· STIX^™ Version 1.2.1. Part 1: Overview. Edited by Sean Barnum, Desiree Beck, Aharon Chernin, and Rich Piazza. Latest version: http://docs.oasis-open.org/cti/stix/v1.2.1/stix-v1.2.1-part1-overview.html.

· CybOX^™ Version 2.1.1. Part 01: Overview. Edited by Trey Darley, Ivan Kirillov, Rich Piazza, and Desiree Beck. Latest version: http://docs.oasis-open.org/cti/cybox/v2.1.1/cybox-v2.1.1-part01-overview.html.

This specification is related to:

· TAXII^™ Version 2.0. Edited by John Wunder, Mark Davidson, and Bret Jordan. Latest version: http://docs.oasis-open.org/cti/taxii/v2.0/taxii-v2.0.html.

Abstract:

Structured Threat Information Expression (STIX^™) is a language for expressing cyber threat and observable information. STIX Cyber Observables are defined in two documents. This document defines concepts that apply across all of STIX Cyber Observables.

Status:

This document was last revised or approved by the OASIS Cyber Threat Intelligence (CTI) TC on the above date. The level of approval is also listed above. Check the “Latest version” location noted above for possible later revisions of this document. Any other numbered Versions and other technical work produced by the Technical Committee (TC) are listed at https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=cti#technical.

TC members should send comments on this specification to the TC’s email list. Others should send comments to the TC’s public comment list, after subscribing to it by following the instructions at the “Send A Comment” button on the TC’s web page at https://www.oasis-open.org/committees/cti/.

This Committee Specification Public Review Draft is provided under the Non-Assertion Mode of the OASIS IPR Policy, the mode chosen when the Technical Committee was established. For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the TC’s web page (https://www.oasis-open.org/committees/cti/ipr.php).

Note that any machine-readable content (Computer Language Definitions) declared Normative for this Work Product is provided in separate plain text files. In the event of a discrepancy between any such plain text file and display content in the Work Product's prose narrative document(s), the content in the separate plain text file prevails.

Citation format:

When referencing this specification the following citation format should be used:

[STIX-v2.0-Pt3-Cyb-Core]

STIX™ Version 2.0. Part 3: Cyber Observable Core Concepts. Edited by Trey Darley and Ivan Kirillov. 03 May 2017. OASIS Committee Specification Draft 02 / Public Review Draft 02. http://docs.oasis-open.org/cti/stix/v2.0/csprd02/part3-cyber-observable-core/stix-v2.0-csprd02-part3-cyber-observable-core.html. Latest version: http://docs.oasis-open.org/cti/stix/v2.0/stix-v2.0-part3-cyber-observable-core.html.

Notices

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STIX™, CYBOX™, AND TAXII™ (STANDARD OR STANDARDS) AND THEIR COMPONENT PARTS ARE PROVIDED "AS IS" WITHOUT ANY WARRANTY OF ANY KIND, EITHER EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTY THAT THESE STANDARDS OR ANY OF THEIR COMPONENT PARTS WILL CONFORM TO SPECIFICATIONS, ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR FREEDOM FROM INFRINGEMENT, ANY WARRANTY THAT THE STANDARDS OR THEIR COMPONENT PARTS WILL BE ERROR FREE, OR ANY WARRANTY THAT THE DOCUMENTATION, IF PROVIDED, WILL CONFORM TO THE STANDARDS OR THEIR COMPONENT PARTS. IN NO EVENT SHALL THE UNITED STATES GOVERNMENT OR ITS CONTRACTORS OR SUBCONTRACTORS BE LIABLE FOR ANY DAMAGES, INCLUDING, BUT NOT LIMITED TO, DIRECT, INDIRECT, SPECIAL OR CONSEQUENTIAL DAMAGES, ARISING OUT OF, RESULTING FROM, OR IN ANY WAY CONNECTED WITH THESE STANDARDS OR THEIR COMPONENT PARTS OR ANY PROVIDED DOCUMENTATION, WHETHER OR NOT BASED UPON WARRANTY, CONTRACT, TORT, OR OTHERWISE, WHETHER OR NOT INJURY WAS SUSTAINED BY PERSONS OR PROPERTY OR OTHERWISE, AND WHETHER OR NOT LOSS WAS SUSTAINED FROM, OR AROSE OUT OF THE RESULTS OF, OR USE OF, THE STANDARDS, THEIR COMPONENT PARTS, AND ANY PROVIDED DOCUMENTATION. THE UNITED STATES GOVERNMENT DISCLAIMS ALL WARRANTIES AND LIABILITIES REGARDING THE STANDARDS OR THEIR COMPONENT PARTS ATTRIBUTABLE TO ANY THIRD PARTY, IF PRESENT IN THE STANDARDS OR THEIR COMPONENT PARTS AND DISTRIBUTES IT OR THEM "AS IS."

