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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 6125

None                                                      P. Saint-Andre
Internet-Draft                                                     Cisco
Intended status: Standards Track                             K. Zeilenga
Expires: April 5, 2010                                     Isode Limited
                                                               J. Hodges
                                                                  PayPal
                                                               R. Morgan
                                                               Internet2
                                                         October 2, 2009


         Server Identity Verification in Application Protocols
                draft-saintandre-tls-server-id-check-02

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Abstract

   Technologies such as Transport Layer Security (TLS) and IPsec enable
   a secure connection between two entities (a "client" and a "server")
   using X.509 certificates.  This document specifies recommended
   procedures for checking the identity of the server in such an
   interaction.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Verification Process . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Comparison Rules . . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  Domain Names . . . . . . . . . . . . . . . . . . . . .  6
       3.2.2.  IP Addresses . . . . . . . . . . . . . . . . . . . . .  7
       3.2.3.  Email Addresses  . . . . . . . . . . . . . . . . . . .  7
       3.2.4.  SIP Addresses  . . . . . . . . . . . . . . . . . . . .  8
       3.2.5.  XMPP Addresses . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   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













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1.  Introduction

   Technologies such as Transport Layer Security [TLS] and [IPSEC]
   enable a secure connection between two entities using the Internet
   X.509 Public Key Infrastructure (PKI) as described in [X509].  In
   such interactions, the entity that initiates the connection is called
   a "client" and the entity that receives the connection is called a
   "server".

      Note: The terms "client" and "server" as used here refer to
      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
      context of an application protocol as deployed on the Internet.

   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
   what it claims to be, verify that there is no attacker in the middle,
   and enforce other relevant security considerations.  Typically this
   checking is done by correlating the information presented in the
   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
   pattern for security purposes have traditionally specified their own
   procedures for checking server identities.  Examples include but are
   not limited to:

   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]

   Unfortunately, this divergence of approaches has caused some
   confusion among developers and protocol designers.  Therefore this



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   document specifies recommended identity checking procedures for
   application protocols produced within the Internet Standards Process,
   for the purpose of codifying secure authentication practices.

   Note: This document is currently limited in scope to the presentation
   of identities in X.509 certificates as issued in the context of the
   Public Key Infrastructure (PKI) and as applied to Transport Layer
   Security [TLS]; a future version of this document might address 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
   other than in the certificate itself (e.g., certificate fingerprints
   for Secure Shell as described in [SSH] or for Datagram Transport
   Layer Security DTLS and Secure Real-time Transport Protocol as
   described in [DTLS-SRTP]), and applications that use security
   technologies other than TLS.


2.  Conventions

   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".

   Most security-related terms are to be understood in the sense defined
   in [SECTERMS]; such terms include, but are not limited to, "attack",
   "authentication", "authorization", "certificate", "credential",
   "fingerprint", "identity", "self-signed certificate", "trust", "trust
   anchor", "trust chain", "validate", and "verify".

   In addition, we define the following terms to assist in understanding
   the process of verifying server identity:

   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
      the client attempts to establish a secure connection to 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, an email address, a SIP address, an XMPP address, or some
      other type (this specification does not yet provide a complete
      taxonomy of identity types).  In the case of domain names, the
      reference identity MUST NOT contain the wildcard character '*'
      (ASCII 42) in the left-most (least significant) domain name
      component or component fragment.





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   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 to 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.  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

   When a client connects to a server, it MUST verify the server's
   identity in order to prevent certain passive and active attacks
   against the connection.  By "verify identity" we mean that the client
   needs to establish that at least one of the presented identities
   matches the reference identity.

3.1.  Overview

   At a high level, the client verifies the server identity in
   accordance with the following rules:

   1.  Before connecting to the server, the client determines the
       identity type 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].
   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 (potentially in an
       application-specific preference order) and compares the value of
       each extension against 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 3.2, including fallbacks to several
       subjectName fields).
   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 either inherently secure
       (e.g., extracting the DNS name from a URI to compare with a



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       subjectAltName of type dNSName or SRVName) or for which the
       mapping is performed in a secure manner (e.g., using [DNSSEC], or
       using a user-configured or admin-configured lookup table for
       host-to-address or address-to-host translations).
   5.  If the identity set has more than one member, a match with any of
       the presented identities is acceptable.

      Note: Beyond the server identity check described in this section,
      clients might complete further checking to ensure that the server
      is authorized to provide the service it is requested to provide.
      The client might need to make use of local policy information in
      making this determination.

3.2.  Comparison Rules

3.2.1.  Domain Names

   If the reference identity is a domain name as defined by [RFC1034]
   and [RFC1035] for "traditional" domain names or by [IDNA] for
   internationalized domain names, then the client can match the
   reference identity against subjectAltName extensions of type dNSName
   and SRVName [SRVNAME] according to the following rules.

   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.

   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 [IDNA]
   before comparison with subjectAltName values of type dNSName;
   specifically, the conversion operation specified in Section 4 of
   [IDNA] 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 [IDNA].

