Network Working Group P. Saint-Andre, Ed.
Internet-Draft Cisco
Intended status: Standards Track J. Hodges, Ed.
Expires: September 9, 2010 PayPal
March 8, 2010
Representation and Verification of Application Server Identity in
Certificates Used with Transport Layer Security (TLS)
draft-saintandre-tls-server-id-check-03
Abstract
Many application technologies enable a secure connection between two
entities using certificates in the context of Transport Layer
Security (TLS). This document specifies procedures for representing
and verifying the identity of application servers in such
interactions.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Contributors . . . . . . . . . . . . . . . . . . . . . . . 4
1.5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 4
2. Architectural Assumptions . . . . . . . . . . . . . . . . . . 5
3. Representation of Server Identity . . . . . . . . . . . . . . 6
4. Verification of Server Identity . . . . . . . . . . . . . . . 7
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Comparison Rules . . . . . . . . . . . . . . . . . . . . . 8
4.2.1. Traditional Domain Names . . . . . . . . . . . . . . . 9
4.2.2. Internationalized Domain Names . . . . . . . . . . . . 9
4.2.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . 9
4.2.4. Common Names . . . . . . . . . . . . . . . . . . . . . 10
4.2.5. Relative Distinguished Names . . . . . . . . . . . . . 10
4.3. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Prior Art . . . . . . . . . . . . . . . . . . . . . . 15
A.1. IMAP, POP3, and ACAP (1999) . . . . . . . . . . . . . . . 15
A.2. HTTP (2000) . . . . . . . . . . . . . . . . . . . . . . . 16
A.3. LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 17
A.4. SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 20
A.5. XMPP (2004) . . . . . . . . . . . . . . . . . . . . . . . 21
A.6. NNTP (2006) . . . . . . . . . . . . . . . . . . . . . . . 22
A.7. NETCONF (2006/2009) . . . . . . . . . . . . . . . . . . . 23
A.8. Syslog (2009) . . . . . . . . . . . . . . . . . . . . . . 25
A.9. SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
1.1. Motivation
When a client wishes to establish a secure communication channel with
an application server (e.g., "the HTTP server for example.com"), at a
minimum the client needs to verify the identity of the server in
order to prevent certain passive and active attacks against the
connection. To establish secure connections, many application
technologies use Transport Layer Security [TLS] with certificates
issued by certification authorities that are part of the Internet
X.509 Public Key Infrastructure (PKI) as described in [X509]; such
application technologies include but are not limited to the
following:
o The Hypertext Transfer Protocol [HTTP], for which see also
[HTTP-TLS]
o The Internet Message Access Protocol [IMAP] and the Post Office
Protocol [POP3], for which see also [USINGTLS]
o The Lightweight Directory Access Protocol [LDAP], for which see
also [LDAP-AUTH] and its predecessor [LDAP-TLS]
o The NETCONF Configuration Protocol [NETCONF], for which see also
[NETCONF-SSH] and [NETCONF-TLS]
o The Network News Transfer Protocol [NNTP], for which see also
[NNTP-TLS]
o The Session Initiation Protocol [SIP], for which see also
[SIP-CERTS]
o The Simple Mail Transfer Protocol [SMTP], for which see also
[SMTP-AUTH] and [SMTP-TLS]
o The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]
o The Extensible Messaging and Presence Protocol [XMPP], for which
see also [XMPPBIS]
Different application protocols have traditionally specified their
own rules for representing and verifying server identities.
Unfortunately, this divergence of approaches has caused some
confusion among certification authorities, protocol designers, and
application developers.
Futhermore, currently the vast majority of deployed application
servers use domain names in their certificates (typically via a
subjectAltName extension of dNSName or a subjectName component of
Common Name). Ideally, service operators would use application
service identities in their certificates (such as an SRVName
[SRVNAME], a URI, or an application-specific name form), since this
would reduce the possibility of attacks against unrelated services at
domain names that provide many different application services.
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To codify secure authentication practices, this document specifies
recommended procedures for representing and verifying server
identities in certificates intended for use in applications that
employ TLS.
1.2. Scope
This document applies only to server identities associated with
domain names, only to TLS, and only to the PKI. Similar
considerations might apply to client identities (e.g., for mutual
authentication), to identities other than domain names (e.g., IP
addresses), to security protocols other than TLS (e.g., [IPSEC] and
[DTLS]), and to keys or certificates created outside the context of
the PKI (e.g., where a dependency on Certificate Revocation Lists or
the Online Certificate Status Protocol might introduce unacceptable
latency or denial of service attacks). Although those scenarios
might be able to re-use some aspects of the representation and
verification rules provided here, they are outside the scope of this
document and need to be addressed by separate specifications.
