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Many application technologies enable a secure connection between two entities using certificates in the context of Transport Layer Security (TLS). This document specifies best current practices for representing and verifying the identity of application servers in such interactions.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”
This Internet-Draft will expire on November 1, 2010.
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
1.
Introduction
1.1.
Motivation
1.2.
Scope
1.3.
Terminology
1.4.
Discussion Venue
1.5.
Contributors
1.6.
Acknowledgements
2.
Representation of Server Identity
2.1.
Subject Naming in PKIX Certificates
2.2.
PKIX Certificate Name Rules
3.
Verification of Server Identity
3.1.
Overview
3.2.
Constructing an Ordered List of Reference Identifiers
3.3.
Seeking a Match
3.4.
Verifying a Domain Name
3.4.1.
Checking of Traditional Domain Names
3.4.2.
Checking of Internationalized Domain Names
3.4.3.
Checking of Wildcard Labels
3.4.4.
Checking of DNS Domain Names in Common Names
3.5.
Verifying an Application Type
3.5.1.
SRV-ID
3.5.2.
URI-ID
3.6.
Outcome
3.6.1.
Case #1: Match Found
3.6.2.
Case #2: No Match Found, Cached Certificate
3.6.3.
Case #3: No Match Found, Uncached Certificate
3.6.3.1.
Interactive User Agent
3.6.3.2.
Automated Client
4.
Security Considerations
4.1.
Service Delegation
4.2.
Wildcard Certificates
4.3.
Internationalized Doman Names
5.
IANA Considerations
6.
References
6.1.
Normative References
6.2.
Informative References
Appendix A.
Prior Art
A.1.
IMAP, POP3, and ACAP (1999)
A.2.
HTTP (2000)
A.3.
LDAP (2000/2006)
A.4.
SMTP (2002/2007)
A.5.
XMPP (2004)
A.6.
NNTP (2006)
A.7.
NETCONF (2006/2009)
A.8.
Syslog (2009)
A.9.
SIP (2010)
A.10.
GIST (2010)
§
Authors' Addresses
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The visible face of the Internet consists of services that employ a client-server architecture in which an interactive or automated client connects to an application server in order to retrieve or upload information, communicate with other entities, or access a broader network of services. When a client connects to an application server using Transport Layer Security [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.), it references some conception of the server's identity while attempting to establish a secure connection (e.g., "the web site at example.com"). Likewise, during TLS negotiation the server presents its conception of the server's identity in the form of a public-key certificate that was issued by a certification authority in the context of the Internet Public Key Infrastructure using X.509 [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.). Only a match between the client's reference identity and the server's presented identity enables the client to be sure that the certificate can legitimately be used to secure the connection.
Many application technologies adhere to the pattern outlined here, including but not limited to the following:
Application protocols have traditionally specified their own rules for representing and verifying server identities. Unfortunately, this divergence of approaches has caused some confusion among certification authorities, application developers, and protocol designers.
To codify best current practices regarding the implementation and deployment of secure PKIX-based authentication, this document specifies recommended procedures for representing and verifying server identities in certificates intended for use in applications employing TLS.
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This document applies only to server identities associated with DNS domain names, only to TLS, and only to PKIX-based systems. Similar considerations might apply to client identities (e.g., for mutual authentication), to identifiers other than DNS domain names (e.g., IP addresses), to security protocols other than TLS (e.g., [IPSEC] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) and [DTLS] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.)), and to keys or certificates created outside the context of PKIX-based systems (e.g., on networks that use a web of trust model based on [OPENPGP] (Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, “OpenPGP Message Format,” November 2007.) or that are not directly connected to the public Internet and therefore cannot depend on Certificate Revocation Lists (CRLs) or the Online Certificate Status Protocol (OCSP)). Although those scenarios might be able to re-use some aspects of the representation and verification rules provided here, they are outside the scope of this document and need to be addressed by separate specifications.
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Because the concept of "identity" is too vague to be actionable in application protocols, we define a set of more concrete terms:
- application server:
- A service on the Internet that enables interactive clients and automated clients to connect for the purpose of retrieving or uploading information, communicate with other entities, or connect to a broader network of services.
- automated client:
- A software agent or device that is not directly controlled by a natural person.
- identifier:
- A particular instance of an identifier type that is either presented by a server in a certificate or referenced by a client for matching purposes.
