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RFC 7495 - Enumeration Reference Format for the Incident Object

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Internet Engineering Task Force (IETF)                      A. Montville
Request for Comments: 7495                                           CIS
Category: Standards Track                                       D. Black
ISSN: 2070-1721                                                      EMC
                                                              March 2015

                      Enumeration Reference Format
      for the Incident Object Description Exchange Format (IODEF)


   The Incident Object Description Exchange Format (IODEF) is an XML
   data representation framework for sharing information about computer
   security incidents.  In IODEF, the Reference class provides
   references to externally specified information such as a
   vulnerability, Intrusion Detection System (IDS) alert, malware
   sample, advisory, or attack technique.  In practice, these references
   are based on external enumeration specifications that define both the
   enumeration format and the specific enumeration values, but the IODEF
   Reference class (as specified in IODEF v1 in RFC 5070) does not
   indicate how to include both of these important pieces of

   This document establishes a stand-alone data format to include both
   the external specification and specific enumeration identification
   value, and establishes an IANA registry to manage external
   enumeration specifications.  While this document does not update
   IODEF v1, this enumeration reference format is used in IODEF v2 and
   is applicable to other formats that support this class of enumeration

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2015 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.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
   2. Referencing External Enumerations ...............................3
      2.1. Reference Name Format ......................................4
      2.2. Reference Method Applicability .............................5
   3. Security Considerations .........................................5
   4. IANA Considerations .............................................6
   5. The ReferenceName Schema ........................................8
   6. References ......................................................9
      6.1. Normative References .......................................9
      6.2. Informative References .....................................9
   Acknowledgements ..................................................10
   Authors' Addresses ................................................10

1.  Introduction

   There is an identified need to specify a format to include relevant
   enumeration values from other data representation formats in an IODEF
   document.  It is anticipated that this requirement will exist in
   other standardization efforts within several IETF Working Groups, but
   the scope of this document pertains solely to IODEF.  This format is
   used in IODEF v2 [IODEFv2], which will replace the original IODEF v1
   [IODEF] specification; this document does not specify use of this
   format in IODEF v1 [IODEF].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Referencing External Enumerations

   The need is to place enumeration identifiers and their enumeration
   format references in IODEF's Reference class.  There are several ways
   to accomplish this goal, but the most appropriate at this point is to
   require a specific structure for the ReferenceName string of the
   IODEF Reference class, and use an IANA registry to manage references
   to specific enumeration reference formats.

   Per IODEF [IODEF], the ReferenceName is of type ML_STRING.  This
   becomes problematic when specific references, especially enumeration
   formats such as Common Vulnerability Enumeration [CVE], Common
   Configuration Enumeration [CCE], Common Platform Enumeration [CPE],
   and so on, are referenced -- how is an implementer to know which type
   of reference this is, and thus how to parse it?  One solution,
   presented here, is to require that ReferenceName follow a particular

   Inclusion of such enumeration values, especially those related to
   security automation, is important to incident communication and
   investigation.  Typically, an enumeration identifier is simply an
   identifier with a specific format as defined by an external party.
   Further, that enumeration identifier is itself a reference to
   specific information associated with the identifier.  Thus, the
   ReferenceName is an identifier that is formatted in a specific manner
   and that identifies some set of associated information.

   For example, a vulnerability identifier following the CVE [CVE]
   formatting specification may be CVE-2014-0001.  That identifier is
   formatted in a specific manner and relates to information about a
   specific vulnerability.  Communicating the format for the identifier
   is the subject of this document.

2.1.  Reference Name Format

   The ReferenceName class provides the XML representation for
   identifying an enumeration and specifying a value from it.  A given
   enumeration is uniquely identified by the specIndex attribute.  Each
   specIndex value corresponds to an entry in the "Enumeration Reference
   Type Identifiers" IANA registry (see Section 4).  The child ID
   element represents a particular value from the corresponding
   enumeration identified by the specIndex attribute.  The format of the
   ID element is described in the IANA registry entry of the

         | ReferenceName           |
         | INTEGER specIndex       |<>----------[ ID    ]

                      Figure 1: The ReferenceName Class

   The aggregate class that constitutes ReferenceName is:

         One.  The identifier assigned to represent the particular
         enumeration object being referenced.