1.3. Terminology
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",
"certificate", "credential", "identity", "self-signed certificate",
"trust", "trust anchor", "trust chain", "validate", and "verify".
The following capitalized keywords are to be interpreted as described
in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
"SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
"OPTIONAL".
1.4. Contributors
The following individuals made significant contributions to the text
of this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.
1.5. Acknowledgements
The editors and contributors wish to thank the following individuals
for their feedback and suggestions: Dave Crocker, Cyrus Daboo, Philip
Guenther, David Harrington, Paul Hoffman, Scott Lawrence, Alexey
Melnikov, Tom Petch, Pete Resnick, Joe Salowey, and Dan Wing.
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2. Architectural Assumptions
Internet applications often use a client-server architecture in which
a client connects to a server in order to retrieve or upload data,
access a broader network of services, or communicate with other
entities on the network. From the security perspective, a client
might be a user agent controlled by a human, an automated process
such as a bot, or a peer server. We assume that an application
server hosts information, enables a provisioned account to perform
authorized services, or provides network access on behalf of a
particular organization or service that is canonically identified by
a domain name (not, e.g., an IP address). The specific application
provided (e.g., a web site or an instant messaging system) might
further restrict the identity type that is represented or verified in
a TLS interaction; such an identity type is often informal (e.g.,
most people expect a service that is provided on port 443 to be a web
server) but can be specified more formally through the use of DNS SRV
records [DNS-SRV], a Uniform Resource Identifier [URI] represented by
a subjectAltName of uniformResourceIdentifier, the SRVName
certificate extension defined in [SRVNAME], or other certificate
extensions that have been defined for particular identity types
(e.g., the XmppAddr extension for use in [XMPP]). The certificate
presented by an application server might contain one identity or it
might contain multiple identities of different types (e.g., one
identity might be a simple dNSName and another might be an SRVName
that restricts the certificate to use in the context of the specified
service).
When a client connects to an application server, it has some
conception of either the server's identity (e.g., "an XMPP server for
example.com") or of the server's location (e.g., "the SMTP server at
mail.example.com"). The client expects at least one of the server's
presented identities to match this reference identity. It is
important to note that the reference identity is provided by a human
user or configured into the client application (e.g., in the domain
part of an "Application Unique String" as described in [SIP-LOC]),
and is not an identity that is derived from the reference identity in
an automated fashion (e.g., an IP address discovered through DNS
resolution of the reference identity). Only a match between the
reference identity and a presented identity enables the client to be
sure that the certificate can legitimately be used to secure the
connection. However, a user-oriented client MAY provide a
configuration setting that enables a human user to explicitly specify
a particular hostname to be checked for connection purposes, thus
explicitly overriding matching rules.
To summarize, we define the following terms to assist in
understanding the process of representing and verifying server
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identity:
identity set: The set of identities that are presented by a server
to a client (in the form of the server's X.509 certificate) when
the client attempts to establish a secure connection with the
server.
identity type: The "natural kind" of identity to which a presented
identity or reference identity belongs. For example, the
reference identity might be a domain name, an IPv4 or IPv6
address, or a particular service in the sense of [DNS-SRV] or
[SRVNAME]. This specification does not provide a complete
taxonomy of identity types but assumes that an application server
is identified by a domain name that could be represented in the
form of several identity types (e.g., a dNSName and an SRVName).
presented identity: A single member of the identity set.
reference identity: The client's conception of the server's identity
before it attempts to establish a secure connection with the
server, i.e. the identity that the client expects the server to
present and to which the client makes reference when attempting to
verify the server's identity. The reference identity MUST be
provided by the human user controlling the client (if any), e.g.
when specifying the server portion of the user's account name on
the server or when explicitly configuring the client to connect to
a particular host. The reference identity MAY be reflected in the
TLS "server_name" extension as specified in [TLS].
3. Representation of Server Identity
The following rules apply to the representation of application server
identities in certificates issued by certification authorities, i.e.,
certificates that are to used for securing connections to application
servers such as web sites, email services, and instant messaging
services.
1. The certification authority MUST issue the certificate based on
the domain name at which the server will provide the relevant
service (not an IP address or host name for a specific machine).
2. An identity MAY contain one instance of the wildcard character
'*' but only as the left-most label. An application protocol
that re-uses the rules specified in this document MUST specify
whether the wildcard character is or is not allowed in
certificates issued for use with that protocol.