- identifier type:
- A formally defined category of identifier that can be included in a certificate and therefore also used for matching purposes; the types covered in this specification are:
- CN-ID = a Relative Distinguished Name (RDN) of type Common Name (CN)
- DNS-ID = a subjectAltName identifier of type dNSName
- SRV-ID = the SRVName form of otherName from the GeneralName structure in SubjectAltName
- URI-ID = a subjectAltName identifier of type uniformResourceName
- interactive client:
- A software agent or device that is directly controlled by a natural person.
- PKIX certificate:
- An X.509v3 certificate generated and employed in the context of the Internet Public Key Infrastructure using X.509 [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.).
- presented identifier:
- An identifier that is presented by a server to a client within the server's PKIX certificate when the client attempts to establish a secure connection with the server; the certificate can include one or more presented identifiers of different types.
- reference identifier:
- An identifier that is used by the client for matching purposes when checking the presented identifiers; the client can attempt to match multiple reference identifiers of different types.
- service type:
- A formal identifier for the application protocol used to provide a particular kind of service at a domain; the service type typically takes the form of a URI scheme or a DNS SRV Service.
- source domain:
- The DNS domain name that a client expects an application server to present in the certificate.
- target domain:
- A domain name or host name that a client has derived from the soruce domain in an automated fashion (e.g., by means of a [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) lookup) or that a natural person directly controlling an interactive client has explicitly configured for connecting to the source domain.
Most security-related terms in this document are to be understood in the sense defined in [SECTERMS] (Shirey, R., “Internet Security Glossary, Version 2,” August 2007.); such terms include, but are not limited to, "attack", "authentication", "authorization", "certification authority", "certificate", "credential", "identity", "self-signed certificate", "trust", "trust anchor", "trust chain", "validate", and "verify".
The following capitalized keywords are to be interpreted as described in [TERMS] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.): "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
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[[ RFC Editor: please remove this section. ]]
The editors are actively seeking input from certification authorities, application developers, and protocol designers regarding the recommendations in this document. Please direct feedback to the editors directly or post to the <certid@ietf.org> mailing list, for which archives and subscription information are available at https://www.ietf.org/mailman/listinfo/certid.
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The following individuals made significant contributions to the text of this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.
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The editors and contributors wish to thank the following individuals for their feedback and suggestions: Nelson Bolyard, Scott Cantor, Dave Crocker, Cyrus Daboo, Philip Guenther, Bruno Harbulot, David Harrington, Paul Hoffman, Scott Lawrence, Alexey Melnikov, Ludwig Nussel, Joe Orton, Tom Petch, Tim Polk, Pete Resnick, Joe Salowey, and Dan Wing.
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This section enumerates the rules for representing application server identity in PKIX certificates. First we provide a brief tutorial about subject naming, then we provide the rules.
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The application server is the subject of a server certificate. As [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) says, "[the] subject field identifies the entity associated with the public key stored in the subject public key field."
The application server is identified by a name or names carried in the subject field and/or the subjectAltName extension of the certificate. See sections 4.1.2.6 and 4.2.1.6 of [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) for details. This section only briefly introduces distinguished names and their components, as well as subjectAltNames and the particular subjectAltName extension types explicitly mentioned in this specification.
The subject field of a PKIX Certificate is defined as a X.501 type Name [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.) and known as a Distinguished Name (DN). See also [X.501] (International Telecommunications Union, “Information Technology - Open Systems Interconnection - The Directory: Models,” February 2001.) A DN is an ordered sequence of attribute type and attribute value pairs (termed "attribute value assertions (AVAs)"), where the attributes are to be those of the subject. Each AVA is also termed a "Relative Distinguished Name (RDN)". The RDNs are ordered in the DN sequence from most general to most specific.
Note that certificates are binary objects -- they are encoded using distinguished encoding rules (DER). Thus, displayable (aka printable) renderings of certificate subject (and issuer) names means that the DER-encoded sequences are decoded and converted into a "string representation" of a DN before being rendered. Often such a DN string representation is ordered from left-to-right, most specific to most general. See [LDAP‑AUTH] (Harrison, R., “Lightweight Directory Access Protocol (LDAP): Authentication Methods and Security Mechanisms,” June 2006.) for details.
Certificate subjects may also have "alternative" names. These are represented within certificates using the SubjectAltName field. This field is a sequence of typed fields, where each type is a distinct type of identifier. For example, the SubjectAltName identifier types noted in this specification are:
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The following rules apply to the representation of application server identities in PKIX certificates issued by certification authorities.
