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IETF RFC 7495
Last modified on Wednesday, March 25th, 2015
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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)
 Abstract
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
information.
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
references.
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
http://www.rfc-editor.org/info/RFC 7495.
Montville & Black Standards Track PAGE 1 
RFC 7495 IODEF Enumeration Reference Format March 2015
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
Montville & Black Standards Track PAGE 2
RFC 7495 IODEF Enumeration Reference Format March 2015
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
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
format.
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.
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RFC 7495 IODEF Enumeration Reference Format March 2015
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
enumeration.
+-------------------------+
| ReferenceName |
+-------------------------+
| INTEGER specIndex |<>----------[ ID ]
+-------------------------+
Figure 1: The ReferenceName Class
The aggregate class that constitutes ReferenceName is:
ID
One. The identifier assigned to represent the particular
enumeration object being referenced.
The ReferenceName class has one attribute.
specIndex
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:
<iodef:Reference>
<enum:ReferenceName specIndex="1">
<enum:ID>CXI-1234-XYZ</enum:ID>
</enum:ReferenceName>
<iodef:URL>http://cxi.example.com</iodef:URL>
<iodef:Description>Foo</iodef:Description>
</iodef:Reference>
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RFC 7495 IODEF Enumeration Reference Format March 2015
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
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RFC 7495 IODEF Enumeration Reference Format March 2015
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
registry:
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 [RFC 20] with individual embedded spaces allowed.
The ABNF [RFC 5234] 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 [RFC 5234] 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 [RFC 20] excepting white space, i.e., from VCHAR
as defined in [RFC 5234]. Some of the characters allowed in the
version string are escaped when that string is used in XML
Montville & Black Standards Track PAGE 6
RFC 7495 IODEF Enumeration Reference Format March 2015
documents (e.g., '<' is represented as <); the registered
version string contains the unescaped ASCII character in all
such cases.
Specification URI/Reference: A list of one or more URIs
[RFC 3986] 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:
https://nvd.nist.gov/download.cfm#CVE_FEED
Allocation Policy: Specification Required [RFC 5226] (which implies
Expert Review [RFC 5226]).
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
request.
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 [RFC 3688].
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RFC 7495 IODEF Enumeration Reference Format March 2015
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
document.
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
document.
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"
elementFormDefault="qualified"
targetNamespace="urn:ietf:params:xml:ns:iodef-enum-1.0"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:enum="urn:ietf:params:xml:ns:iodef-enum-1.0">
<!--
==========================================================
=== ReferenceName ===
==========================================================
-->
<xs:element name="ReferenceName">
<xs:complexType>
<xs:sequence>
<xs:element name="ID" type="xs:NCName"/>
</xs:sequence>
<xs:attribute name="specIndex"
type="xs:integer" use="required"/>
</xs:complexType>
</xs:element>
</xs:schema>
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RFC 7495 IODEF Enumeration Reference Format March 2015
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[IODEF] Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident
Object Description Exchange Format", RFC 5070, December
2007, <http://www.rfc-editor.org/info/RFC 5070>.
[RFC 5226] 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/RFC 5226>.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005,
<http://www.rfc-editor.org/info/RFC 3986>.
[RFC 5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008, <http://www.rfc-editor.org/info/RFC 5234>.
[RFC 3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004, <http://www.rfc-editor.org/info/RFC 3688>.
6.2. Informative References
[RFC 20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, October 1969,
<http://www.rfc-editor.org/info/RFC 20>.
[IODEFv2] Danyliw, R. and P. Stoecker, "The Incident Object
Description Exchange Format v2", Work in Progress,
draft-ietf-mile-RFC 5070-bis-11, March 2015.
[CCE] The MITRE Corporation, "Common Configuration Enumeration
(CCE): Unique Identifiers for Common System Configuration
Issues", website in "Archive" status,
<http://cce.mitre.org>.
[CPE] The MITRE Corporation, "CPE - Common Platform
Enumeration", website in "Archive" status,
<http://cpe.mitre.org>.
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RFC 7495 IODEF Enumeration Reference Format March 2015
[CVE] The MITRE Corporation, "CVE - Common Vulnerabilities and
Exposures", <http://cve.mitre.org>.
Acknowledgements
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
Montville & Black Standards Track PAGE 10
RFC TOTAL SIZE: 19891 bytes
PUBLICATION DATE: Wednesday, March 25th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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