< draft-saintandre-tls-server-id-check-01.txt   draft-saintandre-tls-server-id-check-02.txt >
None P. Saint-Andre None P. Saint-Andre
Internet-Draft Cisco Internet-Draft Cisco
Intended status: Standards Track K. Zeilenga Intended status: Standards Track K. Zeilenga
Expires: March 4, 2010 Isode Limited Expires: April 5, 2010 Isode Limited
J. Hodges J. Hodges
PayPal PayPal
R. Morgan R. Morgan
Internet2 Internet2
August 31, 2009 October 2, 2009
Server Identity Verification in Application Protocols Server Identity Verification in Application Protocols
draft-saintandre-tls-server-id-check-01 draft-saintandre-tls-server-id-check-02
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 4, 2010. This Internet-Draft will expire on April 5, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Verification Process . . . . . . . . . . . . . . . . . . . . . 5 3. Verification Process . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Comparison Rules . . . . . . . . . . . . . . . . . . . . . 6 3.2. Comparison Rules . . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Domain Names . . . . . . . . . . . . . . . . . . . . . 6 3.2.1. Domain Names . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. IP Addresses . . . . . . . . . . . . . . . . . . . . . 7 3.2.2. IP Addresses . . . . . . . . . . . . . . . . . . . . . 7
3.2.3. Email Addresses . . . . . . . . . . . . . . . . . . . 7 3.2.3. Email Addresses . . . . . . . . . . . . . . . . . . . 7
3.2.4. SIP Addresses . . . . . . . . . . . . . . . . . . . . 8 3.2.4. SIP Addresses . . . . . . . . . . . . . . . . . . . . 8
3.2.5. JabberIDs . . . . . . . . . . . . . . . . . . . . . . 8 3.2.5. XMPP Addresses . . . . . . . . . . . . . . . . . . . . 8
3.3. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9 4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . . 9 6.1. Normative References . . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . . 9 6.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Appendix A. Prior Art . . . . . . . . . . . . . . . . . . . . . . 12
A.1. IMAP, POP3, and ACAP (1999) . . . . . . . . . . . . . . . 13
A.2. HTTP (2000) . . . . . . . . . . . . . . . . . . . . . . . 13
A.3. LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 15
A.4. SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 18
A.5. XMPP (2004) . . . . . . . . . . . . . . . . . . . . . . . 19
A.6. NNTP (2006) . . . . . . . . . . . . . . . . . . . . . . . 20
A.7. NETCONF (2006/2009) . . . . . . . . . . . . . . . . . . . 21
A.8. Syslog (2009) . . . . . . . . . . . . . . . . . . . . . . 23
A.9. SIP (2009) . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
Technologies such as Transport Layer Security [TLS] and [IPSEC] Technologies such as Transport Layer Security [TLS] and [IPSEC]
enable a secure connection between two entities using the Internet enable a secure connection between two entities using the Internet
X.509 Public Key Infrastructure (PKI) as described in [X509]. In X.509 Public Key Infrastructure (PKI) as described in [X509]. In
such interactions, the entity that initiates the connection is called such interactions, the entity that initiates the connection is called
a "client" and the entity that receives the connection is called a a "client" and the entity that receives the connection is called a
"server". "server".
Note: The terms "client" and "server" as used here refer to Note: The terms "client" and "server" as used here refer to
security roles, not application roles; a server in the context of security roles, not application roles; a server in the context of
TLS or IPSec might be a "client" (i.e., a user agent) in the TLS or IPSec might be a "client" (i.e., a user agent) in the
context of an application protocol as deployed on the Internet. context of an application protocol as deployed on the Internet.
If a client wishes to connect to a server securely, it needs to check If a client wishes to connect to a server securely, it needs to check
the identity of the server so that it can determine if the server is the identity of the server so that it can determine if the server is
what it claims to be, verify that there is no attacker in the middle, what it claims to be, verify that there is no attacker in the middle,
etc. Typically this is done by correlating the information presented and enforce other relevant security considerations. Typically this
in the server's certificate with information available about the checking is done by correlating the information presented in the
server contained in the Domain Name System (DNS). server's certificate with information available about the server
contained in the Domain Name System (DNS) or provided by a human
user.