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At a high level, the client verifies the server's identity by performing the following actions:
In addition to checking identifiers, a client MAY complete further checks to ensure that the server is authorized to provide the requested service. However, methods for doing so (e.g., consulting local policy information) are out of scope for this document.
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Before connecting to the server, the client MUST construct an ordered list of acceptable reference identifiers.
The inputs here might be a URI that a user has typed into an interface (e.g., an HTTP URL for a web site), configured account information (e.g., the username of an IMAP or POP3 account for retrieving email), or some other combination of information that can yield a source domain and an application type.
The client might need to derive the source domain and application type from the input(s) it has received. The derived data MUST include only information that can be securely parsed out of the inputs (e.g., extracting the DNS domain name from the authority component of a URI or extracting the application type from the scheme of a URI) or information for which the derivation is performed in a secure manner (e.g., using [DNSSEC] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.)).
In some cases the inputs might include more than one DNS domain name, because a user might have explicitly configured the client to associate a target domain with the source domain. Such delegation can occur by means of user-approved DNS SRV records (e.g., _xmpp-server._tcp.im.example.com might yield a hostname of hosting.example.net) or a user-configured lookup table for host-to-address or address-to-host translations (e.g., the Unix "hosts" file). See the Security Considerations for further discussion of service delegation.
Using the combination of DNS domain name(s) and application type, the client constructs a list of reference identifiers in accordance with the following rules:
Note: A client MUST NOT construct a reference identifier corresponding to Relative Distinguished Names (RDNs) other than the Common Name and MUST NOT check for such RDNs in the presented identifiers. In particular, this means that a client MUST NOT check a series of Domain Component (DC) attributes if included in the certificate (because the order of Domain Components is not guaranteed, certain attacks are possible if DC attributes are checked).
The client then orders the list in accordance with the following rules:
To illustrate the ordering rules, consider the case where the inputs are a source domain of "im.example.com" and an application type of "XMPP" (for which application servers are discovered via DNS SRV records) and the user of the client has also explicitly configured a target domain of "hosting.example.net". In this case, the ordered list would be:
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Once the client has constructed its order list of reference identifiers and has received the server's presented identifiers in the form of an PKIX certificate, the client checks its reference identifiers against the presented identifiers for the purpose of finding a match. It does so by seeking a match in preference order and aborting the search if any presented identifier matches one of its reference identifiers. The search fails if the client exhausts its list of reference identifiers without finding a match. Detailed comparison rules for finding a match are provided in the following sections.
Note: A client MUST NOT seek a match for a reference identifier of CN-ID if the presented identifiers include an SRV-ID, URI-ID, DNS-ID, or any application-specific subjectAltName extensions, and MUST NOT check a Common Name in the certificate if it is other than the last Relative Distinguished Name (RDN) in within the sequence of RDNs making up the Distinguished Name within the certificate's subjectName. (Note: The term "last" refers to the DER order, which is often not the string order presented to a user; the order that is applied here MUST be the DER order.)
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This document assumes that each reference identifier contains a DNS domain name that is a "traditional domain name" or an "internationalized domain name". The client MUST match the source domain of a reference identifier according to the following rules.
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The term "traditional domain name" is a contraction of this more formal and accurate name: "traditional US-ASCII letter-digit-hyphen DNS domain name". Traditional domain names are defined in [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) and [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) in conjunction with [RFC1123] (Braden, R., “Requirements for Internet Hosts - Application and Support,” October 1989.) as further explained in [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.). In essence, a traditional domain name consists of a set of one or more labels, with the labels usually shown separated by dots. Labels nominally consist of only [US‑ASCII] (American National Standards Institute, “Coded Character Set - 7-bit American Standard Code for Information Interchange,” 1986.) uppercase and lowercase letters, digits, and hyphen. There are additional qualifications (please refer to the above-referenced specifications for details) but they are not germane to this specification.
If the source domain of a reference identifier is a "traditional domain name", then matching of the reference identifier against the presented identifier is performed by comparing the set of domain components using a case-insensitive ASCII comparison (as clarified by [DNS‑CASE] (Eastlake, D., “Domain Name System (DNS) Case Insensitivity Clarification,” January 2006.)). Each label MUST match in order for the names to be considered to match.