3. If the service at which the certificate will be used deploys a
technology that is discovered by means of DNS SRV records
[DNS-SRV], then the certificate SHOULD include an identity of
type SRVName [SRVNAME].
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4. If the service is associated with a particular URI, then the
certificate MAY include an identity of type
uniformResourceIdentifier (i.e., a subjectAltName extension that
includes a uniformResourceIdentifier name form whose authority
component is the domain name of the service).
5. Although the certificate MAY include other application-specific
identities for types that were defined before specification of
the SRVName extension (e.g., XmppAddr for [XMPP]) or for which
service names do not exist, the SRVName extension is preferred.
6. If the certificate does not include an SRVName,
uniformResourceIdentifier, or other application-specific identity
type, then the certificate MUST include an identity of dNSName.
7. The certificate SHOULD NOT represent the server's domain name in
an identity of Common Name (CN) (see [LDAP-SCHEMA]) in the leaf
Relative Distinguished Name (RDN) of the subjectName, even though
it is recognized that many deployed clients still check this
legacy identity. For example, here the CN is the leaf RDN, which
is acceptable:
cn=www.example.com, ou=Web Services, c=GB
8. The certificate MUST NOT represent the server's domain name in an
identity of Common Name (CN) where the CN is in something other
than the "leaf" (left-most) position within the Relative
Distinguished Names (RDNs) of the subjectName. For example, here
the CN is not the leaf RDN, which is unacceptable:
c=GB, ou=Web Services, cn=www.example.com
9. The certificate MUST NOT represent the server's domain name by
means of a series of Domain Component (DC) attributes (because
the order of Domain Components is not guaranteed, certain attacks
are possible if DC attributes are included).
4. Verification of Server Identity
4.1. Overview
At a high level, the client verifies the server's identity in
accordance with the following rules:
1. Before connecting to the server, the client determines the
possible identity type(s) of the reference identity.
2. During the process of attempting to establish a secure
connection, the server MUST present its identity set to the
client in the form of an X.509 certificate [X509].
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3. Upon being presented with the server's identity set, the client
MUST check the reference identity against the presented
identities for the purpose of finding a match. To do so, the
client iterates through all of the subjectAltName extensions it
recognizes in the server's certificate and compares the value of
each extension to the reference identity until it has either
produced a match or exhausted the identities in the identity set
(comparison rules for matching particular identity types are
provided under Section 4.2, including fallbacks to several
subjectName fields). Even if the application technology does not
define an application-specific preference order for checking of
identities, the client SHOULD check them in order from most
specific (e.g., SRVName) to least specific (e.g., dNSName).
4. Before attempting to find a match in relation to a particular
presented identity, the client MAY map the reference identity to
a different identity type. Such a mapping MAY be performed for
any available subjectAltName type to which the reference identity
can be mapped; however, the reference identity SHOULD be mapped
only to types for which the mapping is inherently secure (e.g.,
extracting the domain name from a URI to match against a
subjectAltName of type dNSName or SRVName).
5. The client MUST NOT attempt to match against an an identity of
type SRVName [SRVNAME] unless the service to which the client has
connected deploys a technology that is discovered by means of DNS
SRV records [DNS-SRV].
6. If the identity set has more than one member, a match with any of
the presented identities is acceptable.
Note: A hostname that is resolved via the Domain Name System MUST
NOT be used as the reference identity unless an administrator or
human user has explicitly configured an application to associate a
particular hostname (and potentially port) with the hostname to
which the application connects (e.g., to "hardcode" an association
between an original hostname of example.net and a configured
hostname of connector.example.com:443).
In addition to the identity check described in this section, a client
might complete further verification to ensure that the server is
authorized to provide the service it is requested to provide.
Methods for doing so (which might include consulting local policy
information) are out of scope for this document.
4.2. Comparison Rules
This document assumes that the reference identity is a domain name as
defined by [RFC1034] and [RFC1035] for "traditional" domain names or
by [IDNA2003] or [IDNA2008] for internationalized domain names. The
client MUST match the reference identity against subjectAltName
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extensions of type dNSName and SRVName [SRVNAME] according to the
following rules.
4.2.1. Traditional Domain Names
If the reference identity is a "traditional" domain name, then
matching of reference identity against the presented identity is
performed by comparing the set of domain components using a case-
insensitive ASCII comparison (as clarified by [DNS-CASE]).
4.2.2. Internationalized Domain Names
Note: This section needs to be updated to reflect [IDNA2008].