Table of Contents

1 Introduction. 7

1.0.1 IPR Policy. 7

1.1 Terminology. 7

1.2 Normative References. 7

1.3 Non-Normative References. 9

1.4 Overview. 10

1.4.1 Cyber Observable Objects. 10

1.4.2 Cyber Observable Relationships. 10

1.4.3 Cyber Observable Extensions. 10

1.4.4 Vocabularies & Enumerations. 10

1.5 Naming Requirements. 10

1.5.1 Property Names and String Literals. 10

1.5.2 Reserved Names. 11

1.6 Document Conventions. 11

1.6.1 Naming Conventions. 11

1.6.2 Font Colors and Style. 11

2 Cyber Observable Specific Data Types. 12

2.1 Binary. 12

2.2 Hexadecimal 13

2.3 Dictionary. 13

2.4 Object Reference. 13

2.5 Observable Objects. 14

3 Cyber Observable Objects. 15

3.1 Common Properties. 15

3.2 Object References. 15

3.3 Object Property Metadata. 15

3.3.1 String Encoding. 15

3.4 Object Relationships. 16

3.5 Predefined Object Extensions. 17

4 Common Vocabularies. 18

4.1 Encryption Algorithm Vocabulary. 18

5 Customizing Cyber Observables. 20

5.1 Custom Observable Objects. 20

5.1.1 Requirements. 20

5.2 Custom Object Extensions. 21

5.2.1 5.2.1 Requirements. 21

5.3 Custom Object Properties. 22

5.3.1 Requirements. 22

6 Reserved Names. 23

7 Conformance. 24

7.1 Producers and Consumers. 24

7.2. 24

Appendix A. Glossary. 25

Appendix B. Acknowledgments. 26

Appendix C. Revision History. 27

1 Introduction

The STIX 2.0 specification defines structured representations for observable objects and their properties in the cyber domain. These can be used to describe data in many different functional domains, including but not limited to:

● Malware characterization

● Intrusion detection

● Incident response & management

● Digital forensics

STIX Cyber Observables document the facts concerning what happened on a network or host, but not necessarily the who or when, and never the why. For example, information about a file that existed, a process that was observed running, or that network traffic occurred between two IPs can all be captured as Cyber Observable data.

STIX Cyber Observables are used by various STIX Domain Objects (SDOs) to provide additional context to the data that they characterize. The Observed Data SDO, for example, indicates that the raw data was observed at a particular time and by a particular party.

The Cyber Observable Objects chosen for inclusion in STIX 2.0 represent a minimally viable product (MVP) that fulfills basic consumer and producer requirements. Objects and properties not included in STIX 2.0, but deemed necessary by the community, will be included in future releases.

This document (STIX™ Version 2.0. Part 3: Cyber Observable Core Concepts) in the STIX specification describes Cyber Observable Core Concepts. STIX™ Version 2.0. Part 4: Cyber Observable Objects contains the definitions for the Cyber Observable Objects.

1.0.1 IPR Policy

This Committee Specification Public Review Draft is provided under the Non-Assertion Mode of the OASIS IPR Policy, the mode chosen when the Technical Committee was established.

For information on whether any patents have been disclosed that may be essential to implementing this specification, and any offers of patent licensing terms, please refer to the Intellectual Property Rights section of the TC’s web page (https://www.oasis-open.org/committees/cti/ipr.php).

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 [RFC2119].

All text is normative except for examples, the overview (section 1.4), and any text marked non-normative.

1.2 Normative References

[Character Sets] "N. Freed and M. Dürst, “Character Sets”, IANA, December 2013, [Online]. Available: http://www.iana.org/assignments/character-sets/character-sets.xhtml

[IEEE 754-2008] "IEEE Standard for Floating-Point Arithmetic", IEEE 754-2008, August 2008. [Online] Available: http://ieeexplore.ieee.org/document/4610935/

[ISO10118] “ISO/IEC 10118-3:2004 Information technology -- Security techniques -- Hash-functions -- Part 3: Dedicated hash-functions”, 2004. [Online]. Available: http://www.iso.org/iso/catalogue_detail.htm?csnumber=39876

[FIPS81] “DES MODES OF OPERATION”, FIPS PUB 81, December 1980, National Institute of Standards and Technology (NIST). [Online]. Available: http://csrc.nist.gov/publications/fips/fips81/fips81.htm

[FIPS186-4] “Digital Signature Standard (DSS)”, FIPS PUB 186-4, July 2013, Information Technology Laboratory, National Institute of Standards and Technology (NIST). [Online]. Available: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf.