Different application protocols that make use of the client-server Different application protocols that make use of the client-server
pattern for security purposes have traditionally specified their own pattern for security purposes have traditionally specified their own
procedures for checking server identities. Examples include but are procedures for checking server identities. Examples include but are
not limited to: not limited to:
o The Hypertext Transfer Protocol [HTTP], for which see also o The Hypertext Transfer Protocol [HTTP], for which see also
[HTTP-TLS] [HTTP-TLS]
o The Internet Message Access Protocol [IMAP] and the Post Office o The Internet Message Access Protocol [IMAP] and the Post Office
Protocol [POP3], for which see also [USINGTLS] Protocol [POP3], for which see also [USINGTLS]
o The Lightweight Directory Access Protocol [LDAP], for which see o The Lightweight Directory Access Protocol [LDAP], for which see
also [LDAP-AUTH] and its predecesor [LDAP-TLS] also [LDAP-AUTH] and its predecessor [LDAP-TLS]
o The NETCONF Configuration Protocol [NETCONF], for which see also o The NETCONF Configuration Protocol [NETCONF], for which see also
[NETCONF-SSH] and [NETCONF-TLS] [NETCONF-SSH] and [NETCONF-TLS]
o The Network News Transfer Protocol [NNTP], for which see also o The Network News Transfer Protocol [NNTP], for which see also
[NNTP-TLS] [NNTP-TLS]
o The Session Initiation Protocol [SIP], for which see also o The Session Initiation Protocol [SIP], for which see also
[SIP-CERTS] [SIP-CERTS]
o The Simple Mail Transfer Protocol [SMTP], for which see also o The Simple Mail Transfer Protocol [SMTP], for which see also
[SMTP-AUTH] and [SMTP-TLS] [SMTP-AUTH] and [SMTP-TLS]
o The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS] o The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]
o The Extensible Messaging and Presence Protocol [XMPP], for which o The Extensible Messaging and Presence Protocol [XMPP], for which
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Note: This document is currently limited in scope to the presentation Note: This document is currently limited in scope to the presentation
of identities in X.509 certificates as issued in the context of the of identities in X.509 certificates as issued in the context of the
Public Key Infrastructure (PKI) and as applied to Transport Layer Public Key Infrastructure (PKI) and as applied to Transport Layer
Security [TLS]; a future version of this document might address X.509 Security [TLS]; a future version of this document might address X.509
certificates as issued outside the context of the PKI, non-X.509 certificates as issued outside the context of the PKI, non-X.509
public keys such as OpenPGP keys, presentation of identities in ways public keys such as OpenPGP keys, presentation of identities in ways
other than in the certificate itself (e.g., certificate fingerprints other than in the certificate itself (e.g., certificate fingerprints
for Secure Shell as described in [SSH] or for Datagram Transport for Secure Shell as described in [SSH] or for Datagram Transport
Layer Security DTLS and Secure Real-time Transport Protocol as Layer Security DTLS and Secure Real-time Transport Protocol as
described in [DTLS-SRTP]), and applications other than TLS. described in [DTLS-SRTP]), and applications that use security
technologies other than TLS.
2. Conventions 2. Conventions
The following capitalized keywords are to be interpreted as described The following capitalized keywords are to be interpreted as described
in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
"SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
"OPTIONAL". "OPTIONAL".
Most security-related terms are to be understood in the sense defined Most security-related terms are to be understood in the sense defined
in [SECTERMS]; such terms include, but are not limited to, in [SECTERMS]; such terms include, but are not limited to, "attack",
"assurance", "attack", "authentication", "authorization", "authentication", "authorization", "certificate", "credential",
"certificate", "certification authority", "confidentiality", "fingerprint", "identity", "self-signed certificate", "trust", "trust
"credential", "downgrade", "encryption", "fingerprint", "hash value", anchor", "trust chain", "validate", and "verify".
"identity", "integrity", "signature", "security perimeter", "self-
signed certificate", "sign", "spoof", "tamper", "trust", "trust
anchor", "trust chain", "validate", "verify".