If the reference identity is an internationalized domain name, then
an implementation MUST convert the reference identity to the ASCII
Compatible Encoding (ACE) format as specified in Section 4 of
[IDNA2003] before comparison with subjectAltName values of type
dNSName; specifically, the conversion operation specified in Section
4 of [IDNA2003] MUST be performed 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.
o In step 5, change all label separators to U+002E (full stop).
After performing the "to-ASCII" conversion with regard to an
internationalized domain name, the DNS labels and names MUST be
compared for equality according to the rules specified in Section 3
of [IDNA2003], i.e. once all label separators are replaced with
U+002E (dot) they are compared in a case-insensitive manner.
4.2.3. Wildcards
Unless otherwise specified by an application protocol that re-uses
the rules specified in this document, the client MAY match against an
identity that contains one instance of the wildcard character '*' as
the left-most label of the domain name. If it does so, the client
MUST match the reference identity against the entire left-most label
only (thus *.example.com matches foo.example.com but not
bar.foo.example.com or example.com itself). The client MUST ignore a
presented identity in which the wildcard character is contained
within a label fragment (e.g., baz*.example.net is not allowed and
MUST NOT be taken to match baz1.example.net and baz2.example.net).
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4.2.4. Common Names
If and only if the identity set does not include subjectAltName
extensions of type dNSName, SRVName, uniformResourceIdentifier (or
other application-specific subjectAltName extensions), the client MAY
as a fallback check the value of the Common Name (CN) if presented as
the leaf (left-most) Relative Distinguished Name (RND) in the
subjectName component of the server's X.509 certificate. In existing
certificates, the CN is often used for representing a domain name;
for example, consider the following subjectName with a leaf RDN of
Common Name:
cn=www.example.com, ou=Web Services, c=GB
Here the Common Name is "www.example.com" and the client could choose
to compare the reference identity against that CN. When doing so,
the client MUST follow the comparison rules described above for
subjectAltName extensions of type dNSName and SRVName.
A client MUST NOT check the Common Name if it is other than the leaf
(left-most) Relative Distinguished Name (RDN) in the subjectName.
4.2.5. Relative Distinguished Names
A client MUST NOT check Relative Distinguished Names (RDNs) other
than the Common Name; in particular, this means that a series of
Domain Component (DC) attributes MUST NOT be checked (because the
order of Domain Components is not guaranteed, certain attacks are
possible if DC attributes are checked).
4.3. Outcome
The outcome of the checking procedure is one of the following:
Case #1: The client finds at least one presented identity that
matches the reference identity; the entity MUST use this as the
validated identity of the server.
Case #2: The client finds no subjectAltName or subjectName that
matches the reference identity but a human user has permanently
accepted the certificate during a previous connection attempt; the
client MUST verify that the cached certificate was presented and
MUST notify the user if the certificate has changed since the last
time that a secure connection was successfully negotiated.
Case #3: The client finds no subjectAltName or subjectName that
matches the reference identity and a human user has not
permanently accepted the certificate during a previous connection
attempt. The client MUST NOT use the presented identity (if any).
Instead, if the client is a user-oriented application, then it
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MUST either (1) automatically terminate the connection with a bad
certificate error or (2) show the certificate (including the
entire certificate chain) to the user and give the user the choice
of terminating the connecting or accepting the certificate
temporarily (i.e., for this connection attempt only) or
permanently (i.e., for all future connection attempts) and then
continuing with the connection; if a user permanently accepts a
certificate in this way, the client MUST cache the certificate (or
some non-forgeable representation such as a hash value) and in
future connection MUST attempt behave as in Case #2. (It is the
resposibility of the human user to verify the hash value or
fingerprint of the certificate with the server over a trusted
communication layer.) If the client is an automated application,
then it SHOULD terminate the connection with a bad certificate
error and log the error to an appropriate audit log; an automated
application MAY provide a configuration setting that disables this
check, but MUST enable the check by default.
5. Security Considerations
This entire document discusses security.
6. IANA Considerations
This document has no actions for the IANA.
7. References
7.1. Normative References
[IDNA2003]
Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[IDNA2008]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol",
draft-ietf-idnabis-protocol-18 (work in progress),
January 2010.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
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specification", STD 13, RFC 1035, November 1987.
[SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.
[TERMS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X509] 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.
7.2. Informative References
[DNS-CASE]
Eastlake, D., "Domain Name System (DNS) Case Insensitivity
Clarification", RFC 4343, January 2006.
[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[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.
[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
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Internet Protocol", RFC 4301, December 2005.
[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[LDAP-AUTH]
Harrison, R., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, June 2006.