[FIPS202] “SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions”, FIPS PUB 202, August 2015, Information Technology Laboratory, National Institute of Standards and Technology (NIST). [Online]. Available: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf

[MD6] Rivest, R. et. al, "The MD6 hash function - A proposal to NIST for SHA-3”, October 2008. [Online]. Available: http://groups.csail.mit.edu/cis/md6/submitted-2008-10-27/Supporting_Documentation/md6_report.pdf

[NIST 800-38A] M. Dworkin, “Recommendation for Block Cipher Modes of Operation Methods and Techniques”, NIST Special Publication 800-38A, 2001. [Online]. Available: http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf

[NIST 800-38D] M. Dworkin, “Recommendation for Block Cipher Modes of Operation:Galois/Counter Mode (GCM) and GMAC”, NIST Special Publication 800-38D, November 2007. [Online]. Available: http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf

[NIST 800-38E] M. Dworkin, “Recommendation for Block Cipher Modes of Operation: The XTS-AES Mode for Confidentiality on Storage Devices”, NIST Special Publication 800-38E, January 2010. [Online]. Available: http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38e.pdf

[NIST 800-67] W. Barker and E. Barker, “Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher”, NIST Special Publication 800-67, January 2012. [Online]. Available: http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-67r1.pdf

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, April 1992, http://www.rfc-editor.org/info/rfc1321.

[RFC2119] Bradner, S., “"Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, http://www.rfc-editor.org/info/rfc2119

[RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144, DOI 10.17487/RFC2144, May 1997, http://www.rfc-editor.org/info/rfc2144.

[RFC2612] Adams, C. and J. Gilchrist, "The CAST-256 Encryption Algorithm", RFC 2612, DOI 10.17487/RFC2612, June 1999, http://www.rfc-editor.org/info/rfc2612.

[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001, http://www.rfc-editor.org/info/rfc3174.

[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, http://www.rfc-editor.org/info/rfc6234.

[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015, http://www.rfc-editor.org/info/rfc7539.

[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016, http://www.rfc-editor.org/info/rfc8017.

[RIPEND-160] H. Dobbertin, A. Bosselaers, and B. Preneel, “RIPEMD-160:A Strengthened Version of RIPEMD”, April 1996, [Online]. Available: http://homes.esat.kuleuven.be/~bosselae/ripemd160/pdf/AB-9601/AB-9601.pdf

[Salsa20] D. Bernstein, “Salsa20 specification” (n.d.). [Online]. Available: https://cr.yp.to/snuffle/spec.pdf

[Salsa20/8 20/12] D. Bernstein, “Salsa20/8 and Salsa20/12” (n.d.). [Online]. Available: https://cr.yp.to/snuffle/812.pdf

[SSDEEP] J. Kornblum, “Identifying Almost Identical Files Using Context Triggered Piecewise Hashing”, Proceedings of The Digital Forensic Research Conference (DFRWS) 2006. [Online].
Available: http://dfrws.org/sites/default/files/session-files/paper-identifying_almost_identical_files_using_context_triggered_piecewise_hashing.pdf

1.3 Non-Normative References

[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014. http://www.rfc-editor.org/info/rfc7159.txt.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, http://www.rfc-editor.org/info/rfc4648.

1.4 Overview

1.4.1 Cyber Observable Objects

STIX 2.0 defines a set of Cyber Observable Objects for characterizing host-based, network, and related entities. Each of these objects correspond to a data point commonly represented in CTI and digital forensics. Using the building blocks of Cyber Observable Objects, in conjunction with relationships between these objects, individuals can create, document, and share comprehensive information about computer systems and their state.

Throughout this document, Cyber Observable Objects are referred to simply as "Observable Objects". These should not be confused with STIX Domain Objects (SDOs), as defined in STIX™ Version 2.0. Part 1: STIX Core Concepts and STIX™ Version 2.0. Part 2: STIX Objects.

1.4.2 Cyber Observable Relationships

A Cyber Observable Relationship is a reference linking two (or more) related Cyber Observable Objects. Cyber Observable Relationships are only resolvable within the same observable-objects container. References are a property on Cyber Observable Objects that contain the ID of a different Cyber Observable Object.

Throughout this document, Cyber Observable Relationships are referred to simply as "Relationships". These should not be confused with STIX Relationship Objects (SROs), as defined in STIX™ Version 2.0. Part 1: STIX Core Concepts and STIX™ Version 2.0. Part 2: STIX Objects.

1.4.3 Cyber Observable Extensions

Each Observable Object defines a set of base properties that are generally applicable across any instance of the Object. However, there is also a need to encode additional data beyond the base definition of the Object data models. To enable this, STIX permits the specification of such additional properties through the set of Predefined Cyber Observable Object Extensions. Where applicable, Predefined Object Extensions are included in the definitions of Objects. For example, the File Object includes Predefined Object Extensions for characterizing PDF files, raster image files, archive files, NTFS files, and Windows PE binary files.

Producers may also define and include their own Custom Object Extensions. For further information, refer to section 5 (Customizing Cyber Observable Objects.)