In addition, we define the following terms to assist in understanding In addition, we define the following terms to assist in understanding
the process of verifying identity: the process of verifying server identity:
identity set: The set of identities that are presented by the server identity set: The set of identities that are presented by the server
to the client (in the form of the server's X.509 certificate) when to the client (in the form of the server's X.509 certificate) when
the client is attempts to establish a secure connection to the the client attempts to establish a secure connection to the
server. server.
identity type: The "natural kind" of identity to which a presented identity type: The "natural kind" of identity to which a presented
identity or reference identity belongs. For example, the identity or reference identity belongs. For example, the
reference identity might be a domain name, an IPv4 or IPv6 reference identity might be a domain name, an IPv4 or IPv6
address, an email address, a SIP address, a JabberID, or some address, an email address, a SIP address, an XMPP address, or some
other type (this specification does not yet provide a complete other type (this specification does not yet provide a complete
taxonomy of identity types). In the case of domain names, the taxonomy of identity types). In the case of domain names, the
reference identity MUST NOT contain the wildcard character '*' reference identity MUST NOT contain the wildcard character '*'
(ASCII 42) in the left-most (least significant) domain name (ASCII 42) in the left-most (least significant) domain name
component or component fragment. component or component fragment.
presented identity: A single member of the identity set. presented identity: A single member of the identity set.
reference identity: The client's conception of the server's identity reference identity: The client's conception of the server's identity
before it attempts to establish a secure connection to the server; before it attempts to establish a secure connection to the server,
this is the identity that the client expects the server to present i.e. the identity that the client expects the server to present
and to which the client makes reference when attempting to verify and to which the client makes reference when attempting to verify
the server's identity. the server's identity. It is either the address to which the
client connected or the explicit value of the TLS "server_name"
extension as specified in [TLS]. The reference identity might be
based on a DNS lookup, user configuration, or some other
mechanism.
3. Verification Process 3. Verification Process
When a client connects to a server, it MUST verify the server's When a client connects to a server, it MUST verify the server's
identity (in order to prevent passive and active attacks against the identity in order to prevent certain passive and active attacks
connection). By "verify identity" we mean that the client needs to against the connection. By "verify identity" we mean that the client
establish that at least one of the identities in the identity set needs to establish that at least one of the presented identities
matches the reference identity. matches the reference identity.
3.1. Overview 3.1. Overview
At a high level, the client verifies the server identity in At a high level, the client verifies the server identity in
accordance with the following rules: accordance with the following rules:
1. Before connecting to the server, the client determines the 1. Before connecting to the server, the client determines the
identity type of the reference identity. identity type of the reference identity.
2. During the process of attempting to establish a secure 2. During the process of attempting to establish a secure
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(comparison rules for matching particular identity types are (comparison rules for matching particular identity types are
provided under Section 3.2, including fallbacks to several provided under Section 3.2, including fallbacks to several
subjectName fields). subjectName fields).
4. Before attempting to find a match in relation to a particular 4. Before attempting to find a match in relation to a particular
presented identity, the client MAY map the reference identity to presented identity, the client MAY map the reference identity to
a different identity type. Such a mapping MAY be performed for a different identity type. Such a mapping MAY be performed for
any available subjectAltName type to which the reference identity any available subjectAltName type to which the reference identity
can be mapped; however, the reference identity SHOULD be mapped can be mapped; however, the reference identity SHOULD be mapped
only to types for which the mapping is either inherently secure only to types for which the mapping is either inherently secure
(e.g., extracting the DNS name from a URI to compare with a (e.g., extracting the DNS name from a URI to compare with a
subjectAltName of type dNSName) or for which the mapping is subjectAltName of type dNSName or SRVName) or for which the
performed in a secure manner (e.g., using DNSSEC, or using user- mapping is performed in a secure manner (e.g., using [DNSSEC], or
configured or admin-configured host-to-address/address-to-host using a user-configured or admin-configured lookup table for
lookup tables). host-to-address or address-to-host translations).
5. If the identity set has more than one member, a match with any of 5. If the identity set has more than one member, a match with any of
the presented identities is acceptable. the presented identities is acceptable.
Note: Beyond the server identity check described in this section, Note: Beyond the server identity check described in this section,
clients might complete further checking to ensure that the server clients might complete further checking to ensure that the server
is authorized to provide the service it is requested to provide. is authorized to provide the service it is requested to provide.
The client might need to make use of local policy information in The client might need to make use of local policy information in
making this determination. making this determination.
3.2. Comparison Rules 3.2. Comparison Rules
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performed by comparing the set of domain components using a case- performed by comparing the set of domain components using a case-
insensitive ASCII comparison. insensitive ASCII comparison.
