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Subject: Re: [cti] CybOX Object Extensions


I generally like where this is going. Could you write down what you would consider the current line of thought in the CybOX pre-draft spec? Capturing your bullet points and example in the document would help me understand them better in context. It will also help us make progress toward our MVP.

I think having overlapping properties is probably OK. For instance, would I rather parse a single EXT2/3 extension than an EXT2 extension and an EXTGeneric extension, even if that meant EXT2/EXT3 had duplicate structures. If the necessary properties for a given thing are spread across multiple extensions that makes it harder to parse IMO.

From my perspective (and I’m curious what others think) I think you are on the right track overall and I’d be interested in seeing more of these thoughts captured in the pre-draft spec even if we don’t talk about it on the list fist. Personally I’d love to be able to come back and say “I think Section XYZ is pretty much right with a couple nits; I think Section ABC is entirely wrong and should be deleted”.

Thank you.

P.S. Minor nit – would the file object have a type field?
P.P.S. Minot nit #2 – I think I might prefer slightly different names for the extensions (e.g., `file.metadata`), but that’s not core to the topic.

From: <cti@lists.oasis-open.org> on behalf of "Kirillov, Ivan A." <ikirillov@mitre.org>
Date: Wednesday, February 10, 2016 at 1:00 PM
To: "cti@lists.oasis-open.org" <cti@lists.oasis-open.org>
Subject: [cti] CybOX Object Extensions

Sending this to the broader CTI list since it’s part of the STIX/CybOX Indicator tranche. 

I don’t believe we have consensus yet on the concept of CybOX extensions, so here’s our current thinking to help summarize where we stand:
  • CybOX Object extensions are intended to replace the existing CybOX Object hierarchy that is defined through classes and subclasses (e.g., the Windows File Object is a subclass of the File Object), in order to address the issues with this approach [1]
  • Extensions can be defined only for a specific Object (i.e., there are no “generic” extensions – the File Object has its own set, the Network Connection Object has its own set, etc.)
  • An Object may have 0..N extensions defined for it
  • The maximum cardinality for a specific extension on an Object instance is 1
  • Certain extensions may be mutually exclusive with each other in Object instances
  • Extensions are captured in an Object instance through the extended-properties field
    • The extended-properties field is a map/dictionary (our previous thinking was that it would be an array, but it was pointed out that having it be a dictionary would make it easier to access data from specific extensions, and also goes along with the policy of only allowing one extension of a particular type in an instance)
Here’s a JSON example of what extensions on a File Object would look like:


   "hashes": [{

       "type": "md5",

       "hash-value": "3773a88f65a5e780c8dff9cdc3a056f3"


   "size": 25537,

   "extended-properties": {

       "FileMetadataExtension": {"mime-type": "vnd.microsoft.portable-executable"},

       "EXT3FileExtension": {"inode": "34483923"},

       "PEBinaryFileExtension": {"exports": [{"name": "foo_app"}]}



Besides some logistical questions around extension management and versioning [2], the biggest open question is around extension design, especially whether we should permit overlapping properties. Our current thinking is that extensions are defined independently and cannot extend/sub-class each other (to avoid the same issues that we’ve had with this approach). What this means in practice is that there could be cases where two extensions share one or more properties; for example, if we have an EXT2FileExtension and EXT3FileExtension, both could have the “inode” property. To get around this, we could create a “generic” EXTFileExtension that has a set of properties common to all EXT file systems, and have the EXT2FileExtension and EXT3FileExtension contain only their unique set of properties.

Are there any thoughts on how we should approach this? Should we permit overlapping properties in extensions? 


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