   The ReferenceName class has one attribute.

         Required.  INTEGER.  Enumeration identifier.  This value
         corresponds to an entry in the "Enumeration Reference Type
         Identifiers" IANA registry with an identical SpecIndex value.

   An example of such a reference is as follows:

         <enum:ReferenceName specIndex="1">

   Information in the IANA table (see Section 4) would include:

      Full Name: Concept X Identifier
      SpecIndex: 1
      Version: any
      Specification URI: http://cxi.example.com/spec_url

2.2.  Reference Method Applicability

   While the scope of this document pertains to IODEF, any standard
   needing to reference an enumeration identified by a specially
   formatted string can use this method of providing structure after the
   standard has been published.  In effect, this method provides a
   standardized interface for enumeration formats, thus allowing a loose
   coupling between a given standard and the enumeration identifiers it
   needs to reference now and in the future.

3.  Security Considerations

   Ensuring a proper mapping of enumeration reference ID elements to the
   correct SpecIndex is important.  Potential consequences of not
   mapping correctly include inaccurate information in references and
   similar distribution of misinformation.

   Use of enumeration reference IDs from trusted sources is preferred to
   mitigate the risk of receiving and/or providing misinformation.
   Trust decisions with respect to enumeration reference providers are
   beyond the scope of this document.  However, receiving an IODEF
   [IODEF] document containing an unknown ReferenceName (i.e., the
   SpecIndex does not exist in the IANA table) may indicate a misled or
   malicious source.

   This document establishes a container for publicly available
   enumeration values to be included in an IODEF [IODEF] document, and
   it is important to note the distinction between the enumeration
   value's format and the information conveyed by the value itself.
   While the enumeration value may hold information deemed to be private
   by relying parties, the enumeration format is likely not subject to
   privacy concerns.

   However, if the Reference class includes an enumeration value in
   combination with other data in an IODEF [IODEF] document, the
   resulting combination could expose information.  An example might
   include attack vectors or system descriptions used in a privacy-
   related incident.  As such, the reader is referred to the IODEF
   [IODEF] Security Considerations section, which explicitly covers
   protecting IODEF [IODEF] documents in transit and at rest, ensuring

   proper recipient authentication, data confidence levels, underlying
   transport security characteristics, and proper use of IODEF's
   restriction attribute.

4.  IANA Considerations

   This document specifies an enumeration reference identifier format.
   All fields, including abbreviation, are mandatory.

   Per this document, IANA has created and maintains the following

      Name of the Registry: "Security External Enumeration Registry"

      Location of Registry: http://www.iana.org/assignments/sec-ext-enum

      Fields to record in the registry:

         Full Name: The full name of the enumeration (i.e., the
         referenced specification) as a string from the printable ASCII
         character set [RFC20] with individual embedded spaces allowed.
         The ABNF [RFC5234] syntax for this field is:

               FULL-NAME = 1*VCHAR *(SP 1*VCHAR)

         Abbreviation: An abbreviation may be an acronym -- it consists
         of uppercase characters (at least two).  Uppercase is used to
         avoid mismatches due to case differences.  It is specified by
         this ABNF [RFC5234] syntax:

               ABBREVIATION = 2*UC-ALPHA      ; At least two
               UC-ALPHA = %x41-5A             ; A-Z

               Multiple registrations MAY use the same Abbreviation but
               MUST have different Versions.

         SpecIndex: This is an IANA-assigned positive integer that
         identifies the registration.  The first entry added to this
         registry uses the value 1, and this value is incremented for
         each subsequent entry added to the registry.