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[[ Editorial Note: This section needs to be updated to reflect [IDNA2008] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” January 2010.). ]]
The term "internationalized domain name" refers to a DNS domain name that conforms to the overall form of a domain name (dot-separated labels) but that can include Unicode code points outside the traditional US-ASCII range, as explained by [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) and [IDNA2008] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” January 2010.).
If the source domain of a reference identifier is an internationalized domain name, then an implementation MUST convert the domain name to the ASCII Compatible Encoding (ACE) format as specified in Section 4 of [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) before comparison; specifically, the conversion operation specified in Section 4 of [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) MUST be performed as follows:
After performing the "to-ASCII" conversion with regard to an internationalized domain name, the DNS labels and names MUST be compared for equality according to the rules specified in Section 3 of [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.), i.e. once all label separators are replaced with U+002E (dot) they are compared in a case-insensitive manner.
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Unless forbidden by an application protocol's own specification(s), an application protocol client employing this specification's rules MAY match the reference identifier against a presented identifier containing one instance of the wildcard character '*' (see [DNS‑WILD] (Lewis, E., “The Role of Wildcards in the Domain Name System,” July 2006.)) as the left-most label of the domain name, e.g. *.example.com (following the definition of "label" from [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.)).
If such a wildcard identifier is presented, the wildcard MUST be used to match only the one position-wise corresponding label (thus *.example.com matches foo.example.com but not bar.foo.example.com or example.com). The client MUST fail to match a presented identifier in which the wildcard character is contained within a label fragment (e.g., baz*.example.net is not allowed and MUST NOT be taken to match baz1.example.net and baz2.example.net), or in which the wildcard character does not comprise the left-most label in the presented identifier (e.g., neither bar.*.example.net nor bar.f*o.example.net are allowed).
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As noted, a client MUST NOT seek a match for a reference identifier of CN-ID if the presented identifiers include an SRV-ID, URI-ID, DNS-ID, or any application-specific subjectAltName extensions.
Therefore, if and only if the identity set does not include subjectAltName extensions of type dNSName, SRVName, or uniformResourceIdentifier (or any application-specific subjectAltName extensions), the client MAY as a fallback check for a DNS domain name in the value of the last Relative Distinguished Name (RDN), if it is of type Common Name (CN), within the sequence of RDNs making up the Distinguished Name within the certificate's subjectName. (Note: The term "last" refers to the DER order, which is often not the string order presented to a user; the order that is applied here MUST be the DER order.)
In existing certificates, the CN is often used for representing a DNS domain name; for example, consider the following subjectName, where the last RDN is a Common Name that is intended to represent a DNS domain name:
ou=Web Services, c=GB, cn=www.example.com
Here the Common Name is "www.example.com" (a string whose form matches that of a DNS domain name) and the client could choose to compare a reference identifier of type CN-ID against that string. When doing so, the client MUST follow the comparison rules for the source domain of an identifier of type DNS-ID, SRV-ID, or URI-ID, as described under Section 3.4 (Verifying a Domain Name).
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As specified under the ordering rules for reference identifiers, a client SHOULD check not only the domain name but also the application type of the service to which it connects. This is a best practice because typically a client is not designed to connect to all kinds of services using all possible application protocols, but instead is designed to connect to a specific kind of service, such as a web site, an email service, or an instant messaging service.
The application type is verified by means of either an SRV-ID or URI-ID.
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The service name portion of an SRV-ID (e.g., "xmpp") MUST be matched in a case-insensitive manner, in accordance with [DNS‑SRV] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.). Note that the "_" character is prepended to the service identifier in DNS SRV records.
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The scheme name portion of a URI-ID (e.g., "sip") MUST be matched in a case-insensitive manner, in accordance with [URI] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.). Note that the ":" character is a separator between the scheme name and the rest of the URI, and therefore does not need to be included in any comparison.
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The outcome of the checking procedure is one of the following cases.
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If the client has found a presented identifier that matches a reference identifier, matching has succeeded. In this case, the client MUST use the matched reference identifier as the validated identity of the server.
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If the client finds no presented identifier that matches any of the reference identifiers but a natural person has permanently accepted the certificate during a previous connection attempt, the certificate is cached. In this case, the client MUST verify that the presented certificate matches the cached certificate and MUST notify the user if the certificate has changed since the last time a secure connection was successfully negotiated.
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If the client finds no presented identifier that matches any of the reference identifiers and a human user has not permanently accepted the certificate during a previous connection attempt, the client MUST NOT consider the certificate to include a validated identity for the application server.