If the reference identity is an internationalized domain name, then If the reference identity is an internationalized domain name, then
an implementation MUST convert the reference identity to the ASCII an implementation MUST convert the reference identity to the ASCII
Compatible Encoding (ACE) format as specified in Section 4 of [IDNA] Compatible Encoding (ACE) format as specified in Section 4 of [IDNA]
before comparison with subjectAltName values of type dNSName; before comparison with subjectAltName values of type dNSName;
specifically, the conversion operation specified in Section 4 of specifically, the conversion operation specified in Section 4 of
[IDNA] MUST be performed as follows: [IDNA] MUST be performed as follows:
o in step 1, the domain name SHALL be considered a "stored string" 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 3, set the flag called "UseSTD3ASCIIRules".
o in step 4, process each label with the "ToASCII" operation o In step 4, process each label with the "ToASCII" operation.
o in step 5, change all label separators to U+002E (full stop) o In step 5, change all label separators to U+002E (full stop).
After performing the "to-ASCII" conversion, the DNS labels and names After performing the "to-ASCII" conversion with regard to an
MUST be compared for equality according to the rules specified in internationalized domain name, the DNS labels and names MUST be
Section 3 of [IDNA]. compared for equality according to the rules specified in Section 3
of [IDNA].
A dNSName MAY contain the wildcard character '*' (ASCII 42). The Unless otherwise specified by an application protocol, the dNSName
wildcard character applies only to the left-most (least significant) MAY contain one instance of the wildcard character '*'. The wildcard
domain name component or component fragment and matches any single character applies only to the left-most domain name component and
component or component fragment. For instance, a dNSName of matches any single component (thus a dNSName of *.example.com matches
*.example.com matches foo.example.com but not bar.foo.example.com or foo.example.com but not bar.foo.example.com or example.com itself).
example.com itself; similarly, a dNSName of baz*.example.net matches The wildcard character is not allowed in component fragments (thus a
baz1.example.net and baz2.example.net but not qux.example.net or dNSName of baz*.example.net is not allowed and shall not be taken to
example.net itself. match baz1.example.net and baz2.example.net).
In addition to checking the subjectAltName extensions of type dNSName In addition to checking the subjectAltName extensions of type dNSName
and SRVNAME, the client MAY as a fallback check the value of the and SRVNAME, the client MAY as a fallback check the value of the
Common Name (CN) (see [LDAP-SCHEMA]) as presented in the subjectName Common Name (CN) (see [LDAP-SCHEMA]) as presented in the subjectName
component of the server's X.509 certificate. In existing component of the server's X.509 certificate. In existing
certificates, the CN is often used for encapsulating a domain name; certificates, the CN is often used for encapsulating a domain name;
for example, consider the following subjectName: for example, consider the following subjectName:
cn=www.example.com, ou=Web Services, c=GB cn=www.example.com, ou=Web Services, c=GB
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contain exactly four octets, and for IP Version 6 the octet string contain exactly four octets, and for IP Version 6 the octet string
will contain exactly sixteen octets. The client then compares this will contain exactly sixteen octets. The client then compares this
octet string, where a match occurs if the reference identity and octet string, where a match occurs if the reference identity and
presented identity octet strings are identical. presented identity octet strings are identical.
3.2.3. Email Addresses 3.2.3. Email Addresses
If the reference identity is an email address as defined by [EMAIL], If the reference identity is an email address as defined by [EMAIL],
then the client SHOULD compare the reference identity against the then the client SHOULD compare the reference identity against the
value of the "rfc822Name" subjectAltName extension described in value of the "rfc822Name" subjectAltName extension described in
[X509]. [X509].
The client MAY also compare the reference identity against the value The client MAY also compare the reference identity against the value
of the "E" attribute of the subjectName as described in [CRMF]. of the "E" attribute of the subjectName as described in [CRMF].
3.2.4. SIP Addresses 3.2.4. SIP Addresses
If the reference identity is a SIP address as defined by [SIP], then If the reference identity is a SIP address as defined by [SIP], then
the client SHOULD compare map the reference identity to a domain name the client SHOULD compare map the reference identity to a domain name
or email address and proceed as described for those identity types, or email address and proceed as described for those identity types,
or proceed as described in [SIP-CERTS]. or proceed as described in [SIP-CERTS].
3.2.5. JabberIDs 3.2.5. XMPP Addresses
If the reference identity is a JabberID as defined by [XMPP], then If the reference identity is an XMPP address ("JabberID") as defined
the client SHOULD compare the reference identity against the value of by [XMPP], then the client SHOULD compare the reference identity
the "id-on-xmppAddr" subjectAltName extension of type otherName against the value of the following subjectAltName extensions, in this
described in [XMPP], or proceed as described in [XMPPBIS]. order: SRVName, dNSName, and (as defined in [XMPP]) "id-on-xmppAddr".