1.4.4 Vocabularies & Enumerations

Many Cyber Observable Objects contain properties whose values are constrained by a predefined enumeration or open vocabulary. In the case of enumerations, this is a requirement that producers must use the values in the enumeration and cannot use any outside values. In the case of open vocabularies, this is a suggestion for producers that permits the use of values outside of the suggested vocabulary. If used consistently, vocabularies make it less likely that, for example, one entity refers to the md5 hashing algorithm as "MD5" and another as "md-5-hash", thereby making comparison and correlation easier.

1.5 Naming Requirements

1.5.1 Property Names and String Literals

In the JSON serialization all property names and string literals MUST be exactly the same, including case, as the names listed in the property tables in this specification. For example, the SDO common property created_by_ref must result in the JSON key name "created_by_ref". Properties marked required in the property tables MUST be present in the JSON serialization.

1.5.2 Reserved Names

Reserved property names are marked with a type called RESERVED and a description text of “RESERVED FOR FUTURE USE”. Any property name that is marked as RESERVED MUST NOT be present in STIX content conforming to this version of the specification.

1.6 Document Conventions

1.6.1 Naming Conventions

All type names, property names, and literals are in lowercase, except when referencing canonical names defined in another standard (e.g., literal values from an IANA registry). Words in property names are separated with an underscore(_), while words in type names and string enumerations are separated with a dash (-). All type names, property names, object names, and vocabulary terms are between three and 250 characters long.

1.6.2 Font Colors and Style

The following color, font and font style conventions are used in this document:

● The Consolas font is used for all type names, property names and literals.

○ type names are in red with a light red background - hashes

○ property names are in bold style - protocols

○ literals (values) are in blue with a blue background - SHA-256

● In an object's property table, if a common property is being redefined in some way, then the background is dark gray.

● All examples in this document are expressed in JSON. They are in Consolas 9-point font, with straight quotes, black text and a light grey background, and 2-space indentation.

● Parts of the example may be omitted for conciseness and clarity. These omitted parts are denoted with the ellipses (...).

2 Cyber Observable Specific Data Types

The Cyber Observable specification within STIX makes use of many common types that are defined in section 2 of STIX™ Version 2.0. Part 2: STIX Objects. In addition, data types specific to` the representation of Cyber Observables are defined in this section. The table below lists common data types from STIX Core with a gray background and the Cyber Observable specific types with a white background.

Type	Description
boolean	A value of true or false.
float	An IEEE 754 [IEEE 754-2008] double-precision number.
hashes	One or more cryptographic hashes.
integer	A whole number.
list	An ordered sequence of values. The phrasing “list of type <type>” is used to indicate that all values within the list MUST conform to the specified type.
open-vocab	A value from a STIX open (open-vocab) or suggested vocabulary.
string	A series of Unicode characters.
timestamp	A time value (date and time).
binary	A sequence of bytes.
hex	An array of octets as hexadecimal.
dictionary	A set of key/value pairs.
object-ref	A local reference to a Cyber Observable Object.
observable-objects	One or more Cyber Observable Objects.

2.1 Binary

Type Name: binary

The binary data type represents a sequence of bytes. In order to allow pattern matching on custom objects, for all properties that use the binary type, the property name MUST end with '_bin'.

The JSON MTI serialization represents this as a base64-encoded string as specified in

[RFC4648]. Other serializations SHOULD use a native binary type, if available.

2.2 Hexadecimal

Type Name: hex

The hex data type encodes an array of octets (8-bit bytes) as hexadecimal. The string MUST consist of an even number of hexadecimal characters, which are the digits '0' through '9' and the letters 'a' through 'f'. In order to allow pattern matching on custom objects, for all properties that use the hex type, the property name MUST end with '_hex'.

Examples

...

"src_flags_hex": "00000002"

...

2.3 Dictionary

Type Name: dictionary

A dictionary captures an arbitrary set of key/value pairs. dictionary keys MUST be unique in each dictionary, MUST be in ASCII, and are limited to the characters a-z (lowercase ASCII), A-Z (uppercase ASCII), numerals 0-9, hyphen (-), and underscore (_). dictionary keys SHOULD be no longer than 30 ASCII characters in length, MUST have a minimum length of 3 ASCII characters, MUST be no longer than 256 ASCII characters in length, and SHOULD be lowercase.

dictionary values MUST be valid property base types.

2.4 Object Reference

Type Name: object-ref

The Object Reference data type specifies a local reference to an Observable Object, that is, one which MUST be valid within the local scope of the Observable Objects (observable-objects) container that holds both the source Observable Object and the Observable Object that it references.

Examples

The following example demonstrates how a Network Traffic Object specifies its destination via a reference to an IPv4 Address Object.

