the Internet.
The primary audience for this document consists of application
protocol designers, who might reference this document instead of
defining their own rules for the representation and verification of
application service identity. Secondarily, the audience consists of
certification authorities, client developers, service providers, and
other members of technology communities that might re-use or
"profile" the recommendations in this document when defining
certificate issuance policies, writing software algorithms for
identity matching, generating certificate signing requests, etc.
1.3. Overview of Recommendations
To orient the reader, this section provides an informational overview
of the recommendations contained in this document.
For the primary audience of application protocol designers, based on
best current practices this document provides recommended procedures
for representation and verification of application service identity
within PKIX certificates used in the context of TLS.
For the secondary audiences, in essence this document encourages
certification authorities, application service providers, and
application client developers to coalesce on the following best
current practices:
o Move away from including and checking strings that look like
domain names in the subject's Common Name.
o Move toward including and checking DNS domain names via the
subjectAlternativeName extension designed for that purpose:
dNSName.
o Move toward including and checking even more specific
subjectAlternativeName extensions where appropriate for the using
protocol (e.g., uniformResourceIdentifier and the otherName form
SRVName).
o Move away from the issuance of so-called wildcard certificates
(e.g., a certificate containing an identifier for
"*.example.com").
These suggestions are not entirely consistent with all practices that
are currently followed by certification authorities, client
developers, and service providers. However, they reflect the best
aspects of current practices and are expected to become more widely
adopted in the coming years.
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1.4. Scope
1.4.1. In Scope
This document applies only to service identities associated with
fully-qualified DNS domain names, only to TLS and DTLS (or the older
Secure Sockets Layer (SSL) technology), 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).
1.4.2. 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.
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 (less than 1%) of issued
certificates. 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 focus here on application service identities, not
specific resources located at such services, e.g., a specific web
page that can be accessed at a particular Uniform Resource
Identifier [URI] whose authority component is the DNS domain name
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of the application service. We also do not address identifiers
derived from Naming Authority Pointer (NAPTR) DNS resource records
[NAPTR] and related technologies such as [S-NAPTR], since such
identifiers cannot be validated in a trusted manner in the absence
of [DNSSEC].
Finally, we do not discuss attributes unrelated to DNS domain
names, such as those defined in [X.520] and other such
specifications (e.g., organizational attributes, geographical
attributes, company logos, and the like).
o Security protocols other than [TLS], [DTLS], or the older Secure
Sockets Layer (SSL) technology.
Although other secure, lower-layer protocols exist and even employ
PKIX certificates at times (e.g., IPsec [IPSEC]), their use cases
can differ from those of TLS-based and 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 [OPENPGP], or use self-signed
certificates, or are deployed on networks that 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 differs essentially from that of X.509, 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.
Furthermore, this document also does not address various
certification authority policies, such as:
o What types or "classes" of certificates to issue and whether to
apply different policies for them (e.g., allow the wildcard
character in certificates issued to individuals who have provided
proof of identity but do not allow the wildcard character in
"Extended Validation" certificates [EV-CERTS]).
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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 SRV-IDs or
URI-IDs as defined in the body of this specification).
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]).
1.5. Terminology
Because many concepts related to "identity" are often too vague to be
actionable in application protocols, we define a set of more concrete
terms for use in this specification.
application service: A service on the Internet that enables
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].
automated client: A software agent or device that is not directly
controlled by a human user.
delegated domain: A domain name or host name that is explicitly
configured for connecting to the source domain, by either (a) the
human user controlling an interactive client or (b) a trusted
administrator. In case (a), one example of delegation is an
account setup that specifies the domain name of a particular host
to be used for retrieving information or connecting to a network
(which might be different from the server portion of the user's
account name). In case (b), one example of delegation is an
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admin-configured host-to-address/address-to-host lookup table.