Instead, the client MUST proceed as follows.
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If the client is an interactive client that is directly controlled by a natural person, then it MUST either do one of the following:
If a user permanently accepts a certificate, the client MUST cache the certificate (or some non-forgeable representation such as a hash value) and in future connection attempts MUST behave as in "Case #2: No Match Found, Cached Certificate".
Note: It is the responsibility of the human user to verify the hash value or fingerprint of the certificate with the server over a trusted communication layer.
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If the client is an automated application that is not directly controlled by a natural person, then it SHOULD terminate the connection with a bad certificate error and log the error to an appropriate audit log. An automated application MAY provide a configuration setting that disables this check, but MUST enable the check by default.
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When the connecting application is an interactive client, the source domain name and application type MUST be provided by a human user (e.g. when specifying the server portion of the user's account name on the server or when explicitly configuring the client to connect to a particular host or URI as in [SIP‑LOC] (Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” June 2002.)) and MUST NOT be derived from the user inputs in an automated fashion (e.g., a hostname address discovered through DNS resolution of the source domain). This rule is important because only a match between the user inputs (in the form of a reference identifier) and a presented identifier enables the client to be sure that the certificate can legitimately be used to secure the connection.
However, an interactive client MAY provide a configuration setting that enables a human user to explicitly specify a particular hostname (called a "target domain") to be checked for connection purposes.
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Allowing the wildcard character in certificates has led to homograph attacks involving non-ASCII characters that look similar to characters commonly included in HTTP URLs, such as "/" and "?"; for discussion, see for example [Defeating‑SSL] (Marlinspike, M., “New Tricks for Defeating SSL in Practice,” February 2009.).
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In addition to the wildcard certificate attacks previously mentioned, allowing internationalized domain names can lead to the inclusion of visually similar (so-called "confusable") characters in certificates; for discussion, see for example [IDNA‑DEFS] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” January 2010.).
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This document has no actions for the IANA.
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(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 "[...]").
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In 1999, [USINGTLS] (Newman, C., “Using TLS with IMAP, POP3 and ACAP,” June 1999.) 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:
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.
######
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In 2000, [HTTP‑TLS] (Rescorla, E., “HTTP Over TLS,” May 2000.) 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] (Housley, R., Ford, W., Polk, T., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and CRL Profile,” January 1999.). 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.
######
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In 2000, [LDAP‑TLS] (Hodges, J., Morgan, R., and M. Wahl, “Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security,” May 2000.) 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:
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.
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In 2006, [LDAP‑AUTH] (Harrison, R., “Lightweight Directory Access Protocol (LDAP): Authentication Methods and Security Mechanisms,” June 2006.) specified the following text regarding server identity verification in LDAP:
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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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.), 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] (Sciberras, A., “Lightweight Directory Access Protocol (LDAP): Schema for User Applications,” June 2006.) value in the last Relative Distinguished Name (RDN) of the subjectName field of the server's certificate (where "last" refers to the DER-encoded order, not the order of presentation in a string representation of DER-encoded data). This comparison is performed using the rules for comparison of DNS names in Section 3.1.3.1, below, with the exception that no wildcard matching is allowed. Although the use of the Common Name value is existing practice, it is deprecated, and Certification Authorities are encouraged to provide subjectAltName values instead. Note that the TLS implementation may represent DNs in certificates according to X.500 or other conventions. For example, some X.500 implementations order the RDNs in a DN using a left-to-right (most significant to least significant) convention instead of LDAP's right-to-left convention.
If the server identity check fails, user-oriented clients SHOULD either notify the user (clients may give the user the opportunity to continue with the LDAP session in this case) or close the transport connection and indicate that the server's identity is suspect. Automated clients SHOULD close the transport connection and then return or log an error indicating that the server's identity is suspect or both.
Beyond the server identity check described in this section, clients should be prepared to do further checking to ensure that the server is authorized to provide the service it is requested to provide. The client may need to make use of local policy information in making this determination.
3.1.3.1. Comparison of DNS Names
If the reference identity is an internationalized domain name, conforming implementations MUST convert it to the ASCII Compatible Encoding (ACE) format as specified in Section 4 of RFC 3490 [IDNA2003] (Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) 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:
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] (Postel, J., “Internet Protocol,” September 1981.) [IPv6] (Deering, S. and R. Hinden, “Internet Protocol, Version 6 (IPv6) Specification,” December 1998.). 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.
