cpe core team: brant cheikes and mary parmelee (mitre); dave waltermire, paul cichonski, harold...
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CPE Core Team: Brant Cheikes and Mary Parmelee (MITRE); Dave Waltermire, Paul Cichonski, Harold Booth and Chris McCormick (NIST); Jim Ronayne and Shane Shaffer (DOD); Seth Hanford (Cisco); Kent Landfield (McAfee); Tim Keanini (nCircle)
CPE Naming Specification Outline
MITRE1
CPE Specification Stack
Naming
Matching
Representation
(Binding)
Language Dictionary
The diagram below illustrates the stack relationship among the various specifications comprising v2.3 of the Common Product Enumeration (CPE) standard.
Naming is at the bottom of the stack—it defines a the general concept of a well-formed name Defines the logical structure of well-formed names, and requirements
on attributes and values used to form names Provides informative guidance relating to the use of names and the
different contexts where they may be used
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CPE 2.3 Naming Specification Executive Summary (1)In v2.3 we introduce new features in CPE
names that make those names non-conformant with the v2.2 specification– We distinguish “v2.2 conformant names” from
“v2.3 conformant names”– We define a mechanical translation between
versions of names
We define a Well-Formed Name (WFN) as a referring expression– Interpretation depends on context of use
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CPE 2.3 Naming Specification Executive Summary (2)We define a WFN as a conceptual data
structure which can be bound to a version-conformant machine-readable representation– We retain the URI binding for backward
compatibility w/ CPE v2.2 and to facilitate interoperability with v2.2 conformant SCAP tools and content
– We define a new formatted string binding for use by CPE v2.3 conformant SCAP tools and content
Naming Specification ScopeIn scope:
– The logical structure of Well-Formed CPE Names– Procedures for binding well-formed names to their
encodings for exchange among machines– Procedures for translating between bindings
Out of scope:– Criteria for determining “correct” or “valid” values
for attributes of products– Procedures for comparing/matching names
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Well-Formed CPE Names
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Well-Formed CPE NamesA well-formed CPE name (WFN) is an
unordered set of attribute-value pairsMust satisfy these criteria:
– Attributes selected from a fixed vocabulary– Each attribute appears at most once in a name– Values of attributes are character strings
Some reserved and special characters
– Some attributes may have specified valid values, for most others we recommended that values be chosen from valid-values lists
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WFNs: Conceptual Data StructuresWFN is a conceptual data structure
– A kind of “normal form” for product identifiers and identifying expressions
– There shall be no requirement that SCAP tools use WFNs internally
When discussing WFNs in the spec, we will use the following written representation:– [a1=“v1”,a2=“v2”,…]
Ex1: [part=“a”,vendor=“adobe”,…]Ex2: [part=“o”,version=“3.*”]
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WFNs: Legal Attributes (1/2)Shall be no mandatory/optional distinction
– All attributes are effectively optional
All seven v2.2 components become allowed attributes of WFNs in 2.3:– Part, Vendor, Product, Version, Update, Edition and
Language
These will have the same meanings in v2.3 as in v2.2
NB: the edition attribute will be deprecated in v2.3—its use allowed only under certain circumstances
WFNs: Legal Attributes (2/2)WFNs shall allow four new legal attributes:
– sw_edition (“software edition”)– target_sw (“target software platform”)– target_hw (“target hardware platform”)– other_edition (“other edition data not included elsewhere”)
We will not convert legacy dictionary content to use these new attributes
If any of these four attributes are used in a WFN, the (deprecated) edition attribute must not be used
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WFNs: Attribute ValuesAttributes values are character stringsUS-ASCII character set
– Excluding whitespace and CTRL characters
We will specify a maximum string lengthReserved characters:
– colon (:), fwd_slash (/), double-quoteThese must be percent-encoded when embedded in
value strings
Special characters:– Question-mark (?), asterisk (*)
WFNs are Referring ExpressionsA WFN is a kind of referring expression
– It denotes something (or set of things) in the world
That which is denoted is called the referentDetermining the referent of a WFN depends
on its context of use– Ex: “The president of the US” has different
referents depending on temporal context
Upper-stack specifications may define contexts of use in which attribute values have special interpretations
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Inventory ContextInventory context is the context of use in
which an asset inventory tool reports lists of names of products believed to be installed within an enterprise
Each WFN on the inventory list is intended to have a single product as its referent, but the inventory tool may only be able to provide a partial description of that product– In this context, a WFN is ambiguous if there is
more than one possible referent
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Catalog ContextCatalog context is the context of use in which
an organization creates an authoritative listing of names of distinct products
Each WFN in the catalog is intended to have a single product as its referent, and is assumed to fully describe that product to the best knowledge of the catalog curator
No requirement that each product in the catalog exists either in the world or in the enterprise’s installed inventory
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Applicability ContextApplicability context is the context of use in
which two WFNs (a “source” and a “target”) are compared to determine whether the referents of the source and target are disjoint
Both the source and the target may be ambiguous—i.e., have multiple referents in the world
Disjointness is determined by reference to a catalog– The catalog serves as a proxy for the world
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Bindings
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TermsTo bind a name means:
– Convert a WFN to a machine-readable representation suitable for interchange among SCAP applications
To unbind a name means:– To convert a machine-readable representation of
a CPE identifier into a WFN
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Overview (1/2)For interoperability purposes, we will define
two alternative bindings for a WFN:– A URI binding for use when exchanging CPE
information with CPE 2.2 conformant SCAP tools– A formatted string binding for use when
exchanging CPE information with CPE 2.3 conformant SCAP tools
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Overview (2/2)In general, the procedure for translating a
CPE 2.2-conformant bound name to a CPE 2.3-conformant bound name takes two steps:– First unbind the name into a WFN– Bind the resulting WFN to the desired target
binding
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Binding a WFN to a formatted string (1/2)The specified binding of WFN shall be a
formatted string prefixed with “cpe-2.3:/”Iterate through WFN attributes in this order:
– part, vendor, product, version, update, edition, sw_edition, target_sw, target_hw, other_edition, languageIf edition attribute is used, treat as equivalent to
sw_edition, and skip target_sw, target_hw, and other_edition; otherwise ignore edition
Binding a WFN to a formatted string (2/2)Concatenate value strings together,
separating each one with a colon:– If the attribute is absent in the WFN, encode its
value as ‘*’
Thus every attribute value appears explicitly in the bound form– TBD: do we need to be able to elide trailing “:*”
substrings?