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; 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]
interactive client: A software agent or device that is directly
controlled by a human user. (Other specifications related to
security and application protocols, such as [WSC-UI], often refer
to this entity as a "user agent"; however that term is neither
entirely accurate nor consistent with the terminology of common
application protocols such as [HTTP].)
pinning: The act of establishing a cached name association between
the application service's certificate and one of the client's
reference identifiers, despite the fact that none of the presented
identifiers matches the given reference identifier. Pinning is
accomplished by allowing a human user to positively accept the
mismatch during an attempt to connect to the application service.
Once a cached name association is established, the certificate is
said to be pinned to the reference identifier and in future
connection attempts the client simply verifies that the service's
presented certificate matches the pinned certificate, as described
under Section 4.6.2. (A similar definition of "pinning" is
provided in [WSC-UI].)
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PKIX: PKIX is a short name for the Internet Public Key
Infrastructure using X.509 defined in RFC 5280 [PKIX], which
comprises a profile of the X.509v3 certificate specifications and
X.509v2 certificate revocation list (CRL) specifications for use
in the Internet.
PKIX-based system: A software implementation or deployed service
that makes use of X.509v3 certificates and X.509v2 certificate
revocation lists (CRLs).
PKIX certificate: An X.509v3 certificate generated and employed in
the context of 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.
reference identifier: An identifier, constructed from a source
domain and optinoally a service type, used by the client for
matching purposes when examining presented identifiers.
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
(e.g., "www.example.com"), typically input by a human user,
configured into a client, or provided by reference such as in a
hyperlink. The combination of a source domain and, optionally, a
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 certificate extension
[PKIX] enabling identifiers of various types to be bound to the
certificate subject -- in addition to, or in place of, identifiers
that may be embedded in or provided as a certificate's subject
field.
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subject field: The subject field of a PKIX certificate identifies
the entity associated with the public key stored in the subject
public key field (see Section 4.1.2.6 of [PKIX]).
subject name: In an overall sense, a subject's name(s) can be
represented by or in the subject field, the subjectAltName
extension, or both (see [PKIX] for details). More specifically,
the term often refers to the composite 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]).
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.
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 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
[KEYWORDS].
1.6. Contributors
The following individuals made important contributions to the text of
this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.
1.7. Acknowledgements
The editors and contributors wish to thank the following individuals
for their feedback and suggestions: Bernard Aboba, Richard Barnes,
Uri Blumenthal, Nelson Bolyard, Kaspar Brand, Anthony Bryan, Ben
Campbell, Scott Cantor, Wan-Teh Chang, Bil Corry, Dave Cridland, Dave
Crocker, Cyrus Daboo, Charles Gardiner, Philip Guenther, Phillip
Hallam-Baker, Bruno Harbulot, Wes Hardaker, David Harrington, Paul
Hoffman, Love Hornquist Astrand, Henry Hotz, Russ Housley, Jeffrey
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Hutzelman, Simon Josefsson, Geoff Keating, John Klensin, Scott
Lawrence, Matt McCutchen, Alexey Melnikov, Eddy Nigg, Ludwig Nussel,
Joe Orton, Tom Petch, Yngve N. Pettersen, Tim Polk, Robert Relyea,
Eric Rescorla, Pete Resnick, Martin Rex, Joe Salowey, Stefan
Santesson, Jim Schaad, Rob Stradling, Michael Stroeder, Andrew
Sullivan, Peter Sylvester, Martin Thomson, Paul Tiemann, Sean Turner,
Nicolas Williams, Dan Wing, Dan Winship, and Stefan Winter.
2. Names
This section discusses naming of application services on the
Internet, followed by a brief tutorial about subject naming in PKIX.
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 most 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 most for certificate issuance.
Therefore:
o A CN-ID is direct (provided by a user) and unrestricted (can be
used for any application).
o A DNS-ID is direct (provided by a user) and unrestricted (can be
used for any application).
o An SRV-ID can be either direct (provided by a user) or more
typically indirect (resolved by a client), and is restricted (can
be used for only a single application).