   Unless otherwise specified by an application protocol, the dNSName
   MAY contain one instance of the wildcard character '*'.  The wildcard
   character applies only to the left-most domain name component and



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   matches any single component (thus a dNSName of *.example.com matches
   foo.example.com but not bar.foo.example.com or example.com itself).
   The wildcard character is not allowed in component fragments (thus a
   dNSName of baz*.example.net is not allowed and shall not be taken to
   match baz1.example.net and baz2.example.net).

   In addition to checking the subjectAltName extensions of type dNSName
   and SRVNAME, the client MAY as a fallback check the value of the
   Common Name (CN) (see [LDAP-SCHEMA]) as presented in the subjectName
   component of the server's X.509 certificate.  In existing
   certificates, the CN is often used for encapsulating a domain name;
   for example, consider the following subjectName:

   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 comparing the referenced identity against the Common Name, the
   client MUST follow the comparison rules described above for
   subjectAltName extensions of type dNSName and SRVName, with the
   exception that no wildcard matching is allowed.

   In order to match domain 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).

3.2.2.  IP Addresses

   If the reference identity is an IP address as defined by [IP] or
   [IPv6], then the client can match the reference identity against
   subjectAltName extensions of type iPaddress according to the
   following rules.

   The reference identity MUST be converted to the "network byte order"
   octet string representation; for IP Version 4 the octet string will
   contain exactly four octets, and for IP Version 6 the octet string
   will contain exactly sixteen octets.  The client then compares this
   octet string, where a match occurs if the reference identity and
   presented identity octet strings are identical.

3.2.3.  Email Addresses

   If the reference identity is an email address as defined by [EMAIL],
   then the client SHOULD compare the reference identity against the
   value of the "rfc822Name" subjectAltName extension described in



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   [X509].

   The client MAY also compare the reference identity against the value
   of the "E" attribute of the subjectName as described in [CRMF].

3.2.4.  SIP Addresses

   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
   or email address and proceed as described for those identity types,
   or proceed as described in [SIP-CERTS].

3.2.5.  XMPP Addresses

   If the reference identity is an XMPP address ("JabberID") as defined
   by [XMPP], then the client SHOULD compare the reference identity
   against the value of the following subjectAltName extensions, in this
   order: SRVName, dNSName, and (as defined in [XMPP]) "id-on-xmppAddr".

3.3.  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 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 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) as the validated
      identity of the server and instead MUST proceed as described in
      the next section.  Instead, if the client is a user-oriented
      application, then it 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 attempts behave as in



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      Case #2.  (It is the resposibility of the human user to verify the
      hash value or fingerprint of the certificate with the peer 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 provide a setting that enables the
      check.


4.  Security Considerations

   This entire document discusses security.


5.  IANA Considerations

   This document has no actions for the IANA.


6.  References

6.1.  Normative References

   [IDNA]     Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [IP]       Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [IPv6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [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.

6.2.  Informative References

   [CRMF]     Schaad, J., "Internet X.509 Public Key Infrastructure
              Certificate Request Message Format (CRMF)", RFC 4211,
              September 2005.




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   [DNSSEC]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [DTLS-SRTP]
              McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",
              draft-ietf-avt-dtls-srtp-07 (work in progress),
              February 2009.

   [EMAIL]    Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [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.

   [IPSEC]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [LDAP]     Sermersheim, J., "Lightweight Directory Access Protocol
              (LDAP): The Protocol", RFC 4511, June 2006.

   [LDAP-AUTH]
              Harrison, R., "Lightweight Directory Access Protocol
              (LDAP): Authentication Methods and Security Mechanisms",
              RFC 4513, June 2006.

   [LDAP-SCHEMA]
              Sciberras, A., "Lightweight Directory Access Protocol
              (LDAP): Schema for User Applications", RFC 4519,
              June 2006.

   [LDAP-TLS]
              Hodges, J., Morgan, R., and M. Wahl, "Lightweight
              Directory Access Protocol (v3): Extension for Transport
              Layer Security", RFC 2830, May 2000.

   [NETCONF]  Enns, R., "NETCONF Configuration Protocol", RFC 4741,
              December 2006.




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   [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.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              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]
              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.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.




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   [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.

   [SRVNAME]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, August 2007.

   [SSH]      Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [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.

   [USINGTLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [XMPP]     Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [XMPPBIS]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-02 (work
              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



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   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



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   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



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   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



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   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



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   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



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   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



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   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



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       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.




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   o  Matching is case-insensitive.
   o  A "*" wildcard character MAY be used as the left-most name
      component in the certificate.  For example, *.example.com would
      match a.example.com, foo.example.com, etc., but would not match
      example.com.
   o  If the certificate contains multiple names (e.g., more than one
      dNSName field), then a match with any one of the fields is
      considered acceptable.

   If the match fails, the client SHOULD either ask for explicit user
   confirmation or terminate the connection 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:




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   ######

   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.



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   ######

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.

   ######



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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

   Peter Saint-Andre
   Cisco

   Email: Peter.SaintAndre@WebEx.com


   Kurt D. Zeilenga
   Isode Limited

   Email: Kurt.Zeilenga@Isode.COM


   Jeff Hodges
   PayPal

   Email: Jeff.Hodges@KingsMountain.com


   RL 'Bob' Morgan
   UWashington/Internet2

   Email: rlmorgan@washington.edu



















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