{

"0": {

"type": "ipv4-addr",

"value": "198.51.100.2"

"1": {

"type": "network-traffic",

"dst_ref": "0"

}

2.5 Observable Objects

Type Name: observable-objects

The Observable Objects type represents 1 or more Observable Objects as a special set of key/value pairs. The keys in the dictionary are references used to refer to the values, which are objects. Each key in the dictionary SHOULD be a non-negative monotonically increasing integer, incrementing by 1 from a starting value of 0, and represented as a string within the JSON MTI serialization. However, implementers MAY elect to use an alternate key format if necessary.

Examples

{

"0": {

"type": "email-addr",

"value": "jdoe@example.com",

"display_name": "John Doe"

"1": {

"type": "email-addr",

"value": "mary@example.com",

"display_name": "Mary Smith"

"2": {

"type": "email-message",

"from_ref": "0",

"to_refs": ["1"],

"date": "1997-11-21T15:55:06Z",

"subject": "Saying Hello"

}

3 Cyber Observable Objects

This section outlines the common properties and behavior across all Cyber Observable Objects.

The JSON MTI serialization uses the JSON object type [RFC7159] when representing Objects.

3.1 Common Properties

Property Name

Type

Description

type (required)

string

Indicates that this object is an Observable Object. The value of this property MUST be a valid Observable Object type name.

extensions (optional)

dictionary

Specifies any extensions of the object, as a dictionary.

Dictionary keys MUST identify the extension type by name.

The corresponding dictionary values MUST contain the contents of the extension instance.

3.2 Object References

Identifiers on Observable Objects are specified as keys in the observable-objects type. For more information on how such keys may be defined, see section 2.6.

The object-ref type is used to define Observable Object properties that are references to other Observable Objects (such as the src_ref property on the Network Traffic Object). Resolving a reference is the process of identifying and obtaining the actual Observable Object referred to by the reference property. References resolve to an object when the value of the property (e.g., src_ref) is an exact match with the key of another Observable Object that resides in the same parent container as the Observable Object that specifies the reference. This specification does not address the implementation of reference resolution.

3.3 Object Property Metadata

3.3.1 String Encoding

Capturing the observed encoding of a particular Observable Object string is useful for attribution, the creation of indicators, and related use cases.

Certain string properties in Observable Objects may contain an additional sibling property with the same base name and a suffix of _enc that captures the name of the original observed encoding of the property value. All _enc properties MUST specify their encoding using the corresponding name from the the IANA character set registry [Character Sets]. If the preferred MIME name for a character set is defined, this value MUST be used; if it is not defined, then the Name value from the registry MUST be used instead.

As an example of how this capability may be used in an Object, the name property in the File Object has the sibling property name_enc, for capturing the observed encoding of the file name string.

Examples

File with Unicode representation of the filename and a corresponding encoding specification

{

"0": {

"type": "file",

"hashes": {

"SHA-256": "effb46bba03f6c8aea5c653f9cf984f170dcdd3bbbe2ff6843c3e5da0e698766"

"name": "quêry.dll",

"name_enc": "windows-1252"

}

3.4 Object Relationships

A Cyber Observable Relationship is a connection between two or more Cyber Observable Objects within the scope of a given Observable Objects dictionary. Cyber Observable relationships are references that are represented as properties of a Cyber Observable Object, containing the keys of the target Cyber Observable Object(s).

Cyber Observable Object relationships are implemented in Object properties as either singletons or lists. In the case of singleton relationships, the name of their Object property MUST end in _ref, whereas for lists of relationships the name of their Object property MUST end in _refs.

The target(s) of Cyber Observable relationships may be restricted to a subset of Cyber Observable Object types, as specified in the description of the Observable Object property that defines the relationship. For example, the belongs_to_refs property on the IPv4 Address Object specifies that the only valid target of the relationship is one or more AS Objects.

Examples

Network Traffic with Source/Destination IPv4 Addresses and AS

{

"0": {

"type": "ipv4-addr",

"value": "1.2.3.4",

"belongs_to_refs": ["3"]

"1": {

"type": "ipv4-addr",

"value": "2.3.4.5"

"2": {

"type": "network-traffic",

"src_ref": "0",

"dst_ref": "1",

}

"3": {

"type": "as"

"number": 42

}

3.5 Predefined Object Extensions

Predefined Object Extensions have a specific purpose in Cyber Observable Objects: defining coherent sets of properties beyond the base, e.g., HTTP request information for a Network Traffic object. Accordingly, each Cyber Observable Object may include one or more Predefined Object Extensions.

Each Predefined Object Extension can be defined at most once on a given Observable Object. In an Observable Object instance, each extension is specified under the extensions property, which is of type dictionary. Note that this means that each extension is specified through a corresponding key in the extensions property. For example, when specified in a File Object instance, the NTFS extension would be specified using the key value of ntfs-ext.