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o A URI-ID is direct (provided by a user) and restricted (can be
used for only a single application).
We summarize this taxonomy in the following table.
+-----------+-----------+---------------+
| | Direct | Restricted |
+-----------+-----------+---------------+
| CN-ID | Yes | No |
+-----------+-----------+---------------+
| DNS-ID | Yes | No |
+-----------+-----------+---------------+
| SRV-ID | Either | 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. Ideally, protocol specifications that
reference this document will specify which identifiers are mandatory-
to-implement by servers and clients, which identifiers ought to be
supported by certificate issuers, and which identifiers ought to be
requested by application service providers. 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 (one such community, the CA/Browser Forum, has
codified such a baseline for "Extended Validation Certificates" in
[EV-CERTS]).
2.2. DNS Domain Names
For the purposes of this specification, the name of an application
service is a DNS domain name that conforms to one of the following
forms:
1. A "traditional domain name", i.e., a fully-qualified domain name
or "FQDN" (see [DNS-CONCEPTS]) all of whose labels are "LDH
labels" as defined in [IDNA-DEFS]. Informally, such labels are
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constrained to [US-ASCII] letters, digits, and the hyphen, with
the hyphen prohibited in the first character position.
Additional qualifications apply (please refer to the above-
referenced specifications for details) but they are not germane
to this specification.
2. An "internationalized domain name", i.e., a DNS domain name that
conforms to the overall form of a domain name (dot-separated
labels) but that can include Unicode code points outside the
traditional US-ASCII range or, more precisely, either U-labels or
A-labels as described in [IDNA-DEFS] and the associated
documents.
2.3. 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 that 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 object or
alias 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 the Relative Distinguished Names of its
superior entries in the tree (all the way down to the root of the
DIT) with one or more specially-nominated attributes of the entry
itself (which together comprise the Relative Distinguished Name (RDN)
of the entry, so-called because it is relative to the Distinguished
Names of the superior entries in the tree). The entry closest to the
root is sometimes referred to as the "most significant" entry and the
entry farthest from the root is sometimes referred to as the "least
significant" entry. 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.
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
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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
identifier types:
o DNS-ID
o SRV-ID
o URI-ID
Existing certificates often use a CN-ID in the subject field to
represent a fully-qualified DNS domain name; for example, consider
the following subject name, 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
In general, this specification recommends and prefers use of
subjectAltName entries (DNS-ID, SRV-ID, URI-ID, etc.) over use of the
subject field (CN-ID) where possible, as more completely described in
the following sections. However, profiles of this specification can
legitimately encourage continued support for the CN-ID identifier
type if they have good reasons to do so, such as backward
compatibility with deployed infrastructure.
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 the certificate issuer, subject field,
and subjectAltName extension, 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 typically preserved in the DN string
representations, although the two most prevalent DN string
representations differ in employing left-to-right vs. right-to-
left ordering. However, because 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 (and the order of attribute-type-and-value pairs can
differ in the RDN string representations or display orders
provided by various implementations). 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-
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most", "first" vs. "last", or "most significant" vs. "least
significant" (see for example [LDAP-DN]). To reduce confusion, in
this specification we avoid such terms and instead use the terms
provided under Section 1.5; in particular, we do not use the term
"(most specific) Common Name field in the Subject field" from
[HTTP-TLS] and instead state that a CN-ID is a Relative
Distinguished Name (RDN) in the certificate subject that contains
one and only one attribute-type-and-value pair of type Common Name
(thus removing the possibility that an RDN might contain multiple
AVAs of type CN, one of which would be considered "most
specific").
3. Representation of Server Identity
3.1. Rules
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" 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"; furthermore, multiple instances of
the "SRV-ID" identifier type are allowed.
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; 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; furthermore, multiple instances of
the "URI-ID" identifier type are allowed.