[LDAP-SCHEMA]
Sciberras, A., "Lightweight Directory Access Protocol
(LDAP): Schema for User Applications", RFC 4519,
June 2006.
[LDAP-TLS]
Hodges, J., Morgan, R., and M. Wahl, "Lightweight
Directory Access Protocol (v3): Extension for Transport
Layer Security", RFC 2830, May 2000.
[NETCONF] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[NETCONF-SSH]
Wasserman, M. and T. Goddard, "Using the NETCONF
Configuration Protocol over Secure SHell (SSH)", RFC 4742,
December 2006.
[NETCONF-TLS]
Badra, M., "NETCONF over Transport Layer Security (TLS)",
RFC 5539, May 2009.
[NNTP] Feather, C., "Network News Transfer Protocol (NNTP)",
RFC 3977, October 2006.
[NNTP-TLS]
Murchison, K., Vinocur, J., and C. Newman, "Using
Transport Layer Security (TLS) with Network News Transfer
Protocol (NNTP)", RFC 4642, October 2006.
[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2459] Housley, R., Ford, W., Polk, T., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, January 1999.
[SECTERMS]
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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 B. Laboratories, "Domain
Certificates in the Session Initiation Protocol (SIP)",
draft-ietf-sip-domain-certs-04 (work in progress),
May 2009.
[SIP-LOC] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[SMTP-AUTH]
Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[SMTP-TLS]
Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[SYSLOG] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[SYSLOG-TLS]
Miao, F., Ma, Y., and J. Salowey, "Transport Layer
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.
[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[USINGTLS]
Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[XMPP] Saint-Andre, P., Ed., "Extensible Messaging and Presence
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Protocol (XMPP): Core", RFC 3920, October 2004.
[XMPPBIS] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-04 (work
in progress), November 2009.
Appendix A. Prior Art
This section is non-normative.
The recommendations in this document are an abstraction from
recommendations in specifications for a wide range of application
protocols. For the purpose of comparison and to delineate the
history of thinking about server identity verification within the
IETF, this informative section gathers together prior art by
including the exact text from various RFCs (the only modifications
are changes to the names of several references to maintain coherence
with the main body of this document, and the elision of irrelevant
text as marked by the characters "[...]").
A.1. IMAP, POP3, and ACAP (1999)
In 1999, [USINGTLS] specified the following text regarding server
identity verification in IMAP, POP3, and ACAP:
######
2.4. Server Identity Check
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use any
form of the server hostname derived from an insecure remote source
(e.g., insecure DNS lookup). CNAME canonicalization is not done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc. but would not match
example.com.
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o If the certificate contains multiple names (e.g. more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation, or terminate the connection and indicate the server's
identity is suspect.
######
A.2. HTTP (2000)
In 2000, [HTTP-TLS] specified the following text regarding server
identity verification in HTTP:
######
3.1. Server Identity
In general, HTTP/TLS requests are generated by dereferencing a URI.
As a consequence, the hostname for the server is known to the client.
If the hostname is available, the client MUST check it against the
server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted. (For instance, a
client may be connecting to a machine whose address and hostname are
dynamic but the client knows the certificate that the server will
present.) In such cases, it is important to narrow the scope of
acceptable certificates as much as possible in order to prevent man
in the middle attacks. In special cases, it may be appropriate for
the client to simply ignore the server's identity, but it must be
understood that this leaves the connection open to active attack.
If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Matching is performed using the matching rules specified by
[RFC2459]. If more than one identity of a given type is present in
the certificate (e.g., more than one dNSName name, a match in any one
of the set is considered acceptable.) Names may contain the wildcard
character * which is considered to match any single domain name
component or component fragment. E.g., *.a.com matches foo.a.com but
not bar.foo.a.com. f*.com matches foo.com but not bar.com.
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In some cases, the URI is specified as an IP address rather than a
hostname. In this case, the iPAddress subjectAltName must be present
in the certificate and must exactly match the IP in the URI.
If the hostname does not match the identity in the certificate, user
oriented clients MUST either notify the user (clients MAY give the
user the opportunity to continue with the connection in any case) or
terminate the connection with a bad certificate error. Automated
clients MUST log the error to an appropriate audit log (if available)
and SHOULD terminate the connection (with a bad certificate error).
Automated clients MAY provide a configuration setting that disables
this check, but MUST provide a setting which enables it.
Note that in many cases the URI itself comes from an untrusted
source. The above-described check provides no protection against
attacks where this source is compromised. For example, if the URI
was obtained by clicking on an HTML page which was itself obtained
without using HTTP/TLS, a man in the middle could have replaced the
URI. In order to prevent this form of attack, users should carefully
examine the certificate presented by the server to determine if it
meets their expectations.
