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[RFC] Risk field extensions [Stage 0] (#2232)
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* Add initial summary of risk field extensions for stage 0

* set RFC # and date

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Co-authored-by: Eric Beahan <[email protected]>
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# 0042: Risk field extensions
<!-- Leave this ID at 0000. The ECS team will assign a unique, contiguous RFC number upon merging the initial stage of this RFC. -->

- Stage: **0 (strawperson)** <!-- Update to reflect target stage. See https://elastic.github.io/ecs/stages.html -->
- Date: **2023-07-13** <!-- The ECS team sets this date at merge time. This is the date of the latest stage advancement. -->

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Stage 0: Provide a high level summary of the premise of these changes. Briefly describe the nature, purpose, and impact of the changes. ~2-5 sentences.
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This RFC seeks to extend the [existing risk fields](https://www.elastic.co/guide/en/ecs/current/ecs-risk.html) [(RFC 0031)](https://github.com/elastic/ecs/pull/2048) to support new/extended Risk Score investigation workflows. The workflows that this RFC intends to enable include all those described in 0031, along with the following:

1. Risk Score Explainability
* We want to provide more insight into the anatomy of a risk score. The first (and simplest) way we intend to do this is by showing the documents (referred to commonly as Risk Inputs) that contributed to a particular risk score. Given that there may be a large number of these documents, we expect to have to choose a representative subset of these documents to persist along with the score (most obviously: top N riskiest inputs).
* Since we cannot realistically persist the _entire_ contributing document along with the risk score (let alone several), we intend to persist just enough information to allow one to uniquely identify those documents at a later point in time (i.e. during investigation/analysis of a risk score), along with any information that would not be present on the original document (e.g. the document's calculated risk score).
2. Categorical Risk Scores
* While the initial iteration of risk scoring ingested Detection Engine Alerts, we intend to expand risk scoring to include more data sources from multiple new categories of data. While we will still present a single risk score for most investigative purposes (composed of all these evaluated data sources), we believe that it will be useful to present individual risk scores _per category_ of data.
* These categories (and their definitions) are still being discussed [in this internal ticket](https://github.com/elastic/security-team/issues/5485), we currently know that categories will have the following traits:
* There will be a finite (<10) number of categories
* These categories' definitions may be _extended_ in the future to include new data sources
* Due to the above category traits, we need to come up with a naming convention for these categorical score fields that allows them to be extended without invalidating the existing field names.

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Stage 1: If the changes include field additions or modifications, please create a folder titled as the RFC number under rfcs/text/. This will be where proposed schema changes as standalone YAML files or extended example mappings and larger source documents will go as the RFC is iterated upon.
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## Fields

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Stage 1: Describe at a high level how this change affects fields. Include new or updated yml field definitions for all of the essential fields in this draft. While not exhaustive, the fields documented here should be comprehensive enough to deeply evaluate the technical considerations of this change. The goal here is to validate the technical details for all essential fields and to provide a basis for adding experimental field definitions to the schema. Use GitHub code blocks with yml syntax formatting, and add them to the corresponding RFC folder.
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## Usage

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Stage 1: Describe at a high-level how these field changes will be used in practice. Real world examples are encouraged. The goal here is to understand how people would leverage these fields to gain insights or solve problems. ~1-3 paragraphs.
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## Source data

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Stage 1: Provide a high-level description of example sources of data. This does not yet need to be a concrete example of a source document, but instead can simply describe a potential source (e.g. nginx access log). This will ultimately be fleshed out to include literal source examples in a future stage. The goal here is to identify practical sources for these fields in the real world. ~1-3 sentences or unordered list.
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## Scope of impact

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## Concerns

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## People

The following are the people that consulted on the contents of this RFC.

* @rylnd | author
* @SourinPaul | SME / EA product manager

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## References

* [existing risk fields](https://www.elastic.co/guide/en/ecs/current/ecs-risk.html)
* [previous risk fields RFC (stage 3)](https://github.com/elastic/ecs/pull/2048)
* [internal risk categories epic](https://github.com/elastic/security-team/issues/5485)

### RFC Pull Requests

<!-- An RFC should link to the PRs for each of it stage advancements. -->

* Stage 0: https://github.com/elastic/ecs/pull/2232

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* Stage 1: https://github.com/elastic/ecs/pull/NNN
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