4. The certificate MAY include other application-specific
identifiers for types that were defined before publication of
[SRVNAME] (e.g., XmppAddr for [XMPP]) or for which service names
or URI schemes do not exist; however, such application-specific
identifiers are not applicable to all application technologies
and therefore are out of scope for this specification.
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5. Even though many deployed clients still check for the CN-ID
within the certificate subject field, the certificate SHOULD NOT
represent the server's fully-qualified DNS domain name in a CN-ID
unless a profile of this specification encourages continued
support for the CN-ID identifier type.
6. The certificate MAY contain more than one DNS-ID, but SHOULD NOT
contain more than one CN-ID.
7. The fully-qualified DNS domain name portion of a presented
identifier SHOULD NOT contain the wildcard character '*', whether
as the complete left-most label within the identifier (following
the definition of "label" from [DNS], e.g., "*.example.com") or
as a fragment thereof (e.g., *oo.example.com, f*o.example.com, or
foo*.example.com). For details, see Section 5.2.
3.2. Examples
A certificate for the website at "www.example.com" might include only
a DNS-ID of "www.example.com" (and, strictly as a fallback for
existing client software, a CN-ID of "www.example.com").
A certificate for the IMAP-accessible email server at
"mail.example.com" might include SRV-IDs of "_imap.mail.example.com"
and "_imaps.mail.example.com" (see [EMAIL-SRV]) and a DNS-ID of
"mail.example.com".
A certificate for the XMPP-compatible instant messaging server at
"im.example.com" might include SRV-IDs of "_xmpp-
client.im.example.com" and "_xmpp-server.im.example.com" (see
[XMPP]), a DNS-ID of "im.example.com", and an XMPP-specific
"XmppAddr" of "im.example.com" (see [XMPP]).
4. Verification of Service Identity
4.1. Overview
At a high level, the client verifies the application service's
identity by performing the actions listed below (which are defined in
the following subsections of this document):
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.
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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 matches the source domain of the
identifiers and, optionally, their service type.
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 application service identity presented in a
certificate, and therefore methods for doing so (e.g., consulting
local policy information) are out of scope for this document.
4.2. Constructing a List of Reference Identifiers
4.2.1. 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 HTTP URL for a web site), configured account information
(e.g., the domain name of a particular host or URI used for
retrieving information or connecting to a network, which might be
different from the server portion of the user's account name), 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 a service type.
The client might need to extract the source domain and 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 (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 extraction is performed in a
manner that is not subject to subversion by network attackers (e.g.,
pulling the data from a delegated domain that 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 that provides both mutual
authentication and integrity checking).
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For an interactive client, it is strongly encouraged that each
reference identifier SHOULD be based on the source domain provided by
the user and SHOULD NOT be based on a derived domain (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 and a presented identifier enables the client to be sure
that the certificate can legitimately be used to secure the
connection. There is only one scenario in which it is acceptable for
an interactive client to override the recommendation in this rule and
therefore connect to a domain name other than the source domain
(e.g., to a derived domain or some other domain name that is
presented by the server in a certificate): because a human user has
"pinned" the application service's certificate to the alternative
domain name as further discussed under Section 4.6.4 and Section 5.1.
In this case, the inputs used by the client to construct its list of
reference identifiers might include more than one fully-qualified DNS
domain name, i.e., both (a) the source domain and (b) the alternative
domain contained in the pinned certificate.
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
derived 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.
o The list MAY include a CN-ID, mainly for the sake of backward
compatibility with existing infrastructure.
The client does not need to construct the foregoing identifiers in
the actual formats found in a certificate (e.g., as ASN.1 types), it
only needs to construct 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
those of type Common Name and MUST NOT check for RDNs other than
those of type Common Name in the presented identifiers.
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4.2.2. Examples
A web browser that is connecting via HTTP to the website at
"www.example.com" might have two reference identifiers: a DNS-ID of
"www.example.com" and, as a fallback, a CN-ID of "www.example.com".
