Suspicious DNS Z Flag Bit Set (via dns)

title: Suspicious DNS Z Flag Bit Set (via dns)
id: c8118d9f-93f0-5d1b-8369-f0d3fa50833f
status: stable
description: 'This rule detects 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, that can be found here: https://www3.corelight.com/corelights-introductory-guide-to-threat-hunting-with-zeek-bro-logs'''
references:
    - https://attack.mitre.org/techniques/T1571/
    - https://attack.mitre.org/techniques/T1095/
    - https://twitter.com/neu5ron/status/1346245602502443009
    - https://tdm.socprime.com/tdm/info/eLbyj4JjI15v#sigma
    - https://tools.ietf.org/html/rfc2929#section-2.1
    - https://www.netresec.com/?page=Blog&month=2021-01&post=Finding-Targeted-SUNBURST-Victims-with-pDNS
author: 'Huntrule Team'
date: 2026-03-31
tags:
    - attack.t1095
    - attack.t1571
    - attack.command-and-control
logsource:
    product: zeek
    service: dns
detection:
    z_flag_unset:
        Z: 0
    most_probable_valid_domain:
        query|contains: '.'
    exclude_tlds:
        query|endswith:
            - '.arpa'
            - '.local'
            - '.ultradns.net'
            - '.twtrdns.net'
            - '.azuredns-prd.info'
            - '.azure-dns.com'
            - '.azuredns-ff.info'
            - '.azuredns-ff.org'
            - '.azuregov-dns.org'
    exclude_query_types:
        qtype_name:
            - 'ns'
            - 'mx'
    exclude_responses:
        answers|endswith: '\\x00'
    exclude_netbios:
        id.resp_p:
            - 137
            - 138
            - 139
    condition: not z_flag_unset and most_probable_valid_domain and not (exclude_tlds or exclude_query_types or exclude_responses or exclude_netbios)
falsepositives:
    - Unknown
level: medium