         Version: The version of the enumeration (i.e., the referenced
         specification) as a free-form string from the printable ASCII
         character set [RFC20] excepting white space, i.e., from VCHAR
         as defined in [RFC5234].  Some of the characters allowed in the
         version string are escaped when that string is used in XML

         documents (e.g., '<' is represented as &lt;); the registered
         version string contains the unescaped ASCII character in all
         such cases.

         Specification URI/Reference: A list of one or more URIs
         [RFC3986] from which the registered specification can be
         obtained.  The registered specification MUST be readily and
         publicly available from that URI.  The URI SHOULD be a stable
         reference to a specific version of the specification.  URIs
         that designate the latest version of a specification (which
         changes when a new version appears) SHOULD NOT be used.

      Initial registry contents:

         Full Name: Common Vulnerabilities and Exposures

         Abbreviation: CVE

         SpecIndex: 1

         Version: 1.0

         Specification URI/Reference:

      Allocation Policy: Specification Required [RFC5226] (which implies
      Expert Review [RFC5226]).

   The Designated Expert is expected to consult with the MILE (Managed
   Incident Lightweight Exchange) working group or its successor if any
   such WG exists (e.g., via email to the working group's mailing list).
   The Designated Expert is expected to review the request and validate
   the appropriateness of the enumeration for the attribute.  This
   review includes review of the specification associated with the

   The Designated Expert is expected to ensure that the Full Name,
   Abbreviation, and Version are appropriate and that the information at
   the Specification URI is sufficient to unambiguously parse
   identifiers based on that specification.  Additionally, the
   Designated Expert should prefer short Abbreviations over long ones.

   This document uses URNs to describe XML namespaces and XML schemas
   conforming to a registry mechanism described in [RFC3688].

   Registration for the IODEF enumeration reference format namespace:

      URI: urn:ietf:params:xml:ns:iodef-enum-1.0

      Registrant Contact: See the "Authors' Addresses" section of this

      XML: None.

   Registration for the IODEF enumeration reference format XML schema:

      URI: urn:ietf:params:xml:schema:iodef-enum-1.0

      Registrant Contact:  See the "Authors' Addresses" section of this

      XML: See Section 5, "The ReferenceName Schema", of this document.

5.  The ReferenceName Schema

      <?xml version="1.0" encoding="UTF-8"?>
      <xs:schema attributeFormDefault="unqualified"

       ===  ReferenceName                                     ===
       <xs:element name="ReferenceName">
             <xs:element name="ID" type="xs:NCName"/>
           <xs:attribute name="specIndex"
                         type="xs:integer" use="required"/>

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,

   [IODEF]    Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident
              Object Description Exchange Format", RFC 5070, December
              2007, <http://www.rfc-editor.org/info/rfc5070>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008, <http://www.rfc-editor.org/info/rfc5226>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005,

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234, January
              2008, <http://www.rfc-editor.org/info/rfc5234>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004, <http://www.rfc-editor.org/info/rfc3688>.

6.2.  Informative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, October 1969,

   [IODEFv2]  Danyliw, R. and P. Stoecker, "The Incident Object
              Description Exchange Format v2", Work in Progress,
              draft-ietf-mile-rfc5070-bis-11, March 2015.

   [CCE]      The MITRE Corporation, "Common Configuration Enumeration
              (CCE): Unique Identifiers for Common System Configuration
              Issues", website in "Archive" status,

   [CPE]      The MITRE Corporation, "CPE - Common Platform
              Enumeration", website in "Archive" status,

   [CVE]      The MITRE Corporation, "CVE - Common Vulnerabilities and
              Exposures", <http://cve.mitre.org>.


   The authors would like to thank Eric Burger for the recommendation to
   rely on XML, Roman D. Danyliw for his schema contribution and
   insight, and Tim Bray, Panos Kampanakis, Barry Leiba, Ted Lemon,
   Alexey Melnikov, Kathleen Moriarty, Takeshi Takahashi, Henry S.
   Thompson, and David Waltermire for their contributions and reviews.

Authors' Addresses

   Adam W. Montville

   EMail: adam.w.montville@gmail.com

   David Black
   EMC Corporation

   EMail: david.black@emc.com


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