Examples

Basic File with NTFS Extension

{

"0": {

"type": "file",

"hashes": {

"MD5": "3773a88f65a5e780c8dff9cdc3a056f3"

"size": 25537,

"extensions": {

"ntfs-ext": {

"sid": "1234567"

}

4 Common Vocabularies

4.1 Encryption Algorithm Vocabulary

Type Name: encryption-algo-ov

An open vocabulary of encryption algorithms.

When specifying an encryption algorithm not already defined within the encryption-algo-ov, wherever an authoritative name for an encryption algorithm name is defined, it should be used as the value. In cases where no authoritative name exists and/or there is variance in the naming of a particular encryption algorithm, producers should exercise their best judgement.

Vocabulary Value	Description
AES128-ECB	Specifies the Advanced Encryption Standard (AES) with Electronic Codebook (ECB) mode, as a defined in [NIST 800-38A ].
AES128-CBC	Specifies the Advanced Encryption Standard (AES) with Cipher Block Chaining (CBC) mode, as a defined in [NIST 800-38A ].
AES128-CFB	Specifies the Advanced Encryption Standard (AES) with Cipher Feedback (CFB) mode, as a defined in [NIST 800-38A ].
AES128-OFB	Specifies the Advanced Encryption Standard (AES) with Output Feedback (OFB) mode, as a defined in [NIST 800-38A ].
AES128-CTR	Specifies the Advanced Encryption Standard (AES) with counter (CTR) mode, as a defined in [NIST 800-38A ].
AES128-XTS	Specifies the Advanced Encryption Standard (AES) with XEX Tweakable Block Cipher with Ciphertext Stealing (XTS) mode, as a defined in [NIST 800-38E ].
AES128-GCM	Specifies the Advanced Encryption Standard (AES) with Galois/Counter (GCM) mode, as a defined in N I ST SP 8I00-38D.
Salsa20	Specifies the Salsa20 stream cipher, as defined in the [Salsa20] specification.
Salsa12	Specifies the Salsa20/12 stream cipher as defined in the [Salsa20/8 20/12] specification.
Salsa8	Specifies the Salsa20/8 stream cipher as defined in the [Salsa20/8 20/12] specification.
ChaCha20-Poly1305	Specifies the ChaCha20-Poly1305 stream cipher, as defined in [RFC 7539 ].
ChaCha20	Specifies the ChaCha20 stream cipher (without poly1305 authentication), as defined in [RFC 7539 ].
DES-CBC	Specifies the Data Encryption Standard algorithm with Cipher Block Chaining (CBC) mode, as defined in [FIPS81 ].
3DES-CBC	Specifies the Triple Data Encryption Standard algorithm with Cipher Block Chaining (CBC) mode, as defined in [NIST 800-67] and [NIST 800-38A].
DES-ECB	Specifies the Data Encryption Standard algorithm with Electronic Codebook (ECB) mode, as defined in [FIPS81 ].
3DES-ECB	Specifies the Triple Data Encryption Standard algorithm with Electronic Codebook (ECB) mode, as defined in [NIST 800-67 ].
CAST128-CBC	Specifies the CAST-128 algorithm with Cipher Block Chaining (CBC) mode, as defined in [RFC 2144 ].
CAST256-CBC	Specifies the CAST-256 algorithm with Cipher Block Chaining (CBC) mode, as defined in [RFC 2612 ].
RSA	Specifies the RSA symmetric encryption algorithm, as defined by [RFC 8017]
DSA	Specifies the Digital Signature Algorithm, as defined by [FIPS186-4 ].

5 Customizing Cyber Observables

There are three means to customize Cyber Observable Objects: custom object extensions, custom observable objects, and custom properties. Custom object extensions provide a mechanism and requirements for the specification of extensions not defined by this specification (including relationships) on Observable Objects. Custom Observable Objects provide a mechanism and requirements to create Observable Objects not defined by this specification. Custom properties, as in the rest of STIX, provide a mechanism to add individual properties anywhere in the data model.

Custom Observable Object properties SHOULD be used for cases where it is necessary to add one or more simple additional properties (i.e. key/value pairs) on an Observable Object. On the other hand, Custom Observable Object extensions SHOULD be used for cases where it is necessary to describe more complex additional properties (i.e., those with potentially multiple levels of hierarchy). As an example, a vendor-specific property that expresses some custom threat score for a File Object should be added directly to the Observable Object as a custom property, whereas a set of properties that represent metadata around a new file system to the File Object should be done as a custom extension.

A consumer that receives a STIX document containing Custom Cyber Observable Properties, Extensions, or Objects it does not understand MAY refuse to process the document or MAY ignore those properties or objects and continue processing the document.

5.1 Custom Observable Objects

There will be cases where certain information exchanges can be improved by adding objects that are not specified nor reserved in this document; these objects are called Custom Observable Objects. This section provides guidance and requirements for how producers can use Custom Observable Objects and how consumers should interpret them in order to extend STIX in an interoperable manner.