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In 2002, [SMTP‑TLS] (Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” February 2002.) specified the following text regarding server identity verification in SMTP:
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4.1 Processing After the STARTTLS Command
[...]
The decision of whether or not to believe the authenticity of the other party in a TLS negotiation is a local matter. However, some general rules for the decisions are:
[...]
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In 2006, [SMTP‑AUTH] (Siemborski, R. and A. Melnikov, “SMTP Service Extension for Authentication,” July 2007.) specified the following text regarding server identity verification in SMTP:
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14. Additional Requirements When Using SASL PLAIN over TLS
[...]
After a successful [TLS] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) negotiation, the client MUST check its understanding of the server hostname against the server's identity as presented in the server Certificate message, in order to prevent man-in-the-middle attacks. If the match fails, the client MUST NOT attempt to authenticate using the SASL PLAIN mechanism. Matching is performed according to the following rules:
The client MUST use the server hostname it used to open the connection as the value to compare against the server name as expressed in the server certificate. The client MUST NOT use any form of the server hostname derived from an insecure remote source (e.g., insecure DNS lookup). CNAME canonicalization is not done.
If a subjectAltName extension of type dNSName is present in the certificate, it SHOULD be used as the source of the server's identity.
Matching is case-insensitive.
A "*" wildcard character MAY be used as the leftmost name component in the certificate. For example, *.example.com would match a.example.com, foo.example.com, etc., but would not match example.com.
If the certificate contains multiple names (e.g., more than one dNSName field), then a match with any one of the fields is considered acceptable.
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In 2004, [XMPP] (Saint-Andre, P., Ed., “Extensible Messaging and Presence Protocol (XMPP): Core,” October 2004.) specified the following text regarding server identity verification in XMPP:
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14.2. Certificate Validation
When an XMPP peer communicates with another peer securely, it MUST validate the peer's certificate. There are three possible cases:
- Case #1:
- The peer contains an End Entity certificate which appears to be certified by a chain of certificates terminating in a trust anchor (as described in Section 6.1 of [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.)).
- 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:
In Case #2 and Case #3, implementations SHOULD act as in (2) above.
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At the time of this writing, [XMPPBIS] (Saint-Andre, P., “Extensible Messaging and Presence Protocol (XMPP): Core,” April 2010.) refers to this document for rules regarding server identity verification in XMPP.
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In 2006, [NNTP‑TLS] (Murchison, K., Vinocur, J., and C. Newman, “Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP),” October 2006.) specified the following text regarding server identity verification in NNTP:
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5. Security Considerations
[...]
During the TLS negotiation, the client MUST check its understanding of the server hostname against the server's identity as presented in the server Certificate message, in order to prevent man-in-the-middle attacks. Matching is performed according to these rules:
If the match fails, the client SHOULD either ask for explicit user confirmation or terminate the connection with a QUIT command and indicate the server's identity is suspect.
Additionally, clients MUST verify the binding between the identity of the servers to which they connect and the public keys presented by those servers. Clients SHOULD implement the algorithm in Section 6 of [PKIX] (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,” May 2008.) for general certificate validation, but MAY supplement that algorithm with other validation methods that achieve equivalent levels of verification (such as comparing the server certificate against a local store of already-verified certificates and identity bindings).
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In 2006, [NETCONF‑SSH] (Wasserman, M. and T. Goddard, “Using the NETCONF Configuration Protocol over Secure SHell (SSH),” December 2006.) specified the following text regarding server identity verification in NETCONF:
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6. Security Considerations
The identity of the server MUST be verified and authenticated by the client according to local policy before password-based authentication data or any configuration or state data is sent to or received from the server. The identity of the client MUST also be verified and authenticated by the server according to local policy to ensure that the incoming client request is legitimate before any configuration or state data is sent to or received from the client. Neither side should establish a NETCONF over SSH connection with an unknown, unexpected, or incorrect identity on the opposite side.
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In 2009, [NETCONF‑TLS] (Badra, M., “NETCONF over Transport Layer Security (TLS),” May 2009.) 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] (Murchison, K., Vinocur, J., and C. Newman, “Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP),” October 2006.)):
If the match fails, the client MUST either ask for explicit user confirmation or terminate the connection and indicate the server's identity is suspect.