3.3. Outcome 3.3. Outcome
The outcome of the checking procedure is one of the following: The outcome of the checking procedure is one of the following:
Case #1: The client finds at least one presented identity that Case #1: The client finds at least one presented identity that
matches the reference identity; the entity MUST use this as the matches the reference identity; the entity MUST use this as the
validated identity of the server. validated identity of the server.
Case #2: The client finds no subjectAltName that matches the Case #2: The client finds no subjectAltName that matches the
reference identity but a human user has permanently accepted the reference identity but a human user has permanently accepted the
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hash value or fingerprint of the certificate with the peer over a hash value or fingerprint of the certificate with the peer over a
trusted communication layer.) If the client is an automated trusted communication layer.) If the client is an automated
application, then it SHOULD terminate the connection with a bad application, then it SHOULD terminate the connection with a bad
certificate error and log the error to an appropriate audit log; certificate error and log the error to an appropriate audit log;
an automated application MAY provide a configuration setting that an automated application MAY provide a configuration setting that
disables this check, but MUST provide a setting that enables the disables this check, but MUST provide a setting that enables the
check. check.
4. Security Considerations 4. Security Considerations
To follow. This entire document discusses security.
5. IANA Considerations 5. IANA Considerations
This document has no actions for the IANA. This document has no actions for the IANA.
6. References 6. References
6.1. Normative References 6.1. Normative References
[IDNA] Faltstrom, P., Hoffman, P., and A. Costello, [IDNA] Faltstrom, P., Hoffman, P., and A. Costello,
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Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, May 2008.
6.2. Informative References 6.2. Informative References
[CRMF] Schaad, J., "Internet X.509 Public Key Infrastructure [CRMF] Schaad, J., "Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)", RFC 4211, Certificate Request Message Format (CRMF)", RFC 4211,
September 2005. September 2005.
[DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[DTLS-SRTP] [DTLS-SRTP]
McGrew, D. and E. Rescorla, "Datagram Transport Layer McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for Secure Security (DTLS) Extension to Establish Keys for Secure
Real-time Transport Protocol (SRTP)", Real-time Transport Protocol (SRTP)",
draft-ietf-avt-dtls-srtp-07 (work in progress), draft-ietf-avt-dtls-srtp-07 (work in progress),
February 2009. February 2009.
[EMAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, [EMAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008. October 2008.
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[POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996. STD 53, RFC 1939, May 1996.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[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] [SECTERMS]
Shirey, R., "Internet Security Glossary, Version 2", Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007. RFC 4949, August 2007.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
[SIP-CERTS] [SIP-CERTS]
skipping to change at page 12, line 24 skipping to change at page 12, line 38
(TLS) Protocol Version 1.2", RFC 5246, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[USINGTLS] [USINGTLS]
Newman, C., "Using TLS with IMAP, POP3 and ACAP", Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999. RFC 2595, June 1999.
[XMPP] Saint-Andre, P., Ed., "Extensible Messaging and Presence [XMPP] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004. Protocol (XMPP): Core", RFC 3920, October 2004.
[XMPPBIS] Saint-Andre, P., "Extensible Messaging and Presence [XMPPBIS] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-01 (work Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-02 (work
in progress), August 2009. in progress), September 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.
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.
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.
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
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 [IDNA]
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";
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:
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:
######
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.
######
As of this writing, [XMPPBIS] specified updated text regarding server
identity verification in XMPP. However, that specification has not
yet been approved by the IESG, and the relevant text might be
replaced by a reference to this document.
A.6. NNTP (2006)
In 2006, [NNTP-TLS] specified the following text regarding server
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 [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:
######
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.
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
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 (2009)
As of this writing, [SIP-CERTS] specified text regarding server
identity verification in SIP. However, that specification has not
yet been approved by the IESG, and the relevant text might be
replaced by a reference to this document.
Authors' Addresses Authors' Addresses
Peter Saint-Andre Peter Saint-Andre
Cisco Cisco
Email: psaintan@cisco.com Email: Peter.SaintAndre@WebEx.com
Kurt D. Zeilenga Kurt D. Zeilenga
Isode Limited Isode Limited
Email: Kurt.Zeilenga@Isode.COM Email: Kurt.Zeilenga@Isode.COM
Jeff Hodges Jeff Hodges
PayPal PayPal
Email: Jeff.Hodges@KingsMountain.com Email: Jeff.Hodges@KingsMountain.com
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