5.1.1 Requirements

● Producers MAY include any number of Custom Observable Objects in an Observable Objects entity.

● The type property in a Custom Observable Object MUST be in ASCII and MUST only contain the characters a-z (lowercase ASCII), 0-9, and hyphen (-).

● The type property MUST NOT contain a hyphen (-) character immediately following another hyphen (-) character.

● Custom Observable Object names MUST have a minimum length of 3 ASCII characters.

● Custom Observable Object names MUST be no longer than 250 ASCII characters in length.

● The value of the type property in a Custom Observable Object SHOULD start with “x-” followed by a source unique identifier (like a domain name with dots replaced by dashes), a dash and then the name. For example: x-example-com-customobject.

● A Custom Observable Object whose name is not prefixed with “x-” MAY be used in a future version of the specification with a different meaning. Therefore, if compatibility with future versions of this specification is required, the “x-” prefix MUST be used.

● A Custom Observable Object MUST have one or more Custom Properties:

○ Custom Property names MUST be in ASCII and MUST only contain the characters a–z (lowercase ASCII), 0–9, and underscore (_).

○ Custom Property names MUST have a minimum length of 3 ASCII characters.

○ Custom Property names MUST be no longer than 250 ASCII characters in length.

● Custom Observable Objects SHOULD only be used when there is no existing Observable Object defined by the STIX specification that fulfills that need.

● Custom Observable Object property values MUST be a valid primitive, type, or a homogenous list of types.

Examples

Simple Custom Observable Object

{

"0": {

"type": "x-example",

"foo": "bar",

"vals": ["this",

"is",

"an",

"example"]

}

5.2 Custom Object Extensions

In addition to the Predefined Cyber Observable Object extensions specified in STIX™ Version 2.0. Part 4: Cyber Observable Objects, STIX supports user-defined custom extensions for Cyber Observable Objects. As with Predefined Object Extensions, custom extension data MUST be conveyed under the extensions property.

5.2.1 5.2.1 Requirements

● An Observable Object MAY have any number of Custom Extensions.

● Custom Extension names MUST be in ASCII and are limited to characters a-z (lowercase ASCII), 0-9, and dash (-).

● Custom Extension names SHOULD start with “x-” followed by a source unique identifier (like a domain name), a dash and then the name. For example: x-example-com-customextension.

● Custom Extension names MUST have a minimum length of 3 ASCII characters.

● Custom Extension names MUST be no longer than 250 ASCII characters in length.

● Custom Extension names that are not prefixed with “x-” may be used in a future version of the specification for a different meaning. If compatibility with future versions of this specification is required, the “x-” prefix MUST be used.

● Custom Extensions SHOULD only be used when there is no existing extension defined by the STIX 2.0 specification that fulfills that need.

● A Custom Extension MUST have one or more Custom Properties:

○ Custom Property names MUST be in ASCII and MUST only contain the characters a–z (lowercase ASCII), 0–9, and underscore (_).

○ Custom Property names MUST have a minimum length of 3 ASCII characters.

○ Custom Property names MUST be no longer than 250 ASCII characters in length.

Examples

Custom File Object Extension

{

"0": {

"type": "file",

"hashes": {

"SHA-256": "effb46bba03f6c8aea5c653f9cf984f170dcdd3bbbe2ff6843c3e5da0e698766"

"extensions": {

"x-example-com-foo": {

"foo_val": "foo",

"bar_val": "bar"

}

5.3 Custom Object Properties

There will be cases where certain information exchanges can be improved by adding properties to Observable Objects that are neither specified nor reserved in this document; these properties are called Custom Object Properties. This section provides guidance and requirements for how producers can use Custom Object Properties and how consumers should interpret them in order to extend Cyber Observable Objects in an interoperable manner.

5.3.1 Requirements

● A Cyber Observable Object MAY have any number of Custom Properties.

● Custom Property names MUST be in ASCII and MUST only contain the characters a–z (lowercase ASCII), 0–9, and underscore (_).

● Custom Property names SHOULD start with “x_” followed by a source unique identifier (such as a domain name with dots replaced by underscores), an underscore and then the name. For example, x_example_com_customfield.

● Custom Property names MUST have a minimum length of 3 ASCII characters.

● Custom Property names MUST be no longer than 250 ASCII characters in length.

● Custom Property names that do not start with “x_” may be used in a future version of the specification for a different meaning. If compatibility with future versions of this specification is required, the “x_” prefix MUST be used.

● Custom Properties SHOULD only be used when there are no existing properties defined by the STIX 2.0 specification that fulfils that need.

● Custom Properties SHOULD only be used to define simple properties (e.g., those of string or integer type)

● For Custom Properties that use the hex type, the property name MUST end with '_hex'.