######
A.3. LDAP (2000/2006)
In 2000, [LDAP-TLS] specified the following text regarding server
identity verification in LDAP:
######
3.6. Server Identity Check
The client MUST check its understanding of the server's hostname
against the server's identity as presented in the server's
Certificate message, in order to prevent man-in-the-middle attacks.
Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the LDAP
connection as the value to compare against the server name as
expressed in the server's certificate. The client MUST NOT use
the server's canonical DNS name or any other derived form of name.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
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o Matching is case-insensitive.
o The "*" wildcard character is allowed. If present, it applies
only to the left-most name component.
E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
bar.com. If more than one identity of a given type is present in the
certificate (e.g. more than one dNSName name), a match in any one of
the set is considered acceptable.
If the hostname does not match the dNSName-based identity in the
certificate per the above check, user-oriented clients SHOULD either
notify the user (clients MAY give the user the opportunity to
continue with the connection in any case) or terminate the connection
and indicate that the server's identity is suspect. Automated
clients SHOULD close the connection, returning and/or logging an
error indicating that the server's identity is suspect.
Beyond the server identity checks described in this section, clients
SHOULD be prepared to do further checking to ensure that the server
is authorized to provide the service it is observed to provide. The
client MAY need to make use of local policy information.
######
In 2006, [LDAP-AUTH] specified the following text regarding server
identity verification in LDAP:
######
3.1.3. Server Identity Check
In order to prevent man-in-the-middle attacks, the client MUST verify
the server's identity (as presented in the server's Certificate
message). In this section, the client's understanding of the
server's identity (typically the identity used to establish the
transport connection) is called the "reference identity".
The client determines the type (e.g., DNS name or IP address) of the
reference identity and performs a comparison between the reference
identity and each subjectAltName value of the corresponding type
until a match is produced. Once a match is produced, the server's
identity has been verified, and the server identity check is
complete. Different subjectAltName types are matched in different
ways. Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
various subjectAltName types.
The client may map the reference identity to a different type prior
to performing a comparison. Mappings may be performed for all
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available subjectAltName types to which the reference identity can be
mapped; however, the reference identity should only be mapped to
types for which the mapping is either inherently secure (e.g.,
extracting the DNS name from a URI to compare with a subjectAltName
of type dNSName) or for which the mapping is performed in a secure
manner (e.g., using [DNSSEC], or using user- or admin-configured
host-to-address/address-to-host lookup tables).
The server's identity may also be verified by comparing the reference
identity to the Common Name (CN) [LDAP-SCHEMA] value in the leaf
Relative Distinguished Name (RDN) of the subjectName field of the
server's certificate. This comparison is performed using the rules
for comparison of DNS names in Section 3.1.3.1, below, with the
exception that no wildcard matching is allowed. Although the use of
the Common Name value is existing practice, it is deprecated, and
Certification Authorities are encouraged to provide subjectAltName
values instead. Note that the TLS implementation may represent DNs
in certificates according to X.500 or other conventions. For
example, some X.500 implementations order the RDNs in a DN using a
left-to-right (most significant to least significant) convention
instead of LDAP's right-to-left convention.
If the server identity check fails, user-oriented clients SHOULD
either notify the user (clients may give the user the opportunity to
continue with the LDAP session in this case) or close the transport
connection and indicate that the server's identity is suspect.
Automated clients SHOULD close the transport connection and then
return or log an error indicating that the server's identity is
suspect or both.
Beyond the server identity check described in this section, clients
should be prepared to do further checking to ensure that the server
is authorized to provide the service it is requested to provide. The
client may need to make use of local policy information in making
this determination.
3.1.3.1. Comparison of DNS Names
If the reference identity is an internationalized domain name,
conforming implementations MUST convert it to the ASCII Compatible
Encoding (ACE) format as specified in Section 4 of RFC 3490
[IDNA2003] before comparison with subjectAltName values of type
dNSName. Specifically, conforming implementations MUST perform the
conversion operation specified in Section 4 of RFC 3490 as follows:
o in step 1, the domain name SHALL be considered a "stored string";
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o in step 3, set the flag called "UseSTD3ASCIIRules";
o in step 4, process each label with the "ToASCII" operation; and
o in step 5, change all label separators to U+002E (full stop).
After performing the "to-ASCII" conversion, the DNS labels and names
MUST be compared for equality according to the rules specified in
Section 3 of RFC3490.
