The DNS Z flag is bit within the DNS protocol header that is, per the IETF design, meant to be used reserved (unused).
Although recently it has been used in DNSSec, the value being set to anything other than 0 should be rare.
Otherwise if it is set to non 0 and DNSSec is being used, then excluding the legitimate domains is low effort and high reward.
Determine if multiple of these files were accessed in a short period of time to further enhance the possibility of seeing if this was a one off or the possibility of larger sensitive file gathering.
This Sigma query is designed to accompany the Corelight Threat Hunting Guide, which can be found here: https://www3.corelight.com/corelights-introductory-guide-to-threat-hunting-with-zeek-bro-logs'
Adversaries may communicate using a protocol and port paring that are typically not associated.
For example, HTTPS over port 8088(Citation: Symantec Elfin Mar 2019) or port 587(Citation: Fortinet Agent Tesla April 2018) as opposed to the traditional port 443.