A mail user agent that is connecting via IMAP to the email service at
"mail.example.com" might have two reference identifiers: an SRV-ID of
"_imaps.mail.example.com" (see [EMAIL-SRV]) and a DNS-ID of
"mail.example.com".
An instant messaging (IM) client that is connecting via XMPP to the
IM service at "im.example.com" might have three reference
identifiers: an SRV-ID of "_xmpp.im.example.com", a DNS-ID of
"im.example.com", and an XMPP-specific "XmppAddr" of "im.example.com"
(see [XMPP]).
4.3. Seeking a Match
Once the client has constructed its list of reference identifiers and
has received the server's presented identifiers in the form of a PKIX
certificate, 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.
Implementation Note: A client might be configured to perform
multiple searches, i.e., to match more than one reference
identifier; although such behavior is not forbidden by this
document, rules for matching multiple reference identifiers are a
matter for implementation or future specification.
Security Note: A client MUST NOT seek a match for a reference
identifier of CN-ID if the presented identifiers include a DNS-ID,
SRV-ID, URI-ID, or any application-specific identifier types
supported by the client.
Detailed comparison rules for finding a match are provided in the
following sections.
4.4. Verifying a Domain Name
The client MUST match the DNS domain name portion of a reference
identifier according to the following rules (and SHOULD also check
the service type as described under Section 4.5). The rules differ
depending on whether the domain to be checked is a "traditional
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domain name" or an "internationalized domain name" (as defined under
Section 2.2). Furthermore, in case the client supports presented
identifiers that contain the wildcard character '*', we define a
supplemental rule for so-called "wildcard domains". We also specify
the circumstances under which it is acceptable to check the "CN-ID"
identifier type.
4.4.1. Checking of Traditional Domain Names
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 is performed by comparing the set of
domain name 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, except as
supplemented by the rule about checking of wildcard labels
(Section 4.4.3).
4.4.2. Checking of Internationalized Domain Names
If the DNS domain name portion of a reference identifier is an
internationalized domain name, then an implementation MUST convert
every label in the domain name to an A-label (as described in
[IDNA-DEFS]) before checking the domain name. 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
(Section 4.4.3).
4.4.3. Checking of Wildcard Labels
A client employing this specification's rules MAY match the reference
identifier against a presented identifier whose DNS domain name
portion contains the wildcard character '*' as part or all of a label
(following the definition of "label" from [DNS]). For information
regarding the security characteristics of wildcard certificates, see
Section 5.2.
If a client matches the reference identifier against a presented
identifier whose DNS domain name portion contains the wildcard
character '*', the following rules apply:
1. The client SHOULD NOT attempt to match a presented identifier in
which the wildcard character comprises a label other than the
left-most label (e.g., do not match bar.*.example.net).
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2. If the wildcard character is the only character of the left-most
label in the presented identifier, the client SHOULD NOT compare
against anything but the left-most label of the reference
identifier (e.g., *.example.com would match foo.example.com but
not bar.foo.example.com or example.com).
3. The client MAY match a presented identifier in which the wildcard
character is not the only character of the label (e.g.,
baz*.example.net and *baz.example.net and b*z.example.net would
be taken to match baz1.example.net and foobaz.example.net and
buzz.example.net, respectively).
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 a DNS-ID, SRV-ID,
URI-ID, or any application-specific identifier types supported by the
client.
Therefore, if and only if the presented identifiers do not include a
DNS-ID, SRV-ID, URI-ID, or any application-specific identifier types
supported by the client, then the client MAY as a last resort 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 DNS domain name portion of an identifier of
type DNS-ID, SRV-ID, or URI-ID, as described under Section 4.4.1,
Section 4.4.2, and Section 4.4.3.
4.5. Verifying an Application Type
When checking identifiers of type SRV-ID and URI-ID, 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 and in SRV-IDs (per [SRVNAME]).