● For Custom Properties that use the binary type, the property name MUST end with '_bin'.

Examples

File Object with Custom Properties

{

"0": {

"type": "file",

"hashes": {

"SHA-256": "effb46bba03f6c8aea5c653f9cf984f170dcdd3bbbe2ff6843c3e5da0e698766"

"x_example_com_foo": "bar",

"x_example_com_bar": 27

}

6 Reserved Names

This section defines names that are reserved for future use in revisions of this document. The names defined in this section MUST NOT be used for the name of any Custom Cyber Observable Object or Property.

The following object names are reserved:

● action

7 Conformance

7.1 Producers and Consumers

A "Cyber Observable Producer" is any software that creates Cyber Observable content and conforms to the following normative requirements:

1. It MUST be able to create content encoded as JSON.

2. All properties marked required in the property table for the Cyber Observable Object or type MUST be present in the created content.

3. All properties MUST conform to the specified data type and normative requirements.

4. It MUST support at least one defined Cyber Observable Object per the Conformance section in STIX™ Version 2.0. Part 4: Cyber Observable Objects.

7.2

A "Cyber Observable Consumer" is any software that consumes Cyber Observable content and conforms to the following normative requirements:

1. It MUST support parsing all required properties for the content that it consumes.

Appendix A. Glossary

CAPEC - Common Attack Pattern Enumeration and Classification

Consumer - Any entity that receives STIX content

CTI - Cyber Threat Intelligence

Embedded Relationship - A link (an "edge" in a graph) between one STIX Object and another represented as a property on one object containing the ID of another object

Entity - Anything that has a separately identifiable existence (e.g., organization, person, group, etc.)

IEP - FIRST (Forum of Incident Response and Security Teams) Information Exchange Policy

Instance - A single occurrence of a STIX object version

MTI - Mandatory To Implement

MVP - Minimally Viable Product

Object Creator - The entity that created or updated a STIX object (see section 3.3 of STIX™ Version 2.0. Part 1: STIX Core Concepts).

Object Representation - An instance of an object version that is serialized as STIX

Producer - Any entity that distributes STIX content, including object creators as well as those passing along existing content

SDO - STIX Domain Object (a "node" in a graph)

SRO - STIX Relationship Object (one mechanism to represent an "edge" in a graph)

STIX - Structured Threat Information Expression

STIX Content - STIX documents, including STIX Objects, STIX Objects grouped as bundles, etc.

STIX Object - A STIX Domain Object (SDO) or STIX Relationship Object (SRO)

STIX Relationship - A link (an "edge" in a graph) between two STIX Objects represented by either an SRO or an embedded relationship

TAXII - An application layer protocol for the communication of cyber threat information

TLP - Traffic Light Protocol

TTP - Tactic, technique, or procedure; behaviors and resources that attackers use to carry out their attacks

Appendix B. Acknowledgments

The contributions of the OASIS Cyber Threat Intelligence (CTI) Technical Committee members, enumerated in STIX™ Version 2.0. Part 1: STIX Core Concepts, are gratefully acknowledged.

Appendix C. Revision History

Revision

Date

Editor

Changes Made

2017-01-20

Bret Jordan,

John Wunder,

Rich Piazza,

Ivan Kirillov,

Trey Darley

Initial Version

2017-04-24

Bret Jordan,

John Wunder,

Rich Piazza,

Ivan Kirillov,

Trey Darley

Changes made from first public review

1 Introduction

1.0.1 IPR Policy

1.1 Terminology

1.2 Normative References

1.3 Non-Normative References

1.4 Overview

1.4.1 Cyber Observable Objects

1.4.2 Cyber Observable Relationships

1.4.3 Cyber Observable Extensions

1.4.4 Vocabularies & Enumerations

1.5 Naming Requirements

1.5.1 Property Names and String Literals

1.5.2 Reserved Names

1.6 Document Conventions

1.6.1 Naming Conventions

1.6.2 Font Colors and Style

2 Cyber Observable Specific Data Types

2.1 Binary

2.2 Hexadecimal

2.3 Dictionary

2.4 Object Reference

2.5 Observable Objects

3 Cyber Observable Objects

3.1 Common Properties

3.2 Object References

3.3 Object Property Metadata

3.3.1 String Encoding

3.4 Object Relationships

3.5 Predefined Object Extensions

4 Common Vocabularies

4.1 Encryption Algorithm Vocabulary

5 Customizing Cyber Observables

5.1 Custom Observable Objects

5.1.1 Requirements

5.2 Custom Object Extensions

5.2.1 ​5.2.1​ Requirements

5.3 Custom Object Properties

5.3.1 Requirements

6 Reserved Names

7 Conformance

7.1 Producers and Consumers

7.2

5.2.1 5.2.1 Requirements