Additionally, clients MUST verify the binding between the identity of the servers to which they connect and the public keys presented by those servers. Clients SHOULD implement the algorithm in Section 6 of [PKIX] (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,” May 2008.) 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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In 2009, [SYSLOG‑TLS] (Miao, F., Ma, Y., and J. Salowey, “Transport Layer Security (TLS) Transport Mapping for Syslog,” March 2009.) specified the following text regarding server identity verification in Syslog:
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5.2. Subject Name Authorization
Implementations MUST support certification path validation [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.). In addition, they MUST support specifying the authorized peers using locally configured host names and matching the name against the certificate as follows.
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At the time of this writing, [SIP‑CERTS] (Gurbani, V., Lawrence, S., and B. Laboratories, “Domain Certificates in the Session Initiation Protocol (SIP),” March 2010.) specified text regarding server identity verification in the Session Initiation Protocol (SIP). However, that specification has not yet been approved by the IESG and text cannot be considered final.
The relevant text follows.
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7.2. Comparing SIP Identities
When an implementation (either client or server) compares two values as SIP domain identities:
Implementations MUST compare only the DNS name component of each SIP domain identifier; an implementation MUST NOT use any scheme or parameters in the comparison.
Implementations MUST compare the values as DNS names, which means that the comparison is case insensitive as specified by [DNS‑CASE] (Eastlake, D., “Domain Name System (DNS) Case Insensitivity Clarification,” January 2006.). Implementations MUST handle Internationalized Domain Names (IDNs) in accordance with Section 7.2 of [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.).
Implementations MUST match the values in their entirety:
Implementations MUST NOT match suffixes. For example, "foo.example.com" does not match "example.com".
Implemenations MUST NOT match any form of wildcard, such as a leading "." or "*." with any other DNS label or sequence of labels. For example, "*.example.com" matches only "*.example.com" but not "foo.example.com". Similarly, ".example.com" matches only ".example.com", and does not match "foo.example.com."
[HTTP‑TLS] (Rescorla, E., “HTTP Over TLS,” May 2000.) allows the dNSName component to contain a wildcard; e.g., "DNS:*.example.com". [PKIX] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.), while not disallowing this explicitly, leaves the interpretation of wildcards to the individual specification. [SIP] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) does not provide any guidelines on the presence of wildcards in certificates. Through the rule above, this document prohibits such wildcards in certificates for SIP domains.
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In 2010, [GIST] (Schulzrinne, H. and M. Stiemerling, “GIST: General Internet Signalling Transport,” June 2009.) specified the following text regarding server identity verification in the General Internet Signalling Transport:
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5.7.3.1. Identity Checking in TLS
After TLS authentication, a node MUST check the identity presented by the peer in order to avoid man-in-the-middle attacks, and verify that the peer is authorised to take part in signalling at the GIST layer. The authorisation check is carried out by comparing the presented identity with each Authorised Peer Database (APD) entry in turn, as discussed in Section 4.4.2. This section defines the identity comparison algorithm for a single APD entry.
For TLS authentication with X.509 certificates, an identity from the DNS namespace MUST be checked against each subjectAltName extension of type dNSName present in the certificate. If no such extension is present, then the identity MUST be compared to the (most specific) Common Name in the Subject field of the certificate. When matching DNS names against dNSName or Common Name fields, matching is case- insensitive. Also, a "*" wildcard character MAY be used as the left- most name component in the certificate or identity in the APD. For example, *.example.com in the APD would match certificates for a.example.com, foo.example.com, *.example.com, etc., but would not match example.com. Similarly, a certificate for *.example.com would be valid for APD identities of a.example.com, foo.example.com, *.example.com, etc., but not example.com.
Additionally, a node MUST verify the binding between the identity of the peer to which it connects and the public key presented by that peer. Nodes SHOULD implement the algorithm in Section 6 of [PKIX] for general certificate validation, but MAY supplement that algorithm with other validation methods that achieve equivalent levels of verification (such as comparing the server certificate against a local store of already-verified certificates and identity bindings).
For TLS authentication with pre-shared keys, the identity in the psk_identity_hint (for the server identity, i.e. the Responding node) or psk_identity (for the client identity, i.e. the Querying node) MUST be compared to the identities in the APD.
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| Peter Saint-Andre (editor) | |
| Cisco | |
| Email: | psaintan@cisco.com |
| Jeff Hodges (editor) | |
| PayPal | |
| Email: | Jeff.Hodges@PayPal.com |