The '*' (ASCII 42) wildcard character is allowed in subjectAltName
values of type dNSName, and then only as the left-most (least
significant) DNS label in that value. This wildcard matches any
left-most DNS label in the server name. That is, the subject
*.example.com matches the server names a.example.com and
b.example.com, but does not match example.com or a.b.example.com.
3.1.3.2. Comparison of IP Addresses
When the reference identity is an IP address, the identity MUST be
converted to the "network byte order" octet string representation
[IP] [IPv6]. For IP Version 4, as specified in RFC 791, the octet
string will contain exactly four octets. For IP Version 6, as
specified in RFC 2460, the octet string will contain exactly sixteen
octets. This octet string is then compared against subjectAltName
values of type iPAddress. A match occurs if the reference identity
octet string and value octet strings are identical.
3.1.3.3. Comparison of Other subjectName Types
Client implementations MAY support matching against subjectAltName
values of other types as described in other documents.
######
A.4. SMTP (2002/2007)
In 2002, [SMTP-TLS] specified the following text regarding server
identity verification in SMTP:
######
4.1 Processing After the STARTTLS Command
[...]
The decision of whether or not to believe the authenticity of the
other party in a TLS negotiation is a local matter. However, some
general rules for the decisions are:
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o A SMTP client would probably only want to authenticate an SMTP
server whose server certificate has a domain name that is the
domain name that the client thought it was connecting to.
[...]
######
In 2006, [SMTP-AUTH] specified the following text regarding server
identity verification in SMTP:
######
14. Additional Requirements When Using SASL PLAIN over TLS
[...]
After a successful [TLS] negotiation, the client MUST check its
understanding of the server hostname against the server's identity as
presented in the server Certificate message, in order to prevent man-
in-the-middle attacks. If the match fails, the client MUST NOT
attempt to authenticate using the SASL PLAIN mechanism. Matching is
performed according to the following rules:
The client MUST use the server hostname it used to open the
connection as the value to compare against the server name as
expressed in the server certificate. The client MUST NOT use any
form of the server hostname derived from an insecure remote source
(e.g., insecure DNS lookup). CNAME canonicalization is not done.
If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
Matching is case-insensitive.
A "*" wildcard character MAY be used as the leftmost name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
######
A.5. XMPP (2004)
In 2004, [XMPP] specified the following text regarding server
identity verification in XMPP:
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######
14.2. Certificate Validation
When an XMPP peer communicates with another peer securely, it MUST
validate the peer's certificate. There are three possible cases:
Case #1: The peer contains an End Entity certificate which appears
to be certified by a chain of certificates terminating in a trust
anchor (as described in Section 6.1 of [X509]).
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 [X509].
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 certificate chain. The peer MUST cache the
certificate (or some non-forgeable representation such as a
hash). In future connections, the peer MUST verify that the same
certificate was presented and MUST notify the user if it has
changed.
In Case #2 and Case #3, implementations SHOULD act as in (2) above.
######
At the time of this writing, [XMPPBIS] refers to this document for
rules regarding server identity verification in XMPP.
A.6. NNTP (2006)
In 2006, [NNTP-TLS] specified the following text regarding server
identity verification in NNTP:
######
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5. Security Considerations
[...]
During the TLS negotiation, the client MUST check its understanding
of the server hostname against the server's identity as presented in
the server Certificate message, in order to prevent man-in-the-middle
attacks. Matching is performed according to these rules:
o The client MUST use the server hostname it used to open the
connection (or the hostname specified in TLS "server_name"
extension [TLS]) as the value to compare against the server name
as expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is not
done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it SHOULD be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the left-most name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
o If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client SHOULD either ask for explicit user
confirmation or terminate the connection with a QUIT command and
indicate the server's identity is suspect.
Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [X509] for general certificate validation, but MAY supplement that
algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
######
A.7. NETCONF (2006/2009)
In 2006, [NETCONF-SSH] specified the following text regarding server
identity verification in NETCONF:
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######
6. Security Considerations
The identity of the server MUST be verified and authenticated by the
client according to local policy before password-based authentication
data or any configuration or state data is sent to or received from
the server. The identity of the client MUST also be verified and
authenticated by the server according to local policy to ensure that
the incoming client request is legitimate before any configuration or
state data is sent to or received from the client. Neither side
should establish a NETCONF over SSH connection with an unknown,
unexpected, or incorrect identity on the opposite side.
######
In 2009, [NETCONF-TLS] specified the following text regarding server
identity verification in NETCONF:
######
3.1. Server Identity
During the TLS negotiation, the client MUST carefully examine the
certificate presented by the server to determine if it meets the
client's expectations. Particularly, the client MUST check its
understanding of the server hostname against the server's identity as
presented in the server Certificate message, in order to prevent man-
in-the-middle attacks.
