Kibana arbitrary code execution via YAML deserialization

A deserialization issue in Kibana can lead to arbitrary code execution when Kibana attempts to parse a YAML document containing a crafted payload. A successful attack requires a malicious user to have a combination of both specific Elasticsearch indices privileges https://www.elastic.co/guide/en/elasticsearch/reference/current/defining-roles.html#roles-indices-priv  and Kibana privileges https://www.elastic.co/guide/en/fleet/current/fleet-roles-and-privileges.html  assigned to them.

The following Elasticsearch indices permissions are required

* write privilege on the system indices .kibana_ingest*
* The allow_restricted_indices flag is set to true

Any of the following Kibana privileges are additionally required

* Under Fleet the All privilege is granted
* Under Integration the Read or All privilege is granted
* Access to the fleet-setup privilege is gained through the Fleet Server’s service account token