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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 application service.
4.6.2. Case #2: No Match Found, Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers but the client has previously pinned the
application service's certificate to one of the reference identifiers
in the list it constructed for this connection attempt (as "pinning"
is explained under Section 1.5), and the presented certificate
matches the pinned certificate, then the service identity check
succeeds.
4.6.3. Case #3: No Match Found, No Pinned Certificate
If the client does not find a presented identifier matching any of
the reference identifiers and the client has not previously pinned
the certificate to one of the reference identifiers in the list it
constructed for this connection attempt, then the client MUST proceed
as described under Section 4.6.4.
4.6.4. Fallback
If the client is an interactive client that is directly controlled by
a human user, then it SHOULD inform the user of the identity mismatch
and automatically terminate the connection with a bad certificate
error; this behavior is preferable because it prevents users from
inadvertently bypassing security protections in hostile situations.
Security Note: Some interactive clients give advanced users the
option of proceeding with acceptance despite the identity
mismatch, thereby "pinning" the certificate to one of the
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reference identifiers in the list constructed by the client for
this connection attempt. Although this behavior can be
appropriate in certain specialized circumstances, in general it
ought to be exposed only to advanced users. Even then it needs to
be handled with extreme caution, for example by first encouraging
even an advanced user to terminate the connection and, if the
advanced user chooses to proceed anyway, by forcing the user to
view the entire certification path and only then allowing the user
to pin the certificate (on a temporary or permanent basis, at the
user's option).
Otherwise, if the client is an automated application not directly
controlled by a human user, then it SHOULD terminate the connection
with a bad certificate error and log the error appropriately. An
automated application MAY provide a configuration setting that
disables this behavior, but MUST enable the behavior by default.
5. Security Considerations
5.1. Pinned Certificates
As defined under Section 1.5, a certificate is said to be "pinned" to
a DNS domain name when a user has explicitly chosen to associate a
service's certificate with the that domain name despite the fact that
the certificate contains some other DNS domain name (e.g., the user
has explicitly approved "apps.example.net" as a domain associated
with a source domain of "example.com"). The cached name association
MUST take account of both the certificate presented and the context
in which it was accepted or configured (where the "context" includes
the chain of certificates from the presented certificate to the trust
anchor, the source domain, the service type, the service's derived
domain and port number, and other relevant information such as the
subject field's Common Name, Organization, and Country).
5.2. Wildcard Certificates
This document states that the wildcard character '*' SHOULD NOT be
included in presented identifiers but MAY be checked by application
clients (mainly for the sake of backward compatibility with existing
infrastructure); as a result, the rules provided in this document are
more restrictive than the rules for many existing application
technologies (see Appendix A). Several security considerations
justify tightening the rules:
o The inclusion of 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 "?";
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for discussion, see for example [Defeating-SSL] (beginning at
slide 91).
o The ability to obtain a certificate containing the wildcard
character might broaden the range of applications services that an
attacker could forge, thus increasing (a) the chances that
attackers would attempt to steal credentials needed for obtaining
certificates and (b) the potential damage that would result from a
successful attack.
o Specifications for existing application technologies are not clear
about whether the wildcard character is allowed only as the
complete left-most label (e.g., *.example.com), some fragment of
the left-most label (e.g., foo*.example.com, f*o.example.com, or
*oo.example.com), or even all or part of a label other than the
left-most label (e.g., www.*.example.com or www.foo*.example.com);
nor are they clear about whether a presented identifier can
include more than one instance of the wildcard character (e.g.,
f*b*r.example.com or *.*.example.com). These ambiguities might
introduce exploitable differences in identity checking behavior
among client implementations and necessitate overly complex and
inefficient identity checking algorithms.
Notwithstanding the foregoing security considerations, profiles of
this specification can legitimately encourage continued support for
the wildcard character if they have good reasons to do so, such as
backward compatibility with deployed infrastructure.