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Ex: WFN to formatted string“Foo Bar for C++ Professional Edition
version 1.3 for 32-bit systems”WFN: [part=“a”,vendor=“foo”,
product=“bar_for_c++”,version=“1.3”,update=“-”,sw_edition=“professional”,target_hw=“x32”]
Bound form:– cpe-2.3:/a:foo:bar_for_c++:1.3:-:professional:*:x32:*:*– target_sw, other_edition and language unspecified, bound
to wildcards
Unbinding a CPE-2.3 NameStraightforward process:
– Parse out the attribute value strings in order:Part, vendor, product, version, update, sw_edition,
target_sw, target_hw, other_edition, language
No need to bother with percent-encoded characters since attribute value strings are in normal form
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Binding a WFN to a URI (1/2)Step 1: normalize all value strings
– Delete each occurrence of the 2.3-defined special characters (‘?’ and ‘*’)
– Percent-encode all RFC-3986 “reserved” characters
Step 2: If the edition attribute is not used in the WFN, create a value for it by “packing” the four other edition-related attributes (next two slides)
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Packing (1/2)Initialize the edition attribute of the WFN to
be the empty stringIterate over the four edition-related attributes:
– sw_edition, target_sw, target_hw, other_edition– Append to the edition string:
concatenate “-” and the attribute valueIf the attribute value is empty or not specified, use “”
NB: the result is a new string, prefixed with a hyphen, in which each edition-related attribute is concatenated in a fixed order separated by hyphens
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Packing (2/2): Examples“…:-professional-winxp-x64-v88:…” (all four)“…:---x32-:…” (only target HW; three leading
hyphens, one trailing)“…:--winxp-x32-:…” (middle two)
Binding a WFN to a URI (2/2)Step 3: Populate the URI template:
– cpe:/<part>:<vendor>:<product> … etc.
Step 4: Step thru each corresponding attribute in the WFN, retrieving the corresponding attribute-value pair from the input WFN– If the attribute is absent in the WFN, encode it as a blank
component value
Step 5 (opt): After the template is fully populated:– From the right-most end, delete trailing colons (“:”) until the
first non-colon is reached
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Unbinding a 2.2 NameStep 1: Parse out the seven componentsStep 2: Unpack the edition component (next
slide)Step 3: Decode all percent-encoded
characters except colon, slash, dquoteStep 4: Delete each occurrence of the 2.3
special characters (“?” and “*”)Step 5: Replace blank values with “*”
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UnpackingUnpacking performed when unbinding a 2.2
name into a WFNThe “edition” attribute of the 2.2 name is
inspected for a leading hyphen, and if present, the four subdelimited values are parsed out into the four 2.3 edition-related attributes
If no leading hyphen is found, the 2.2 edition attribute is simply copied to the (deprecated) 2.3 edition attribute
Example: WFN to URIWFN: [vendor=“microsoft”,product=“c#”,
update=“-”,target_hw=“x64”]Normalize the strings and pack the edition
attribute: [vendor=“microsoft”,product=“c%23”,update=“-”,edition=“---x64”]
URI after step 4:– cpe:/:microsoft:c%23::-:---x64:
URI after step 5:– cpe:/:microsoft:c%23::-:---x64
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Conclusion
MITRE
How does this solution address community issues?
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Does this provide a solution to community issues? No prefix property—no defined hierarchical
relationship among attributesV2.2 URI binding supportedV2.3 introduces a formatted string bindingV2.3 names may incorporate special characters
which may have special meaningsV2.3 names minimize the need for percent encodingWe’ve narrowly scoped the spec to focus on
structure and format, leaving meanings and interpretations to upper-stack specs
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