Matching is performed according to the rules below (following the
example of [NNTP-TLS]):
o The client MUST use the server hostname it used to open the
connection (or the hostname specified in the TLS "server_name"
extension [TLS]) as the value to compare against the server name
as expressed in the server certificate. The client MUST NOT use
any form of the server hostname derived from an insecure remote
source (e.g., insecure DNS lookup). CNAME canonicalization is not
done.
o If a subjectAltName extension of type dNSName is present in the
certificate, it MUST be used as the source of the server's
identity.
o Matching is case-insensitive.
o A "*" wildcard character MAY be used as the leftmost name
component in the certificate. For example, *.example.com would
match a.example.com, foo.example.com, etc., but would not match
example.com.
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o If the certificate contains multiple names (e.g., more than one
dNSName field), then a match with any one of the fields is
considered acceptable.
If the match fails, the client MUST either ask for explicit user
confirmation or terminate the connection and indicate the server's
identity is suspect.
Additionally, clients MUST verify the binding between the identity of
the servers to which they connect and the public keys presented by
those servers. Clients SHOULD implement the algorithm in Section 6
of [X509] for general certificate validation, but MAY supplement that
algorithm with other validation methods that achieve equivalent
levels of verification (such as comparing the server certificate
against a local store of already-verified certificates and identity
bindings).
If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted.
######
A.8. Syslog (2009)
In 2009, [SYSLOG-TLS] specified the following text regarding server
identity verification in Syslog:
######
5.2. Subject Name Authorization
Implementations MUST support certification path validation [X509].
In addition, they MUST support specifying the authorized peers using
locally configured host names and matching the name against the
certificate as follows.
o Implementations MUST support matching the locally configured host
name against a dNSName in the subjectAltName extension field and
SHOULD support checking the name against the common name portion
of the subject distinguished name.
o The '*' (ASCII 42) wildcard character is allowed in the dNSName of
the subjectAltName extension (and in common name, if used to store
the host name), but only as the left-most (least significant) DNS
label in that value. This wildcard matches any left-most DNS
label in the server name. That is, the subject *.example.com
matches the server names a.example.com and b.example.com, but does
not match example.com or a.b.example.com. Implementations MUST
support wildcards in certificates as specified above, but MAY
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provide a configuration option to disable them.
o Locally configured names MAY contain the wildcard character to
match a range of values. The types of wildcards supported MAY be
more flexible than those allowed in subject names, making it
possible to support various policies for different environments.
For example, a policy could allow for a trust-root-based
authorization where all credentials issued by a particular CA
trust root are authorized.
o If the locally configured name is an internationalized domain
name, conforming implementations MUST convert it to the ASCII
Compatible Encoding (ACE) format for performing comparisons, as
specified in Section 7 of [X509].
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 [X509], Section
4.2.1.6. A match occurs if this octet string is equal to the
value of iPAddress in the subjectAltName extension.
######
A.9. SIP (2010)
At the time of this writing, [SIP-CERTS] specified text regarding
server identity verification in the Session Initiation Protocol
(SIP). However, that specification has not yet been approved by the
IESG and text cannot be considered final.
The relevant text follows.
######
7.2. Comparing SIP Identities
When an implementation (either client or server) compares two values
as SIP domain identities:
Implementations MUST compare only the DNS name component of each
SIP domain identifier; an implementation MUST NOT use any scheme
or parameters in the comparison.
Implementations MUST compare the values as DNS names, which means
that the comparison is case insensitive as specified by
[DNS-CASE]. Implementations MUST handle Internationalized Domain
Names (IDNs) in accordance with Section 7.2 of [X509].
Implementations MUST match the values in their entirety:
Implementations MUST NOT match suffixes. For example,
"foo.example.com" does not match "example.com".
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Implemenations MUST NOT match any form of wildcard, such as a
leading "." or "*." with any other DNS label or sequence of
labels. For example, "*.example.com" matches only
"*.example.com" but not "foo.example.com". Similarly,
".example.com" matches only ".example.com", and does not match
"foo.example.com."
[HTTP-TLS] allows the dNSName component to contain a
wildcard; e.g., "DNS:*.example.com". [X509], 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.
######
Authors' Addresses
Peter Saint-Andre (editor)
Cisco
Email: psaintan@cisco.com
Jeff Hodges (editor)
PayPal
Email: Jeff.Hodges@PayPal.com
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