5.3. Internationalized Domain Names
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].
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]
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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.
[IDNA-DEFS]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 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.
[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.
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[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[EMAIL-SRV]
Daboo, C., "Use of SRV Records for Locating Email
Submission/Access services", draft-daboo-srv-email-05
(work in progress), May 2010.
[EV-CERTS]
CA/Browser Forum, "Guidelines For The Issuance And
Management Of Extended Validation Certificates",
October 2009,
<http://www.cabforum.org/Guidelines_v1_2.pdf>.
[GIST] Schulzrinne, H. and R. Hancock, "GIST: General Internet
Signalling Transport", RFC 5971, October 2010.
[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.
[IDNA2003]
Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[IP] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[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.
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Internet-Draft Service Identity October 2010
[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.
[NAPTR] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, October 2002.
[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.
[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.
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[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.
[S-NAPTR] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, January 2005.
[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.
[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.
[SNMP] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[SNMP-TLS]
Hardaker, W., "Transport Layer Security (TLS) Transport
Model for the Simple Network Management Protocol (SNMP)",
RFC 5953, August 2010.
[SYSLOG] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[SYSLOG-DTLS]
Salowey, J., Petch, T., Gerhards, R., and H. Feng,
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"Datagram Transport Layer Security (DTLS) Transport
Mapping for Syslog", RFC 6012, October 2010.
[SYSLOG-TLS]
Miao, F., Ma, Y., and J. Salowey, "Transport Layer
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
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Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690, ISO Standard 8825-1, August 2008.
[XMPP] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-17 (work
in progress), October 2010.
[XMPP-OLD]
Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
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 application service 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
application service 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.
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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 and indicate the server's
identity is suspect.
######
A.2. HTTP (2000)
In 2000, [HTTP-TLS] specified the following text regarding
application service 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
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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.
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
application service 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
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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.
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
application service 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
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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
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 subject 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
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dNSName. Specifically, conforming implementations MUST perform the
conversion operation specified in Section 4 of RFC 3490 as follows:
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
application service identity verification in SMTP:
######
4.1 Processing After the STARTTLS Command
[...]
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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:
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
application service 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.
######
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A.5. XMPP (2004)
In 2004, [XMPP-OLD] specified the following text regarding
application service identity verification in XMPP:
######
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, [XMPP] refers to this document for rules
regarding application service identity verification in XMPP.
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A.6. NNTP (2006)
In 2006, [NNTP-TLS] specified the following text regarding
application service identity verification in NNTP:
######
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).
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######
A.7. NETCONF (2006/2009)
In 2006, [NETCONF-SSH] specified the following text regarding
application service identity verification in NETCONF:
######
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
application service 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.
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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.
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
application service 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.
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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
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)
In 2010, [SIP-CERTS] specified the following text regarding
application service identity verification in SIP:
######
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].
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Implementations MUST match the values in their entirety:
Implementations MUST NOT match suffixes. For example,
"foo.example.com" does not match "example.com".
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. SNMP (2010)
In 2010, [SNMP-TLS] specified the following text regarding
application service identity verification in SNMP:
######
If the server's presented certificate has passed certification path
validation [PKIX] to a configured trust anchor, and an active row
exists with a zero-length snmpTlstmAddrServerFingerprint value, then
the snmpTlstmAddrServerIdentity column contains the expected host
name. This expected host name is then compared against the server's
certificate as follows:
o Implementations MUST support matching the expected host name
against a dNSName in the subjectAltName extension field and MAY
support checking the name against the CommonName portion of the
subject distinguished name.
o The '*' (ASCII 0x2a) 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 provide a configuration option to disable
them.
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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].
If the expected host name fails these conditions then the connection
MUST be closed.
######
A.11. GIST (2010)
In 2010, [GIST] specified the following text regarding application
service 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
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
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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
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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